[Guide to Industrial Sound] [From the U.S. Government Publishing Office, www.gpo.gov] GUIDE TO INDUSTRIAL SOUND Published By OFFICE OF PRODUCTION RESEARCH AND DEVELOPMENT OF THE WAR PRODUCTION BOARD Sponsored By THE INDUSTRY ADVISORY COMMITTEE FOR INDUSTRIAL SOUND EQUIPMENT AND THE RADIO AND RADAR DIVISION OF THE WAR PRODUCTION BOARD FOREWORD Industrial efficiency is many things. It is production according to plan. It is short straight paths of movement. It is timing. It is control of the process. It is frictionless operation. It is the spirit of the man. The degree of our industrial efficiency will determine the length of the war. When we have made enough things, and the ships to carry them, we shall win and not until then. If we have enough tomorrow, we shall win tomorrow. If we have enough in five years, we shall win in five years, and during the intervening time we must fight and bleed and die, and wait for our industry. Insofar as industrial sound promotes industrial efficiency, it is vital now. And when the time comes for us to compete again industrially with less fortunate nations, our industrial efficiency will be the measure of our prosperity. To the extent that industrial sound promotes efficiency, it is essential tomorrow. But neither victory of arms nor material prosperity can bring us freedom and dignity and peace again. Those are things of the spirit. They deny regimentation. They assert man's importance as an individual. They reflect his community of interest with his fellow man. They cannot thrive where man works solely for the pay envelope to spend in leisure. jUiey have a chance only when man is attuned to his work; when work is not a deplorable necessity, but living; when the man in the shop is as conscious of his metier as is the concert violinist. The spirit of men made our nation, our wealth,, our way of life. It is the one element indispensable to our civilization. It must be the first consideration in our industrial planning. If we insure the human values first; work to do, strength to do it $nd pride in«its accomplishment, we shall have accomplished by industrial evolution more than any social revolution dares promise. Music in the shop and in the office is at once a means to this end and a portent. It belongs to the spirit and it makes the wheels go round. It fuzes work and art. Where music helps to make the work sing it is insurance on the best elements of the American way of life. H. B-M WASHINGTON 19 June 1944 BY: Harold Burris-Meyer TABLE OF CONTENTS PAGF Numerical Index of Tables ............................................ i Table of Illustrations, Charts and Diagrams . ... ..................... ii Chapter 1. INTRODUCTION..................................... 1 Chapter 2. ACOUSTICS ....................................... 9 Chapter 3. SOUND SYSTEM COMPONENTS .......................... 20 Loudspeakers..................................... 20 Baffles....................................... 22 Horns..............................................25 Amplifiers........................................27 -nput Sources - Microphones ............. 27 Input Sources - Radio ........ ; ..... 29 Input Sources - Recorded Material.......... . .;. 30 Reproducing Devices ........................... 32 Equalization '. . .............................. 33 Chapter 4. USES OF INDUSTRIAL SOUND.......................... 35 Chapter 5. SURVEY ..... .............................. ..... 39 Chapter 6. SPECIFICATION.......... .........................42 Durability .......................................42 Safety.......................................... 42 Fidelity....................................... 42 Loudspeakers. ........................... Power Amplifiers............................... 44 Voltage Amplifiers.............................. 45 Networks - Special Circuits ....... ............. 45 Input Sources . ................................ 47 Studio* Treatment. ............................. 47 Switching Control.......................... . . 48 Intensity Control ............................. 48 Wiring......................... 1............... 49 Connections .................................... 50 Chapter 7. ADMINISTRATION 52 Chapter 8. OPERATION........................................ 54 Intensity Range ........................... . . 54 Selective Area Paging . ....................... 54 Conflict of Paging and Music . . . . .............55 Chapter 9. PROGRAMMING WORK MUSIC .............................. 56 What to Play....................................... 56 When to Play..................................... . 57 How to Play . ....................................58 Research .............................. ;............5g Music Library................................. . . 59 BIBLIOGRAPHY FOR FURTHER READING.................................... 6L NUMERICAL INDEX OF TABLES TA^LE PAGE I MICROPHONES................................................ 28 II DISC RECORDING ........................................... 32 III USES OF INDUSTRIAL SOUND - A................................ 36 IF USES OF INDUSTRIAL SOUND - B............................... 37 ▼ SURVEY OBJECTIVES - (Check List) .......................... 40 VI SAMPLE PRIORITY SCHEDULE OF TRANSMITTED MATERIAL .... 55 TABLE OF ILLUSTRATIONS, CHARTS AND DIAGRAMS FIGURE PAGE 1. Console for Plant Broadcasting System ..................... 3 2« Music at Work - 1.............................................. 3 3« Music at Work -2................................................. 4 Ma Power Plant Superintendent Directs Operations ....... 5 5« Power Plant Switchboard ......................................... 6 ¿•-Boiler Control ................................................ 7 7« Complete Power Plant Intercommunication Inatallation ... 8 8a The Piano Keyboard............................................... H 9a Intensities of Familiar Sounds ................................. 12 10a Intensity and Frequency Ranges of Speech ....................... 13 11. Intensity and Frequency Ranges of Music .................... 13 12a Basic Sound System ............................................ *5 13. Effect of Frequency Cut-off on Speech Intelligibility ... 17 18a Effect of Frequency Cut-off on Music Definition ...... 18 IJa Effect of Frequency Band Width on Definition ............ 19 16a Cone Loudspeaker Unit ...........................................21 17. Diaphragm Loudspeaker Unit . . 21 18. Baffles ....................................................... 24 19. Horns......................................................... 26 20. Simple Plant Broadcast System ............................. ... 3^ 21. Complex Plant Broadcast System ................................ 3# 22. Typical Intercommunications System ............................ 35 23« Effect of Frequency Range on Industrial Music Definition 28. Wire Sixes for Loudspeaker Circuits............................ 50 25. Administration Diagram .................................. 53 INTRODUCTION Industrial Sound is the electronic transmission of speech, music, or both in industrial plants. The term embraces intercommunication and plant broadcasting. v It has been of such importance that its use has increased much more rapidly thanxhas the understanding of its complete scope and limitations. The purpose of this manual is to present a comprehensive picture of industrial sound, while at the same time limiting the text to items which are basic a nd ess en t i a 1. The basic elements of any sound system are: microphones or other input sources such as radio, recordings or wired-in programs; amplifiers, loudspeakers, and switching devices. These are available in a wide variety of types and sizes which must be carefully selected to produce the desired result. The term intercommunication is applied to the exchange of information between two or more points - generally involving complete or partial two-way communication. The functions of such systems are basically operational and are employed for t t^nsmi t ting orders, and reports, coordinating processes and controlling operations. Plant Broadcasting, on the other hand, involves the transmission of voice or music from one or more points to the entire plant or to selected areas. In this type of application no provision is made for talking back. Its uses are partly operational and partly aimed at the well-being of the employee. Plant Broadcasting is employed for paging personnel, broadcasting morale building talks, playing music for the relief of fatigue and boredom, and for making emergency announcements. It is important to note that the functions of intercommunication and plant broadcasting are basically different, requiring entirely different treatment and in most cases different equipment. While some of the functions of each may be combined in a common system, this should be done only to meet some special condition. Sound System is a production tool to be selected on the basis of what it will do, not what it costs. It may be called upon to perform tasks which were not originally in-tdHded. Because an inadequate system may have to be entirely 1 replaced, all of its probable uses must be predetermined and provision made for future expansion. Equipment designed solely for voice-paging will seldom be satisfactory for the distribution of music. The reverse may^ also be t rue. Satisfactory results will depend on the cqre exercised in planning, installing and operating the system. A Sound System must be audible, it must be intelligible, it must be capable of continuous operation; it must be able to meet any conditions encountered, such as noise, dust, vibration, corrosive fumes or extremes of temperature and humidity. A competent sound engineer will be able to prescribe suitable equipment for any such conditions. Management must provide an intimate knowledge of the plant and basic planning of what the system is to be used for. The Sound System should be looked upon as an integral part of the plant. Procedures and processes should be modified, where required, to take full advantage of its possibilities. Much has been said about the advantages of Industrial Sound. A word of caution, therefore, seems to be in order. An Intercommunication System in not a telephone and will Aot perform the same functions. It is designed to provide communication between selected persons who have frequent need for rapid or continuous exchange of information. Plant Broadcasting is directed primarily at improving the conditions under which people must work. It must not be distracting. Too much paging is a source of annoyance. The system is best divided into areas so that the person is paged where he is likely to be found. Poor quality of music, repetition, or inappropriate selections are more harmful than advantageous. Monopoly of the system by management for too frequent pep talks engenders resentment. The employee must be made to feel that the system is there for his benefit. He should participate in its use. The following chapters are devoted to a discussion of the basic principles of sound, the equipment involved and some suggestion's as to the most effective use of a sound system. A thorough study of these pages will form the foundation for an effective understanding of this new production tool, anS will be of material assistance in selecting equipment, planning programs and deriving the maximum benefit from Industrial Sound. 2 Figure 2 3 617852 0 - 44 -2 Figure 3 4 Figure 4 5 Figure 5 6 Figure 6 7 Figure 7 8 ACOUSTICS Before discussing the parts of a sound system, it is well to review some fundamentals of acoustics which must be kept constantly in mind when approaching any problem connected with Industrial Sound. Sound is the term we apply to the sensation experienced when vibrating air particles touch our eardrums. In science, the term is also applied to the air vibration itself, as well as to the vibrations of the object originating the sound. If a taut string, such as used on a musical instrument, is plucked, it vibrates back and forth. As it moves, air particles ahead of it are pushed out of the way and those behind it are pulled along in its wake. When the particles move close together, the air undergoes a compression. When they recede from each other, a rare fact ion takes place. Air is elastic and the displaced particles tend to return to their position of rest. However, they in turn act upon succeeding air particles, and so the original motion is transmitted over a distance. The back and forth motion of the air particles is considered a vibration of the air. The impulses tend to spread out in all directions in the same manner that ripples recede from a stone dropped in a pond. The water particles move only a short distance from their rest position, creating hills and valleys, while each ripple moves away in the form of a constantly expanding ring. The eardrum responds to the slight pressure variations. By an action part mechanical, part electrical, and part inexplicable, it analyzes the sound wave and sends the message along to the brain through a nerve fiber. In air the impulse travels about 1100 feet per second. Since the original energy is spreading over a constantly increasing area, the particle displacement becomes less and less as the distance from the sound source becomes greater. This effects a reduction in the sound intensity, so that as we move away from the source, the sound becomes fainter. Returning to the pond, it can be noted also that i f two or more stones are dropped, the wave patterns from each may cross each other without notable interference. So in air, a 9 number of waves can travel at the same time and arrive at their destinations without appreciable change. Though most sound waves are of a highly complex nature, they can be broken down into a number of simpler components. What we actually hear is the effect of all these components being added together. The strings of musical instruments move a relatively small quantity of air and would of themselves produce only a weak sound. Such instruments are therefore equipped with sounding boards or diaphragms to which the vibration of the string is imparted. The diaphragm moves a larger number of air particles, by comparison, and thus reinforces the sound of the string. CHARACTERISTICS OF SOUND Now, there are three characteristics which chiefly distinguish one sound from another. They are Loudness, Pitch and Qu a 1 i t y. Loudness, or intensity, is determined by the amount of motion imparted to the air: louder sound for greater motion. Pitch is determined, in general, by the number of compressions in a given time interval: the more per second the higher the pitch. Technically, this is known as Frequency, and is expressed in cycles per second, abbreviated c.p.s. The Quality of a sound is a more complicated matter to explain. Quality» in the sense used here, does not refer to whether a sound is pleasant or unpleasant. It refers to the sound’s actual composition. As already mentioned, this is usually very complex. Consider a string vibrating at a definite frequency ■ say 400 cycles per second. It is possible, and it is the case, that portions of the string can simultaneously vibrate at exact multiples of this frequency - 800, 1200, 1600, 2000, cycles per second and so on. These are known as harmonics, partials, or overtones of the fundamental f requ ency. The resulting sound is the cumulative effect of the harmonic vibrations being added together. Variations in their relative sizes cause changes in tone quality. This accounts for the difference in tone of various musical instruments playing the same note. Remove the overtones and all instruments would sound alike. 10 The human ear is sensitive to the range of frequencies from to about 20,000 c.p.s. The following chart shows the fundamental frequencies of the notes of a piano scale« It should be noted here that there are several pitch standards >for tuning instruments. Another commonly accepted standard tunes A to 440 c.p.s.. whereby middle C becomes 261.6 c.p.s. instead of 250.6 c.p.s. as shown here. Figure 8 11 617852 0- 44-3 As to loudness, the ear is capable of handling a truly amazing range. Tones can be discerned up to the point wh^e the intensity of sound causes pain. The amount of acoustic ppwer concerned heré is greater than a trillion times the power of the weakest sound which the ear will perceive. In order to handle adequately these astronomical values, the acoustics and communication engineers use a term called décibel, abbreviated db. The decibel is not a finite entity such as an inch, gallon, or a pound, but expresses a propor* tion. 60 decibels means an intensity ratio of one million to one, and 70 decibels represents a ratio of ten million to one. The decibel system of notation therefore requires a starting point of a definite value. On the decibel scale this reference point is taken as zero, with higher values expressed as positive and lower values expressed as negative. In use, decibels are added or subtracted from each other. Intensity values in decibels of some familiar sounds will be found in Figured. The reference level used has been adopted as standard for sound level measurement. Figure 9 INTENSITIES OF FAMILIAR SOUDS NOISE OUT-OF-OOORS NOISE IN BUI L0INGS DB —130 THRESHOLD OF PAIHFUL SOUR» AIRPLANE - 1600 IPH, 18 FT -120- THUNDER —110-' »OILER FACTORY RIVETER - 35 FT — SUBWAV - LOCAL STATION WITH *"*100 EXPRESS PASSING ELEVATE» TRAIN - 15 FT NOISIEST SPOT AT NIAGARA FALLS „ LION’S ROAR - BRONX ZOO NOUSE - — 90 - 18 FT VERY HEAVY STREET TRAFFIC • 15 FT AVERAGE MOTOR TRUCE - 15 FT — — 80 VERY LOU» RADIO IH HOME AVERAGE OF 6 FACTORY LOCATIONS AVERAGE AUTOMOBILE * 15 FT — -70- — ORDINARY CONVERSATION - 3 FT — DEPARTMENT STORE, NOISY OFFICE QUIET RESIDENTIAL STREET, — -60- N.Y.C. - 15-300 FT — AVERAGE OFFICE -50 - MINIMUM STREET NOISE, Ml»-TOWN. QUIET OFFICE — 40 AVERAGE RESIDENCE N.Y.C. - 50-500 FT — 30 QUIETEST RESIOEHCE MEASURED QUIET GARDEN - LON»»N —20 — QUIET WHISPER - 5 FT RUSTLE OF LEAVES IN A GENTLE BREEZE OUT OF 000R MINIMUM — I®- THRESHOLD OF HEARIRG OF STREET NOISE REFERENCE LEVEL — 0 s THRESHOLD OF HEARING "0" LEVEL = 10-16 WATTS PER SQUARE CENTIMETER 12 Figure 10 Figure 11 13 The use of this device enables the presentation of data on charts of reasonable size. Figures 10 and 11 show the intensity and frequency ranges of speech and music in the upper portions of the charts. The wavy line near the top deline-ates the intensity at which sound begins to hurt, or threshold of pain. The curve below the dotted portion represents sounds which are just strong enough to be heard, or the threshold of hearing. The lower portions of the charts show the frequency ranges of various instruments. A sound source can be anything which sets the air particles in motion. Stringed instruments employ strings and sounding boards for this purpose. Drums and percussion instruments use vibrating surfaces of one sort or another. In the case of wind instruments vibration of a column of air within a tube or pipe does the work. Such a column has a natural frequency of vibration which is dependent on its dimensions. Its in fluence i s so strong that the reed is forced to vibrate at the same rate, interrupting the flow of air into the instrument. The dimensions of the air column are changed by uncovering openings in the side of the pipe, as in clarinets, flutes, etc., or by inserting different pipe lengths, as in most brass instruments such as trumpets. The purpose of a sound system is to reproduce, at another place, the complex set of air vibrations generated by the source. If a diaphragm could be constructed and operated so that it produced a set of compressions identical with the original, the resulting sound would be of absolute fidelity. This is the goal toward which we are constantly striving. Unfortunately, it has not as yet been attained. A reasonably high fidelity has been achieved, however, and in many instances it is difficult to distinguish a reproduced sound from its ori ginal. A speaking tube is the simplest example of a sound system. You talk into it at one end and the sound comes out of the other. Nothing to get out of order, always ready to operate, but subject to severe limitations of use. The term "Sound System" today applies to various means of accomplishing the same end electronically. 14 Basically, a sound system consists of three parts: B■ A device that produces an electrical or voltage wave which/varies in accordance with a sound wave impressed upon it. Microphone. 2» A device which increases that voltage wave to a much larger size. Amplifier. 3. A device which changes the electrical impulses to mechanical ones, which are impressed upon the surrounding air. Loudspeaker. These are connected in sequence as shown in Figure 12. Figure 12 BASIC SOUND SYSTEM Step Number 2, amplification, is necessary because the voltage generated by the microphone is very minute—entire1y inadequate to drive a loudspeaker. In some cases, as in short distance telephones, amplification is not necessary. DISTORTION Any deviation from a faithful reproduction of an original sound wave is called distortion. There are several .different types of distortion, and they may be introduced into the system by any of its components. The term frequency distortion applies when a unit does not reproduce or amplify all frequencies in the same proportion. The most common deficiency of any sound system in this respect is its inability 15 to handle the complete range of audio frequencies: 16 to 20,000 cycles per second. In an amplifier which is flat from 60 to 8,000 c.p.s., amplification, or gain, is the same for all frequencies between the two points. Amplification is reduced or increased for frequencies outside the range. Microphones and loudspeakers have similar frequency response characteristics, in that the electrical voltages deviate from the acoustic intensities. So-called high fidelity systems will reproduce upwards of 10,000 cycles per second. From Figure 10 it can be seen that a frequency range of 90 to 8,000 c.p.s., is necessary for the perfect reproduction of speech. For the perfect reproduction of music, this range must be extended from 40 to about 16,000 cycles per second« as shown in Figure 11. The actual effect of reducing the frequency transmission can be seen in the following charts. Figures 13, 14 and 15. Figure 13 shows how speech intelligibility is reduced by removing low or high frequencies. If all frequencies below 1000 c.p.s. are cut off intelligibility is reduced to 85%. If all frequencies above 1000 c.p.s. are cut off, intelligibility is reduced to 40%. Music transmission is similarly treated in Figure 14 on th< basis of quality, or definition. Figure 15 is calculated from the two preceding charts anc shows the frequency band width necessary to achieve anj given percentage of definition. For 80% definition in music transmission a frequency range of 75 to 7800 c.p.s. it needed. For 80% definition of speech a band width of onlj 750 to 3800 c.p.s. is required. Non-Linear Distortion results from harmonics which are not in the original sound, being generated by parts of the system. A certain amount is always present, but usually oi such small magnitude it can be ignored. It is an important consideration in amplifier operation, however, because there are many ways in which it can occur within the amplifier. For this reason amplifiers have a distortion rating includec in their performance specifications. 5% distortion in ar amplifier means that the output voltage caused by the induced harmonics is 5% of the total voltage output. This, when the amplifier is operating at full rated load. 16 5% is the maximum allowable distortion that should be con-|sidered for any system. The lower the distortion percentage* the better the quality of the•reproduced sound. Above 5%» the reproduced sound becomes objectionable. Distortion increases as the load on the amplifier is increased. Therefore, operating an amplifier at less than its rated load leads to improved sound quality. Having touched upon the most important acoustical principles, let* us now examine the individual parts of a sound system. Further acoustic phenomena will be discussed as the need arises. Figure 13 FREQUENCY IN CYCLES PER SECOND 17 Figure 14 18 Figure 15 617852 0 - 44 -4 19 SOUND SYSTEM COMPONENTS In discussing the parts of a sound system We will start wit^ loudspeakers and work backwards toward the source. Such procedure is necessary when laying out a sound system because all parts of the system are dependent upon the number and size of loudspeakers required. LOUDSPEAKERS A loudspeaker is the thing we actually hear when listening to a sound system. It imparts vibrations to the air which conform to variations of the electrical power driving it. Its action is similar in principle to an electric mo^or: when an electric current is passed through a wire located in a magnetic field, the wire is caused to move. In a loudspeaker the current carrying wire is in the form of a light coil fastened to a diaphragm or cone. It is called the voice coil. Consideration of the magnetic field surrounding the voice coil brings us to a fundamental difference in loudspeaker operation. The field can be created in two ways: by the action of an electric current or by the design of a structure made of a permanently magnetized material. Speakers em^ ploying the first principle are known as "Electrodynamic". They require a Direct Current source of moderately high voltage and additional wiring to supply this field current. For this reason they are seldom used in industrial sound systems. Therefore, from here on we will consider only the Permanent Magnet or PM type of speaker. Development of new alloys with extremely high magnetic properties in recent years has led to the design of PM speakers whose performance is just as good as the Electromagnetic type. Because they are so simple to install and use they are almost universally used in sound systems today. The driving mechanisms of loudspeakers fall into two classifications: Cone and Diaphragm types, of which cut-away drawings are shown herewith. The cone speaker is the more familiar, being used in all home radio sets. In this type of loudspeaker the voice coil is attached to the center of the cone and is held suspended in the air gap of the magnetic field by a flexible support and centering device at that point. The outer rim of th$, 20 Figure 16 21 cone is fastened to the speaker housing, either directly or by a rim of flexible material. The cone itself is usually I made of stiffened paper. Important factors in the design of the cone speaker are the weight of the moving elements, the stiffness of the supports, and the cone itself, the size of the voice coil, and the dimensions of the air gap in the magnetic structure. Tile diaphragm type of unit has a magnetic field and voice coil assembly essentially similar to the cone speaker. The small diaphragm sets only a small quantity of air in motion compared to the larger cone. The cone, on the other hand, encounters a greater resistance to motion because of its greater loads. In addition, the moving elements are heavier and it is difficult for them to follow the rapid fluctuations of the voice coil current. In order to produce low frequency tones a large quantity of air must be acted upon. This would seem to favor the cone over the diaphragm type of unit. However, another difficulty is presented which warrants inspection. When the cone moves forward, creating a compression, it is causing a rarefaction at the back. The two effects, traveling in the form of spreading waves, meet and tend to cancel1 each.other. This is particularly true at low frequencies, where the rarefaction from the rear has time to get around to the front of the cone before the next compression is started on its way. It would seem that neither type of unit by itself would operate satisfactorily. What can be done, then, to make them work? BAFFLES In the case of cone speakers, careful design is the only re: course in improving the high frequency response. The low frequency response seems to be chiefly limited by pulses at the rear of the cone sneaking' around and cancelling the pulses from the front. The answer fo this problem is simple enough: don’t let them do it. If we increase the distance that a rarefaction pulse from the rear has to travel before it can meet a compression at the front, we have baffled its ability to cause a cancellation. As it travels it is dissipating its energy in all directions and is in no condition to battle a freshly born compression. Any structure or de-< vice designed to lengthen thia path can be called a baffle. 22 Ideally complete baiffling would result if a wall were placed around the outer edge o* the cone and stretched endlessly in all directions. Then the rear wave would never reach the front and the baffling would be infinite. In practical use, speakers are sometimes mounted in the walls be tween rooms with very good results. The distance the rear wave has to travel is so great that cancellation effects are very slight. The chief disadvantage of this practice in Industrial Sound Systems is that speakers can seldom be located where they are usually needed—toward the center of the room or area. Suppose the speaker were mounted so that the back of the cone was in a completely enclosed box. Would not the rear wave be prevented from reaching the front, and thus infinite baffling be achieved? Yes — but at the same time another serious interference has been set up. The pressure pulses at the rear, being confined, react upon the cone itself and impede its motion. This is particularly so at lower frequencies where the motion of the cone is relatively large. The final step, then, is to put into the box something which will absorb this rear wave. A padding of properly selected sound absorbent material serves very well. Loudspeaker enclosures of this type are known as Infinite Baffles. When carefully designed they have an excellent frequency response. There are two features about them which may be a disadvantage. The first is size. The box must have an appreciable cubic content to aid the absorption of the rear wave—minimum 15,000 cubic inches for a 12* cone. The other is the fact that half the power fed to the speaker is wasted in absorption . The simplest and most commonly used cone speaker baffle is the flat baffle. This is merely a rigid wall of finite dimensions. The usual home radio with an open back cabinet is an example. Bear in mind that the smaller the housing the Less baffling there is. The shorter air path from front to back of the cone results in decreased bass response. Some housings are so small that their baffling action is practically nil, and their only purpose is to afford a means of supporting the loudspeaker. Another type of speaker enclosure which may prove useful for industry is known as Bass Reflex or Acoustic Phase Inverter. 23 Figure 18 24 In this type the rear wave re in forces those low frequencies which are not efficiently reproduced by the speaker. It ^thus broadens the response of smaller cone units. Cone speakers mounted in baffles as above radiate sound in all directions. Sometimes it is desired to beam or direct this sound in a certain way. To accomplish this the cont is placed at the end of a horn. Horns will be discussed in more detail in the next section, but for the present let us assume that, like a megaphone, they confine the sound waves, which ordinarily w.ould spread in all directions, into a more concentrated path. Horns used with cone speakers also act as baffles, and the baffling is dependent of the length of the horn. If the speaker unit is enclosed in a sealed chamber there is also the effect of the rear wave reacting on the cone at low frequencies. Several types of cone speaker baffles are shown here. HORNS Let us review for a moment the operation of a diaphragm speaker unit. The light weight and small size of its moving ^elements favored the reproduction of high frequency sounds. On the other hand, the small diaphragm area is not efficient for low frequency sounds, as it does not move enough air. The purpose of the horn is to couple the small diaphragm area to a much larger air surface. By preventing the pressure impulses from spreading in all directions, the successive air particles within the horn are given a greater push than they would ordinarily receive. Thus, the surface of air across the mouth becomes in effect a large sized diaphragm. The flare of the horn, or rate at which it increases in cross-section size, is one of the most important factors in horn design. The most efficient flare is exponential, and entails a horn of considerable length to achieve a given mouth opening. Size and design of the throat, or small end of the horn, is also very important. In order to overcome the cumbersome length of horns, they are frequently folded or doubled back upon themselves. This results in a more compact unit, which simplifies installation and portability. The structure of two common types of horns is shown in the following drawings. 25 Figure 19 Horns tend to concentrate the projected sound into a beam, more so with higher frequencies. The farther a listener is from the center of tne beam, the weaker will be the sound he hears. Within the beam the sound will be much louder than that produced by a cone speaker of equivalent power. This may favor or rule against the use of a horn, depending upon the circumstances in which a speaker is to be used. All horns have a definite low frequency cut-off—a point below which low frequencies cannot be produced. Factors governing the position of this point are chiefly length and mouth diameter. 26 AMPLIFIERS Proceeding back along the line of our basic sound system, the next element we come to is the amplifier. Amplifiers are made in a multitude of sizes and internal designs, to serve many specific uses. Basically their principles of operation are the same: a small electrical voltage is used to control the flow of a larger amount of electrical power. Power Amplifiers deliver the large amounts of power necessary to drive loudspeakers. The voltage generated by the microphone or other input device is too small to operate a power amplifier.. Therefore, a second unit called a Preamplifier or Voltage Amplif ier is used as a driving source. It increases the voltage output of the microphone greatly, but cannot drive a very large load. Still other amplifiers, called Boosters, are sometimes used to compensate for losses brought about by carrying the signal over long lengths of wire. The amplifier is the very heart of the sound system. It employs complex circuits of vacuum, tubes, inductances, resistors and condensers. While it has no moving parts and may appear to be a very sturdy piece of apparatus, it is subject to more troubles than any other portion of the system. To name a few: excessive harmonic distortion, poor frequency 'response, hum, or other noise in the system, breakdown due to failure of any part. Overheatingbecause of faulty design or inadequate ventilation is a frequent reason for such failure. Servicing an amplifier is not always a case of mere tube replacement but often involves a great deal of time. INPUT SOURCES — MICROPHONES There are many types of microphones, each having its advantages for specific uses. Those in general use for Sound Systems, known as Dynamic, Crystal and Velocity, act as small generators, producing a minute electrical voltage which varies with the sound reaching it. Another type, used in telephones, but impractical for sound systems, regulates the flow of current produced by another source. A third is known as a Condenser Microphone. Although it has many favorable characteristics, an amplifier must be built into the housing, making it undesirable for anything but laboratory use. 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Ano the r g r oup emp 1 oy s the phenomenon that when certain types of crystals, such as Rochelle Salt, are bent, they generate a minute voltage between opposite faces. This is known as the Piezo-electric effect. All microphones have a Directional characteristic. That is, they may pick up sounds anywhere from a complete circle down to a relatively narrow angle. The terms Directional and Um-Directional are applied to those microphones which pick up from a small angle only. The term Non-Directiona1 is applied to those which pick up from a relatively wide area. The directional characteristic of a microphone is an important factor in selecting one for a particu1 ar use. In public address systems in particular, if the microphone picks up the sound produced by the loudspeaker it is driving, a condition known as. Feed-Back is set up. Sounds from the loudspeaker are re-amplified and sent through the system again and again, causing a howl. INPUT SOURCES —RADIO When it is desired to reproduce radio programs over a sound system, a radio tuner is used. Essentially the front end of a complete radio set, the tuner selects the particular station wanted and transforms its signal into a voltage wave at audio frequencies. At the present time there are two distinct types of broadcasting signals in use. Amplitude Modulation, or AM, is the type which is known to everyone. Recently a new type of radio, known as Frequency Modulation, or FM, has been developed. It embodies radically different principles from AM and the receiving device must be designed accordingly. FM signals cannot be received on an AM tuner, nor can AM signals be received by FM tuners. Radios have been placed on the market which will receive both signals, but these are in reality two separate tuners, each feeding the audio part of the system. AM radio stations are limited in the audio frequency range they can broadcast. They are also limited in the dynamic range, or spread from soft to loud, which they can handle. In addition they are affected by static. 29 The FM system gains a distinct advantage in all of these limitations. Increased dynamic and audio frequency ranges make possible a more lifelike fidelity. Reduction of static' enables operation regardless of weather conditions. FM broadcasting shows promise of becoming of major importance in the future. FM signals, however, cannot be received satisfactorily at distances greater than 50 miles. Most home radio sets employ a circuit known as Superheterodyne, in receiving the AM signal. This circuit is particularly selective and is very useful in discriminating between stations which are near to each other on the dial. It is absolutely necessary in receiving stations which are some distance away. However, the superheterodyne circuit has the disadvantage of further reducing the limited audio frequency spectrum put out by the broadcast station. In nearly all system applications, local or nearby radio stations broadcast the desired material. There is seldom a need to tune-in any station which is far away. A receiving circuit known as a Band-pass, Tuned Radio Frequency, or T.R.F., is generally used. It will receive the full range of audio frequencies broadcast but is not useful for distant pick-up. In some areas there may be interference between stations which are too close to each other on the dial. This results in a high-pitched note being injected into the program. The interfering signal can be filtered out by an electrical ci rcui t.bu ilt into the tuner or added to the system. INPUT SOURCES—RECORDED MATERIAL There are many ways in which sound can be preserved for future use. A complete discussion of any of them would require volumes. In connection with industrial sound systems we can limit ourselves to disc recording because of the availability of program material. In disc recording, such as a phonograph record, the sound wave is, in effect, captused physically in the shape of the groove. In playing the record a needle or stylus follows the undulations of the groove. The motion of the stylus is transformed into minute voltage fluctuations by a pick-up or , reproducer, just as a microphone functions under the influence of an airborne sound wave. 30 Phonograph records are pressed out of a hard shellac compound in 10 and 12 inch sizes. The sound wave is inscribed in 'a spiral groove laterally; that is, the needle tracks from side to side in following the groove. Because of the composition of the moulding compound, phonograph records have an appreciable surfa-ce noise. The average range of audio frequencies which is reproduced runs approximately from 100 to 5,000 cycles per Second. They are played at a speed of 78 revolutions per minute. Transcriptions were developed primarily for the motion picture and radio industries as a high fidelity means of preserving sound for future play-back. Like phonograph records, they are in disci form with spiral grooves, but there are many points of dissimilarity. They are reproduced at a speed of 33-1/3 revolutions per minute and thus can hold a longer program than a phonograph record for a given size. Transcriptions are made in two sizes — -12 and 16 inch, the 16 inch size holding up to 15 minutes of recording on a side. Surface noise is very ,low and the frequency range is extended to the approximate limits of 50 to 10,000 cycles per second. Two types of transcription are available: ^lateral, as previously described, and vertical, where the stylus motion is up and down. On the other hand, transcriptions require special equipment for proper reproduction. Because of their slow speed, turntables must be balanced and motors large. Otherwise, speed fluctuations would show up in the form of wavering notes, or wows . Reproducing equipment for transcriptions is more expensive than that needed for phonograph records. It is often very delicate and subject to easy damage. The following table will show the merits of the different types of recorded material. 31 TABLE II DISC RECORDING TYPE OPERATION ADVANTAGES DISADVANTAGES PHONOGRAPH RECORDS Recording is cut by laterally vibrating stylus 78 R.P.M. Availability of recorded material Availability of reproducing equipment Low cost per disc Limited frequency range Breakage due to brittleness Short playing time Surface noise LATERAL TRANSCRIPTION Recording is cut by laterally vibrating stylus 33-1/3 R.P.M. Wide frequency response Long playing time Low surface nojse Reduced breakage liability Requires special reproducing equipment Softer discs-easily scratched Discs not distributed through local dealers VERTICAL TRANSCRIPTION. "HILL AND DALE". Recording is cut by up and down motion of stylus 33-1/3 R.P.M. Wide frequency response Long playing time Low surface noise Reduced breakage liability Availability of material recorded specifically for industrial use Requires special reproducing equipment Must be insulated from floor vibration Softer Discs - easily scratched Dises not distributed through local dealers REPRODUCING DEVICES Pick-ups commonly used for playing discs employ two of the same principles of voltage generation as do microphones:^ motion of a wire in a magnetic field and the piezo-electric effect of crystals. The former are known as magnetic or dynamic pick-ups and the latter are called crystal. Crystal pick-ups are usually less expensive than magnetic ones. In order that the stylus should not press too heavily on the disc, causing excessive wear, the arms are counterbalanced or spring-suspended. Low stylus pressure is an important design factor in connection with the frequency response of the unit. It must not be so low, however, that the stylus fails to track, properly, or jumps the groove. Some pickups use replaceable points or needles, while others have a permanent stylus builjt into the unit. In using replaceable points shadowgraphed steel needles are best. These have been carefully ground to conform to the groove shape, and individually inspected before packaging. As a rule, they are not considered long-playing. Low pressure pickups with permanent styli generally employ a jewel such as sapphire or diamond for the point. The jewels are microscopically ground and polished and will play.; 32 thousands of records without wear. They must be handled Very .carefully, as the jewels are brittle and fracture easily. If a cracked jewel point is used, it will ruin the record. As in the case of crystal microphones , crystal pickups should not be used under varying temperature conditions, as their frequency characteristics are altered. At high temperatures they are subject to complete breakdown. EQUALIZATION In the recording process the amplifier driving the cutter conforms to a definite frequency response which is not flat. The reasons are beyond the scope of this manual, but the point to be remembered is that there are several recording characteristics in common use. In addition, the nature of the disc material, and the pickup have their effects of the reproduced frequency response. An electrical circuit known as an equalizer must be inserted between the pickup and amplifier to compensate for these effects. The total effect of the recording characteristic, disc material, pickup response, and equalizer should result in a flat characteristic being delivered to the amplifier. If two types of tecordings are used, such as records and transcriptions, it is necessary to have a separate equalizer for each. In connecting various units together the impedances of each must be properly matched. All devices have a tendency to resist the flow of electric current. This characteristic in an alternating current device is termed Impedance. Units are matched when their respective impedances are such that the most efficient current flow is achieved. Mis-matching of units results in serious distortion of the frequency response. The addition'of switching and control units allows for almost limitless possibilities in the layout of sound systems. Block diagrams of two plant broadcasting systems are shown here, together with a typical intercommunication layout. 33 Figure 20 Figure 21 34 Figure 22 mentioned in the Introduction the uses of an industrial sound system must be fully determined before it is installed. There is no such thing as a universal industrial sound system. The uses and conditions of operation vary so greatly that each installation must be considered on the basis of its own requirements. The following tables are based on functional categories and will prove helpful in obtaining the most efficient use of a system. You will note that the first table is concerned with functions of an industrial sound system which are operational in nature. The second lists functions that relate to the well-being of the employees, rather than process or product. In practice, industrial sound systems frequently combine several of these categories. The items on Table IV and the first three items on Table III, or any combination of them, are often grouped together under the more general designation of Plant Broadcasting. Stories of typical installations have received wide publicity. A number of them are .included in class 4 of the appended bibliography. 35 TABLE III USES OF INDUSTRIAL SOUND - / DESIGNATION SOURCE OF SOUND ULTIMATE DESTINATION FUNCTION AND COMMENTS 1. PLANT NETWORK a. Paging Telephone -or special paging-operator One or more areas of plant. Selective control of destination is desirable Locating personnel b. Emergency and Alarm Paging operator or strategically placed microphones with automatic priority or "sequence "circuits Ditto Fire, damage, and accident control c. General Announcements Paging operator, or special microphone for executive Ditto Replaces or aids bulletin board for messages pertaining to plant operations 2. INTERCOMMUNICATION Strategically placed microphones Message source not limited to single location Ditto Highly specialised use of sound system, involving principle of talk-back. Not generally contained with other uses. Used for coordinating manpower and material flow, or interdependent operations as in steel mills, power plants, etc. All item» on thio table involve the tranamiaaion of SPEECH only Music in industry has had such rapid growth in recent years that it merits a special word at this point. By relieving fatigue and boredom, dispelling nervous tension, raising the job to a more important status in man's life, industrial music has been of inestimable value. Tangible results in the form of reduced labor turnover, decreased accident rate, increased production, and improved product quality, to name a few, have proven its worth many times over. The conditions in which the industrial sound system operates require that careful consideration be given to all its parts and proposed uses. A child can be taught to tie various units together so that a sound will come out of a loudspeaker When someone talks into a microphone. In an industrial plant, however, the system needs to be carefully 36 TABLE IV USES OF INDUSTRIAL SOUND > B DESIGNATION SOURCE OF SOUND ULTIMATE DESTINATION FUNCTION AND COMMENTS 1. WORK MUSIC Recordings Wired-In service Radio programs One or more areas of plant. Selective control of destination may be desirable To increase efficiency of personnel by planned music programs during work period 2. ENTERTAINMENT Recordings Wired-In service Radio programs Live talent, employee Groups or visiting celebrities Ditto To increase and maintain morale of employees. Not used during work periods j. MORALE BUILDING Recordings Rad io programs Talks by individuals Ditto ♦To build morale through inspiritiona1 messages, drives, safety campaigns, announcements of general interest, etc. To bring about more personal contract in personnel relationship ^Distinction is stade between Entertainment and Morale Building Uses, on the basis of material transmitted. En ter tainment usual ly invo Ives Music AMD Speech. In this tabulât ion Mor al e Bui Iding is concerned wi th speech transmission only. engineered, in order that the reproduced sound will have definition and will reach the locations where it is re-qu i r ed . Thé importance of a competent sound engineer to make a plant survey and lay out the detai.Is of a system cannot be too greatly emphasized. In many cases the plant survey should involve the use of sound measuring and analyzing instruments. Factory noise, th^ greatest obstacle to overcome, is usually very loud and highly complex in nature. The reproduced sound must have an acceptable percentage of definition against this background, in order that verbal messages may be understood and music may be effective. The psychology of audience approach has an entirely different aspect from most public address applications. The employees are not assembled for the sole purpose of listening to a program. Yet, when their attention is needed, it must be gained promptly. A sound system built for only one purpose usually will not perform satisfactorily for another. Speech can be transmitted with perfect clarity over a system which falls down when required to reproduce music. 37 Because of the variable factors the cost of an industrial sound system cannot be. given here. The many uses, condi-( tions of noise, employee versus plant area ratios, render it impossible to compute a unit-cost on any usable base. Regardless of the actual cost, whether it be five hundred or fifty thousand dollars, it would be well to consider it in the light of the actual and potential benefits it brings to your company. Some existing installations have paid for themselves in a few hours through their ability to handle emergency situations. Finally, consideration should be given to what is involved in installation, operation, and upkeep. Factory sound systems are required to operate continuously. Therefore they must be ruggedly built, critical parts made for heavy duty, amplifiers well ventilated. Standard parts and tubes should be used to simplify replacement. Placing all units for easy accessibility makes for easier servicing and inspection. Periodic inspections obviate many potent ial breakdowns. Consider the installation of certain stand-by units along with the main equipment. The additional cost often proves to be cheap insurance against serious damage that might be brought about through a communications breakdown. 38 SURVEY The purpose of a plant survey is to facilite the selection of the right equipment to do the job. There are many makes, types, and sizes of each component to choose from. The right equipment for your plant is that which will answer Your needs satisfactorily, not necessarily someone else’s. If the system does not fulfill your needs, it is unsatisfactory and can only be corrected by a process of cut and try, often resulting in excessive replacement of units. On the other hand, sufficient equipment can be put in at the start to do the work many times over. To spend money for something you don’t use is economically unsound. The survey, then, is the actual starting point for the engineer who lays out the sound system. Before he begins his work, however, he must be made thoroughly conversant with all the ways’in which the system is to be used. The decision as to use can be made on the basis of Tables III and IV (pages 36 and 37), and must be complete and final. As the use of a sound system materially affects its equipment requirements, any change of thought about them will oblige ^iim to start over again. The engineer then proceeds to examine the plant in order to determine the requirements of the system. The objectives of a complete survey are listed here in tabular form for use as a check list. In making the survey the engineer should be accompanied by a person familiar with the operations of the plant and will need a floor diagram of the plant. A study of the noise conditions will be made in all areas. The ear alone is not capable of registering this exactly. Usually a sound level meter will be required. This is a portable unit consisting of a microphone, amplifier, and indicating meter which shows quickly and accurately the noise intensity at any location. The location of particular noise sources will be noted, together with comments as to the character of the noise. Any anticipated rearrangement of equipment or partitions should be taken into consideration. In addition, the dimensions of the various areas will be noted, including ceiling heights, as well as the nature of the wall surfaces and sound absorbent material within the areas. 39 1. TABLE V SURVEY OBJECTIVES Noise--all areas Location of Sources Levels in decibels Frequency analysis where necessary 2. Coverage Speaker selection and placement determined 3* Plant Zoning Areas zoned with reference to industrial operations for determining switching requirements Sectional control of system 4. Location Input sources Amplifier equipment Controls Studio--if any 5* Special Considerations Temporary microphone positions Recording apparatus Sequence circuits for priority control of messages Others The foregoing information leads to a determination of the sizes and types of loudspeaker units to be used, and their 1oca t i on s• Plant zoning consists of dividing the plant into operational or departmental units. This is necessary to determine the output, switching, and control requirements of the sound system, so that material can be transmitted to the various areas selectively. Together with the loudspeaker requirements, it leads to the selection of the number and sizes of power amplifiers necessary. Sectional or area control over portions of the sound system is an important consideration in plant zoning. A department or building superintendent may find it necessary to use the part in his domain for operational uses, without interfering with the rest of the system. 40 The location of amplifiers, input, and control equipment will be decided upon and noted. If a studio is needed for reproducing live talent, its location will also be determined at this point. Finally, the survey will include consideration of any special features included in the overall requirements of the system. One might be the sequence of priority of one type of message over another and the possibility of arranging the switching so that such priority is automatic. A second might be the need for a recording machine so that news broadcasts, special events or radio programs could be reproduced in the plant at more convenient times. Another special consideration would be the desire for temporary microphones to be used at various points in the plant. 41 SPECIFICATION Having completed the survey, the engineer is ready to 1 ay^ out the sound system. The performance, scope, and limitations of most of the parts have, to some extent, been discussed. Specification is the process of choosing, among available types and sizes, the units most suitable for the job. In selecting the parts of a sound system there are certain considerations which are applicable to all. These will be discussed first, and from then on it will be assumed they apply to the individual units mentioned. DURABILITY Continuous operation is the rule with industrial sound systems. They are turned on in the morning and run without stopping throughout the work day. In many cases this is a twenty-four hour period and thus the system never gets a rest. Like everything else in this world, sound systems are subject to a certain amount of wear and tear through use. Components which were not designed for continuous operation may perform very satisfactorily for intermittent use — a hours at a time. They usually fail miserably, however, when called upon to function as a part of an industrial sound sys tern. Breakdown of the system not only results in inconvenience but may be responsible for serious damage. Therefore, rug gedness and oversized parts are requisite. SAFETY All units, particularly controls, must be as foolproof as possible. The persons called upon to operate the system may not know a patch cord from a soldering iron. Therefore, there must be no chance of damage to any parts by throwing a switch in the wrong direction. The system must be protected from possible harm from curious or careless individuals. And vice versa. FIDELITY» All portions of the system must live up to certain fidelity requirements. Speech can be intelligibly reproduced over a limited frequency range of 300 to 3000 cycles per second. 42 Music transmission requires a broad band of frequencies. This is not to delude the workers into believing they are hearing the real thing and that the plant has put an orchestra on the payroll. The reason for fidelity in a plant is based upon a more potent consideration than aesthetic appreciation. When music is played in a noisy background many of the tones are masked. Increasing the loudness of the music will overcome this effect but might make it a source of annoyance. Broadening the frequency band is a better way to add definition. Reduction of music intensity is possible and is a desirable result. The effect of frequency range on music intensity for minimum acceptable definition can be seen in the following chart, for a particular case. The shaded area shows the envelope of the noise background. Figure 23 The chain of a sound transmission system is no stronger than its weakest link. If any element fails to reproduce the desired frequency range, the whole system is reduced to. that level. Maximum ranges ever needed are 90 to 8,000 c.p.s. for. speech, 30 to 16,000 c.pt-a. for music. Losses due to 43 reduced frequency range can be determined from charts 13, 14 or IS.' Experience has indicated in all fields of reproduced sound that the restricted frequency range is never as satisfactory as the wide range. Though many potent arguments are put forth in favor of the former, all development has tended toward increasing the range. Increased fidelity entails increased costs. A balance between price and effectiveness of the system is governed by the circumstances of each installation. LOUDSPEAKERS Starting with plant noise levels, the size, type, number and locations of loudspeakers are determined. Horns are generally used in particularly noisy areas because of their ability to concentrate the sound. On the other hand, frequency range is limited and sound distribution apt to be uneven. Well baffled cone speakers generally can be used in areas where the noise level is around 90 decibels or less. Many of them have a reasonably good frequency response and can be arranged to give excellent distribution. Unbaffled cone speakers have a very tinny sound and should, be used only in a limited way for speech transmission. Loudspeakers are usually connected to the line through matching transformers, to obtain the proper impedance relationship. If the matching is not correct, the power and frequency response will be affected. Mounting loudspeakers close to noisy machines is better practice than combating the noise from a distance. In the latter case the sound from the loudspeaker will be much too great in its immediate vicinity. In the former, the sound from the speaker and the noise from the machine are dispersed together. They undergo equivalent reduction with distance and area coverage can be made more uniform. POWER AMPLIFIERS A single power amplifier is capable of driving a number of loudspeakers, depending on their size and power requirements. An additional determining factor in amplifier specification is the number and location of the speakers which will always be carrying the same program. 44 Five per cent is the maximum allowable non-linear distortion, lower values preferred. Distortion factor may be reduced by operating an amplifier at less than rated load. Power amplifiers should have flat frequency response within the given range. An allowable exception is where a frequency distortion is deliberately introduced to overcome local acoustic conditions. (See later discussion on Networks.) Areas of differing noise levels sometimes require separate power amplifiers so that volume levels can be independently adjusted. They may be placed in a single location for convenience. This practice simplifies operation and maintenance. However, it is often advantageous to mount each one near the loudspeakers it is operating. This allows more convenient adjustment to varying noise conditions. Also, in transmitting the signal over long lengths of wire unavoidable losses occur. These losses are lower for the small power carried to the amplifier than for the larger power delivered to the speakers. Remote power amplifiers can be turned on and off from the control point by means of relay circuits. VOLTAGE AMPLIFIERS Voltage amplifiers are generally required to drive the power amplifiers. Their number, size and locations depend mainly on the uses of the system, the place, and nature of input sources. In the majority of cases they are placed together, near the controls. Most of the above discussion on power amplifiers applies to voltage amplifiers as well. Special circuits and networks, however, are often incorporated in the voltage amplifier to control its frequency characteristics. NETWORKS— SPECIAL CIRCUITS In general, a network is an electrical circuit that has a definite reaction upon the frequencies passed through it. Its location in the sound system depends upon the reason for its use and the nature of the circuit employed. A low cut-off or High-pass filter is sometimes used in a sound system as a safety device. It has already been pointed out that a horn has a definite low frequency cut-off. To overcome the loss of bass there is a tendency for the operator to increase the signal to the driving unit. This results in increased diaphragm displacement (which is es 45 pecially large at low frequencies) and might break it. The low cut-off filter removes these low frequencies from the signal, reducing the possibility of damage. Those in general use for the purpose cut off below 250 c.p.s.. This does not interfere with speech intelligibility. It does affect music, however, as can be seen by referring to Figure 14. The music quality, or definition is reduced to 50%. High pass filters with other cut-off points are sometimes used to improve the intelligibility of paging calls made by female operators. They must not be used for music. Referring again to Figure 14, it can be seen that as the cutoff is raised, music definition goes down rapidly. At 600 cycle cut-off, the definition is down to 10%. Giving additional amplification to frequencies in the neighborhood of 1000 cycles per second has been found helpful in the reproduction of male speech. It adds a certain crispness and clarity to the voice.. This is accomplished by a special circuit which is usually built into the pr eampli-fier. If the same preamplifier is to be used in connection with a music source, the circuit must be removed so that amplification again becomes flat. Control of the circuit can be maintained by a switch on the microphone stand. Equalizers compensate for the combined effect of recording characteristic, disc material, and pickup response in reproducing records and transcriptions. These networks are usually located in the turntable unit, but may be built into a preamplifier. Sometimes resonance of the loudspeaker area interferes with the performance of a sound system. The condition can often be improved by reducing the same resonant frequencies in the sound system. It is accomplished by a network circuit, which is best placed in the power amplifier so that other areas will not be affected. One other special circuit which might be built into a preamplifier is volume compression, or automatic volume control. The principle of operation is that as a signal going to the amplifier increases the amplification is reduced, and vice versa. Such a circuit proves helpful in maintaining music programs within certain limits of loudness — a necessary feature in industry. 46 INPUT SOURCES The characteristics and uses of various microphones have been pointed out on Table I page 28. High impedance microphones must be used within a tew feet of an amplifier. If there is a need for a microphone located any distance away, a low impedance type must be chosen. Close-talking microphones are desirable for noisy locations to reduce the pickup of machinery noise. The turntable used for reproducing recorded material needs a heavy duty motor. It must be relatively free from speed fluctuation. In addition, the motor must have sufficient torque or driving force, so that the drag of the pickup will not affect the speed. The drag has a much greater effect at the outer edge of the disc than toward the center. Turntables used for transcriptions are weighted and dynamically balanced. Wows show up much more in a slow speed transcription than in a phonograph record at 78 r.p.m. Remember that record- or transcription-reproducing equipment must be ruggedly built and able to take a beating. Home phonographs are never subjected to the service that reproducing equipment undergoes in a factory. The subject of radio tuners for input sources has been discussed on page 29. In general the Band-pass tuner will be found most useful for industry. STUDIO TREATMENT Some thought must be given to the acoustics of the room or studio where live sound originates. Singing in the bathtub is a great American pastime. The effect of the sound bouncing back from hard walls is very satisfying to the artist, both professional and amateur. Not so to others, however, particularly when reproduced over a sound system. Absorptive material placed on floor, walls and ceiling will reduce the reflections. On the other hand, if too much absorption is used, the sound will have an undesirably dead quality. In addition, the acoustic power reaching the microphone is greatly reduced. Liveness of the sound is to be desired, provided the reflections do not cause too much echo or reverberation, resulting in reduced definition. A recent development in studio treatment is to construct the wall surfaces in the shape of convex half-cy1inders. These surfaces reflect the 47 sound, giving it a live quality, but scatter and break it up so that the effect of echo is not present. SWITCHING CONTROLS The operation ofan industrial sound system involves controls which may be anything from a simple switch and knob to a setup more complicated than a telephone switchboard. The control elements are determined by the uses of the system, and the location and nature of its components. Simplicity of controls, as well as their concentration at a single point is desirable, but not always possible or practical. In larger systems a means must be provided for input switching-connecting the sources to the p r e amp'l i f i e r units. Switching of the preamplifiers to the lines running to power amplifiers is essential. In sectional or area control, means must be provided so that a portion of the system can be disconnected from the whole and fed from an independent source. Many of the switching controls can be made to operate through the use of electrical relays. One example of this is kndwn as a Sequence Circuit. Control of the system is given to messages according to apredetermined schedule of importance. Operation is automatic and merely depends upon which input source is used. For instance, a priority sequence might be arranged so that in the following list,,each item would have precedence over the succeeding ones: (1) Emergency alarm; (2) Fire alarm; (3) Guard headquarters; (4) Music; (5) General paging. INTENSITY CONTROL Loudness of the transmitted sound at the listener's position is regulated by Volume Controls. The volume control on a power amplifier adjusts intensity in the battery of loudspeakers connected to it. It thus affords control over a large area. In practice this volume control is adjusted to the area noise conditions and left there permanently. The only exception is when the noise conditions vary, as might be caused by changes in machine operation or the number of workers present,. In some cases it may be desirable to have a means of controlling intensities of individual loudspeakers, so that they can be more closely adjusted to conditions. 48 Volume controls on preamplifiers serve to regulate the in* tensities of the signal being fed to the power amplifiers. These are the controls which are in most constant use» and should be placed for the convenience of the operator. It is not necessary that they be actually built into the preamplifier units. They can be made to operate remotely sp that all controls and switches are often mounted on a single panel. Volume controls which may be a part of phonograph units and radio tuners are best adjusted to a proper setting and then left alone. The control room will be fitted with a monitor speaker so that the operator can listen to all material being transmitted. A volume indicator is a desirable adjunct to the control room equipment. This is a meter which gives a visual indication of the electrical signal strength and is helpful in monitoring music programs. HIRING Shielded wire is used on all low-level circuits; that is» where the circuit is still subject to amplification. The shield is a metallic basket-wtave sheath and is connected to the ground. It prevents the signal wire from picking up stray impulses which, when amplified would cause noise in the loudspeaker. The shields should be insulated along their length from contact with metal and must be grounded at only One end. Low signal level wires must be isolated from wires carrying output signals or ordirtary power circuits. Wiring to loudspeakers does not need shielding and, in general, can be the same type of wire used for ordinary electrical purposes. Voltages are low so that usually it does not have to be enclosed in metal conduits. 'Special consideration must be given to its size for two reasons: the current may by high, causing excessive heating, and the resistance of the wire causes loss of power. The larger the wire, the less its resi stance. ♦ A reasonable power loss to allow in the loudspeaker wiring is one*half decibel. The following chart is a convenient means of selecting the right size wire for this limitation. If the conditions indicate a point between two wire sizes, choose the larger, or one at the right. If the point falls 49 to the right of the line for #14 wire, then the impedance of the speaker is too low for the length of run required. Such a case requires a higher impedance connection at the amplifier and a corresponding matching transformer at the loudspeaker. A 250 or 500 ohm loudspeaker circuit is generally useful for industrial purposes. Figure 24 WIRE SIZES FOR LOUD SPEAKER CIRCUITS ASSUMING MAXIMUM LOSS OF 0.5DB CONNECTIONS It is well to use special connectors on all sound system wiring except that which connects to regular power outlets. More than one loudspeaker has been ruined because it was fitted with an ordinary domestic plug and someone thoughtlessly.put a hundred and ten volts across it. 50 There are many types and sizes of special connectors made for radio and sound system use. It is a good plan to have different types for power supply, microphone inputs, phonograph input, and loudspeaker connections, so that there will be no possibility of the wrong units being joined together. It also simplifies the actual connecting. If the loudspeaker plug on an amplifier is distinctive from all other connections, you can tell at a glance where the wire goes. All connectors should be of a locking type, particularly any which may be put in a loudspeaker line. This will prevent their accidental separation. Permanent connections should be soldered and carefully insulated* Improper insulation of connections has caused trouble in many a sound system. 51 ADMINISTRATION The potentialities of a plant sound system are similar iFo those of a radio station in many respects. It exerts a major influence over the plant personne1• In contrast to their own home radios, the employees do not have a choice of stations, nor do they have access to the switch that turns it off, which some consider the most important part of a radio set. Operating an industrial sound system is operating something as complex and which reaches as large an audience as many a radio station. The control of such a medium, therefore, entails a great responsibility. Now, one significant element of radio is its use, or abuse, in disseminating propaganda. Propaganda by definition is "any given opinion or doctrine which is spread by an organ* ized movement*,. It thus includes nearly everything you hear on the radio, from the soap-opera to the station announcement. There i s nothing wrong wi th propaganda, intelligently handled. However, in industrial sound the approach to -it must be most cautious, as there is potential dynamite in the harvest. All material not strictly operational in nature ought to be approved before it goes over the sound system. One person should be directly responsible for the operation of the system. He will be authorized to pass judgment on material that is to be transmitted. Let us call him the System Director. Under him will be the paging and music operators. In larger installations a separate paging operator is desirable, apart from the regular telephone operator. This operator will need a phone connection to the switchboard to receive incoming calls. A separate operator for the music program is necessary also. The controls of the system must be under constant supervision while a music program is on. You cannot efficiently operate a telephone switchboard and run a music program at the same time. The functions of paging and music operators may sometimes be combined in the same person, provided there is no conflict in duties. The subject of programming music for industry will be discussed in another section. Its value results from a*?d 52 warrants a great deal of thought. An individual whose business shall be the development and observation of programs is essential to achieve satisfactory results. The Program Director fits in between the System Director and the music operator. The function is separate, but might be performed by the System Director. The following chart shows graphically how such an administration would be set up. It is, of course, subject to variation in accordance with the scope of the system and individual plant characteristics. Figure 25 58 OPERATION With the Administration organized the system is now ready to to operate* In this, let the installation company's instructions be your guide. Keep them handy and refer to them often. Techniques will vary according to the complexity of the system. Only the highlights, applicable to all systems, Will be discussed here. The discussion will be on the basis of manual control —automatic functions being made to conform with the basic principles. INTENSITY RANGE Paging calls and announcements are put through with ample intensity to be understood and to attract attention. The time involved for a message is short—only a few seconds for each. Work music, on the other hand, must be played at as low a level as is compatible with good definition. The more it blends into the background, the less possibility there is of its becoming a source of annoyance. People’s ears rapidly become adjusted to the noise of their surroundings. Extraneous sound, even though not clear to outsiders, stands out with surprising definition. Therefore, the audience, judgment will be the best indication of normal intensity setting for music. Work music needs to be monitored. Most music available has dynamic ranges which are too great for industrial use. Loud passages are too loud and soft passages are too soft, being lost entirely in the machinery noise. Dynamic range must be reduced as far as possible without robbing the music of its character. Entertainment, talks, -special events, etc., may be transmitted on a level between work music and paging. This type of program is generally given when the employees are not at work. Watch out for the reduction in factory noise due to machines being closed down. SELECTIVE AREA PAGING Much has been said, and data computed, concerning the man-houis saved in locating personnel through voice-paging systems. No one has mentioned, much less figured, the annoyance caused by excessive paging. Therefore, it is wise to send paging calls only to areas where the individual is likely to be found. 54 CONFLICT OF PAGING AND MUSIC »The problem of how to prevent paging and work music frdm interfering with each other is a very serious one. It is of particular importance in the case where a paging system has been operating for some time, and a music program is inaugurated. The full value of a music program cannot be achieved if it is subject to frequent interruption. The practice of stopping the music for a call or superimposing the call on it will not serve where there are many calls per day. Voice-paging has proven so effective that systems are apt to be loaded with calls not really necessary. The first step in avoiding conflict, then, is to eliminate all unnecessary me s s age s. Next make sure that every call is routed to most likely area for locating the desired party. This will avoid conflict with the music program in at least some of the areas. Finally, establish and maintain some sort of a priority schedule of material in order of its importance. A sample of such a schedule and its operation follows. TABLE VI SAMPLE PRIORITY SCHEDULE OF TRANSMITTED MATERIAL PRIORITY RATING TYPE EXAMPLE OPERATION 1. General emergency Fire or major catastrophe sufficient to stop all work Cut off music abruptly Announcement following will have full attention 2. Individual emergency including long distance telephone calls Machine breakdown, needing immediate attention Accident requiring immediate attendance of plant physician Cut off music momentarily, or superimpose call on music 3. Work music Do not interrupt except for conditions above ». Calls of high importance Individuals required for conference Insert between selections of music program 5. Calls of lesser iinportance General announcements Individuals required, but not urgently Hold call until end of music period 6. Entertainment and morale programs Employee groups. Guests Not given during work periods 55 PROGRAMMING WORK MUSIC All that remains now ii to tell what music to play and when and howtoplay it. That is the job of the Program Director, and Ft is a difficult one. Unfortunately, there is no Industrial Music Program which is universally applicable. If there were, it could be set down here and that would be the end of the matter. But plants vary in conditions, operations, personnel, and location. In order to receive the utmost benefit from industrial music the program must suit the needs of the plant. Programming can be approached from a number of different angles. It might be based on any one of the following: 1. Principles of showmanship 2» Psychological studies 3. Analytical studies (directed toward relating stimulus to measurable effect) 4. Employe* preference 5. By guess and by gosh 6* Random selection of material Numbers 5 and 6 are rather absurd, but they have been tried in more than one instance—and found wanting. The best programming so far has come from a judicious mixture ot the first four. There are not many fixed rules to follow. Some basic principles, however, have become established through accepted use and a few others by controlled experimental research. WHAT TO PLAY Based upon employee preference, about sixty percent of the music generally used consists of popular dance music. There are seldom more than fifteen new tunes which are hits at any one time, so it becomes necessary to use older selections also, which have become standards. The other forty per cent is mainly composed of Viennese waltzes, popular operetta melodies, polkas,* novelties, marches and Latin American musit. Sometimes a small percentage of heavier works, selections from Grand Opera, and symphonic pieces are called for. The ¿bòve mixture is not to be considered specific. Employee preference is a very unpredictable thing. It is affected by many factors—racial extractions, age, sex, geographical 56 location, musical ability, and environment —to name a few. IA is also subject to change. Remember that variety is most important in industrial music. Trying to please everyone at once is a hard job and the solution is a compromise. Include a proportionate amount of what everyone (not just the majority) wants and mix the selections so that no standard pattern is discernible. The employees hear the music over and over again in the course of time and get to recognize and object to a pattern. In general, the music used for industry should be familiar and have well defined melody and rhythm. Full orchestrations are to be preferred to arrangements featuring solo instruments. In the latter case the tune may become lost in the machinery noise. Avoid selections that have startling or hard to follow passages. The man concentrating on his job follows the music in his head, if not vocally. In fact, it is generally considered a criterion of industrial music’s effectiveness if the employees hum or sing along with it. Hot music or Jive is best avoided also. The demand for it ay be great, but too much of it is overstimulating. It must be used only sparingly. The question of whether or not to use vocals has never been placed on a firm basis. They are used in some plants and definitely taboo in others. There appears to be a relation between the use of vocals and the amount of mental effort involved in the work. Generally» it is wise to proceed cautiously in the use of vocals. WHEN TO PLAY A maximum of about 2-1/2 hours of music per 8 hour shift has been found to be sufficient. More music, even though the employees request it, causes a surfeit resulting in decrease^ effectiveness. It is wise to follow the showman’s principle of leaving the audience wanting more. Then the seats will be packed for the next production. The usual practice is to start the morning off with a short period of march music or other peppy tunes. This helps to wipe away the morning gloom. rhe scheduling of playing periods is best determined as a result of studies made in the plant to locate the times or 57 greatest and least fatigue. Music played just before and during a fatigue period serves as a pick-up. An effective way of grouping selections is to arrange them in order of increasing stimulus. The mood of a selection is a rather intangible thing, not exactly dependant on tempo. If selections are arranged with progressing mood the total effect of the group will carry over for some time. Employee requests are included in the program, but only with discrimination. Those which do not fit in with the over-all plan are played during lunch or rest periods. HOW TO PLAY The volume level at which the music is played is generally kept low. The importance of this also increases with the amount of mental effort put forth on the job. Beginning each group at an even lower level and bringing it up to normal over a fifteen second period is a good plan. About 10 to 15 seconds between selections help to cover changes in key, orchestrations, and music types that might be objectionable if heard in too rapid sequence. ft also allows for paging calls to be made without interrupting the music. The use of automatic record changers is not advisable. The presence of the operator near the controls is demanded by manual operation. A record changer is apt to give rise to laxity in programming. It is so easy to place the first discs that come to hand on it and let the machine do the work. RESEARCH The developing and maintaining of an industrial music program entails a certain amount of research activity. The program director must keep abreast of his program, determining its effects, and making changes where necessary. Bibliographical references in categories 4 and 5 show various techniques used in program research. Subjective studies, such as questionnaires, are useful but often do not lead to results which can beproperly evaluated. Objective studies, such as time or output studies, are arduous but well worth the effort where they can. reasonably be set up. They give statistical results which show at a glancfi. the comparison between one type of program and another. S8 The program director is the mainspring of the industrial music program. Its effectiveness is in direct proportion to his diligence. MUSIC LIBRARY The music repertoire for an industrial program needs to be quite extensive. One of the most common complaints about a continuous program of this sort is in connection with repetition. A large number of selections and orchestra types is necessary to keep repetition from becoming objectionable. A library of more than a thousand selections is none too large for desirable fluidity and variety of programs. There are five possible sources of music to consider for industrial purposes. The’se are: 1. A live orchestra 2. Radio programs 3. Phonograph records 4« Transcription libraries 5. Wired music services A live orchestra for work music is inadvisable because of limited repertoire and lack of variety. The second possibility, radio, offers all that can be desired in repertoire and variety. On the other hand, programs are designed for a listening audience. They contain many undesirable features from the industrial music point of view. Those best suited for industrial purposes are not usually on the air when industry needs them. One notable exception is the case of certain FM stations. Several of these are making an intelligent and effective attempt to produce music programs tailored to the needs of local indus*tries. Phonograph records offer a large repertoire and the possibility of tailoring the program to suit the needs of the plant. The same is true of transcriptions — with the addition of improved quality, which is advantageous, even necessary, in many installations. A wired music service offers several features which may be found advantageous. Such a service eliminates operation and maintenance of extensive music libraries, and if transcrio-tions are used as the music source it brings high quality, the importance of which has already been pointed out. Per 59 haps the most important advantage of wired music service is the programming talent it brings to the plant. Many industries use a combination of the above music sources. A wired music service is used for the bulk of the days' programs, records and radio programs for entertainment, special events and requests. Regardless of the music source, however, it is the Program Director who fits the pieces together into a unified whole. Programming music for industry is a new occupation. It cannot be reduced to a-set of simple rules, nor can it be effectively done by adhering solely to principles established in the field of radio, the concert stage, or the theatre. The program director of ability need not be a concert artist. But he will have a wide knowledge of musical literature« imagination and the courage to use it, some practical psychology, together with an understanding of what goes on in the plant. An engineering approach will help him to turn out the most efficient programs, and an understanding of acoustical principles will help in the selection of music for its penetrative quality. Industrial sound is considered by many to be a war baby. This is not quite true. The infant existed before the war and sprang into adolescence after Pearl Harbor. It will not reach its full maturity until long after the war and industrial activities are back to normal once again. It is then, when electronic materials aré no longer considered strategic and are available to all, that industrial sound will come into its own. Though this manual has been written and published in wartime, it deals with basic and fundamental principles. If it proves of value now, it will be equally important in the days of peace to come. BY: Richmond L. Cardinell and Glenn C. Henry 60 BIBLIOGRAPHY FOR INDUSTRIAL SOUND In compiling this bibliography for further reading, an attempt has been made to classify the references as to subject matter. Because there are so many overlapping references, they are listed here alphabetically by author or publication, rather than by groups. Subject classification is shown at the right, according to the following categories: 1. Acoustic Theory 2. Electronic Equipment and Circuits - Technical References 3. Bhysiological or psychological material relevant to the effects of sound 4. Popular articles, case histories, reports 5. Specific information on operational techniques CATEGORY Academy of Motion Picture Arts & Sciences Research Council 2 Motion Picture Sound Engineering New York: D. Van Nostrand, 1938 Adiss, Th. Blood Pressure and Pulse Rate Levels Arch. Int. Medicine, April, 1922 3 American Magazine, August, 1941 Step on it to Music 4 American Weekly, January 24, 1943 Music Has Charm - to Speed Up Production 4 Antrim, Doron K. Music All Out For Production Forbes, August 15, 1942 4 Archambault, LaSalle Effect of Noise on the Nervous System New York State Journal of Medicine October, 1932 3 Assagioli, R Music as a Cause of Disease and as a Healing Agent j Educational Cinematography, 1933, Year V No. 9 Barmack, Joseph E. Boredom and Other Factors in the Physiology of Mental Effort Archives of Psychology, July, 1937 3 Bartholomew, W. T. Acoustics of Music New York: Prentice-Hall, 1942 1 Barton, E. H. A Textbook on Sound London: Macmillan & Co., 1908 1 61 CATEGORY Beckett, Wheeler Music in War Plants Washington: War Production Drive Headquarters W.P.B., 1943 4 Benedict, Milo E. What Music Does to Us Boston: Small, Maynard Co., 1924 3 Britain, H. A. The Power of Music Jour, of Philosophy, Psychology, and Scientific Method Vol. 5, No. 13, 1908 3,4 Burris-Meyer, H. Music in Industry Mechanical Engineering, January, 1943 4 Birris-Meyer H., Mallory V., & Cardinell, R. Sound in the Theatre, Report No. 6 Stevens Institute of Technology, 1942 2,4 Business Week, April 3, 1943 Industrial Music 4 Cardinell, R. L. Statistical Method in Determining the Effects of Music in Industry Jour, of Acoustical Soc. of America, October, 1943 5 A Guide to Music in Industry Factory Management & Maintenance, October, 1943 5 The Nature and Development of Work Music New York: Amer. Soc. Composers, Authors, and Publishers, 1944 4 Selection of Equipment for Music in Industry New York: Amer. Soc. Composers, Authors, and Publishers, 1944 5 Principles of Programming Music in Industry New York: Amer. Soc. Composers, Authors, and Publishers, 1944 5 Chomet, H. The Influence of Music on Health and Life (Trans, by Mrs. Laura A. Flint) New York: G. P. Putnam & Sons, 1875 3 The Christian Science Monitor, August 4, 1942 N^w Jersey Industrial Plants Find Music a Real Asset 4 Clark, Kenneth S. Music in Industry The Gamut, November, 1929 4 L. B. Cooke Manual of Sound Systems Bell Telephone Laboratories, 1939 2 62 CATEGORY Crandall. I. B. ' Theory of Vibrating Systems and Sound New York: D. Van Nostrand 8k Co. , 1935 1 Davis. A. H. Acoustics of Buildings London: G. Bell & Son, Ltd., 1927 1 Modern Acoustics London: G. Bell & Sons, Ltd., 1934 1 Davis, Roland C. Factors Affecting the Galvanic Reflex Arch, of Psychology, No. 115, July, 1930 3 Dearborn, G. V. Some Practical Notes on Blood Pressure Medical Record, Sept. 16, 1916 3 Diserens, C. M. Reaction to Musical Stimuli Psychological Bulletin, Vol. 20, No. 4, April, 1923 3 > ' ■ , Influence of Music on Human Behavior Princeton University Press, 1926 3,4 Dunbar, H. F. Emotions and Bodily Changes New York: Columbia University Press, 1938 3 Emerson, F. P. * Gradual Change in Otologist's Conception of Conduction and Perception of Sound Impulses Ann. Otology, Rhinology fit Laryngology, 40 September 1931 1,3 Etude, March 1943 Music an Industrial Asset 4 Everitt, W. L. Communication Engineering New York: McGraw-Hill, 1937 . 2 Factory Management and Maintenance, February 1943 Music in the Plant 4 Fletcher, H. Speech and Hearing New York: D. Van Nostrand Co., 1929 1.3 Hearing, the Determining Factor for High Fidelity Transmission Proc. I. R. E., Vol. 30, June, 1942 1.2 Gilbreth, Frank B. Fatigue Study New York: MacMillan & Co., 1919 3 Halpin, D. D. Industrial Music and Morale Jour, of Acoustical Soc. of America 4 63 CATEGORY Helmholtz, H. L. F. On the Sensation of Tone (2nd English Edition) London: Longmans Green & Co., 1885 1,3 The Mechanism of the Ossicles of the Ear and Membrane Tympana Baltimore: W. Wood & Co., 1873 1 Henney, K. Radio Engineering Handbook New York: McGraw-Hill, 1933 2 Hyde, I. H. and Scalapino, W. Influence of Music on Electro Cardiograms and Blood Pressure American Journal of Psychology, Vol. 46, No. 1, 1918 2 Industrial Recreation Association, 1944 Music in Industry 4,5 Jeans, J. H. Science and Music Cambridge, England: The University Press, 1937 1 Kerr, W. A. Attitudes Toward Types of Industrial Music Jour, of Acoustical Soc. of America, October, 1943 4,5 Psychological Research in Industrial Music and Plant Broadcasting Journal of Psychology, 1944, 17, 243-261 4,5 Knudson, V. C. Architectural Acoustics New York: John Wiley & Sons, Inc., 1932 1 Krakov, S. V. Influence of Sound Upon Light and Color Sensibility of the Eye Acta Opthal, 14, 1936 3 Landis, C & Hunt, Wm. A. The Startle Pattern New York: Farrar & Rhinehard, 1939 3 Luria, A. R. The Nature of Human Conflicts New York: Liveright, Inc., 1932 3 Mallory, Vincent Stevens Sound Control System Stevens Inst, of Technology,. 1940 2 Mark, Jeffery Music and Industry The Gamut, November, 1929 4 Miles, Walter R. (Editor) Psychological Studies of Human Variability Psychological Monographs, No. 212, 1936 3 Miller, D. C. Science of Musical Sounds New York: Macmillan Co., 1916 1 64 CATEGORY Miller, D. C. (Continued) History of the Science of Sound to the 20th Century New York: Macmillan Co., 1916 1 Mills, John A Fugue in Cycles and Bels New York: D. Van Nostrand, 1935 1 Modern Industry, September, 1942 Plant Broadcasting 4 Morse, P. W. Vibration and Sound New York: McGraw-Hill, 1936 1 New York Times Magazine Riveting to Rhythm August 31, 1941 4 Rhythm for the Job March 14, 1943 4 Nixon, G. M., Rackey, C. A., Hanson, O. B. Down to Earth on “High Fidelity** New York: Nat*l Broadcasting Co., 1944 1,2 / O'Brine, J. Controlled Sound - New Wartime Tonic Popular Science, February, 1924 4 O'Connor, Johnson Psychometrics Harvard University Press, 1934 3 Olson, H. F. Elements of Acoustical Engineering. New York: D. Van Nostrand, 1940 1,2 Olson, H. F. and Massa, B. Applied Acoustics Philadelphia: P. Blakiston’s Sons & Co., 1939 1,2 Podolsky, Edward The Doctor Prescribes Music New York: Frederick A. Stokes, 1939 3,4 RCA Mfg. Co. Sound Engineering Notes Camden, N. J., RCA Mfg. Co., 1940 2 Radio Retailing Today, February, 1943 Music on the MacArthur Shift 4 Rempel, Henry D. Physical Findings Among Certain Groups of Workers Bulletin of the Employment Stabilization Research Institute: Vol. Ill, No. 1 University of Minnesota 3 Sabine, P. E. Acoustics and Architecture New York flk London: McGraw-Hill Co., 1932 1 65 CATEGORY Schoen, Max (Editor) The Effects of Music New York: Harcourt Brace kCo., 1927 3,4 Selvin, Ben Programming Music for Industry Jour, of Acoustical Soc. of America, October, 1943 4,5 Smith, F. L. Radiotron Designer’s Handbook Harrison, N. J.: RCA Mfg. Co., 1941 2 Stevens, S. S., and Davis, A. H. Hearing New York: John Wiley St'Sons, 1938 1,3 Talbot, E. B. and Darlington, L. Distraction by Musical Sound, The Effect of Pitch upon*Attention Amer. Jour, of Psychology, 1897, Vol. XX, pp.332-343 3 Terman, F. Radio Engineering (Second Edition) New York: McGraw-Hill, 1937 2 Time Music While You Work April 13, 1942 4 Productive Melody November 2, 1942 4 Tremaine, G. M. Music in Industry New York: Nat’l. Bureau for the Advancement of Music 4 Vorhees, S. F. Sound System at the World's Fair Arch. Record, 84, 1938 2,4 Washcoe, Alec. Jr. Effects of Music on Pulse Rate, Blood Pressure, and Mental Imagery Temple University, 1933 3 Wat son, F. R. Sound New York: John Wiley & Sons, 1935 1,3 Wood, A. B. A Textbook of Sound London: G. Bell & Sons, 1930 1 Wood, Alexander Acoustics New York: Interscience Publishers, 1940 1 Wyatt, S., and Langdon, J. N. Fatigue and Boredom in Repetitive Work London: H. ,M. Stationery Office, 1938 4 66