[Federal Register Volume 64, Number 134 (Wednesday, July 14, 1999)]
[Notices]
[Pages 38006-38018]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 99-17907]


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DEPARTMENT OF THE INTERIOR

National Park Service


Change in Noise Evaluation Methodology for Air Tour Operations 
Over Grand Canyon National Park

AGENCY: National Park Service, Interior.

ACTION: Notice of Disposition of Public Comments and Adoption of Final 
Noise.

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Evaluation Methodology for Air Tour Operations Over Grand Canyon 
National Park

SUMMARY: On January 26, 1999, the National Park Service (NPS) published 
a Public Notice of agency policy in the Federal Register with the above 
title (64 FR 3969-3972), requesting comments on refinements to NPS' 
noise evaluation (i.e., impact assessment) methodology for air tour 
operations over Grand Canyon National Park (GCNP). Specifically, the 
refinements contemplated a two-zone system for assessing impacts 
related to substantial restoration of natural quiet at GCNP. In Zone 
One, which would encompass about one-third of the Park's area, the 
threshold of noticeability previously used in noise modeling for 
environmental analyses related to GCNP air tours would continue to be 
used (i.e., the average A-weighted natural ambient level plus 3 
decibels). In Zone Two, which would encompass about two-thirds of the 
Park's area, the threshold for the onset of impact would be audibility 
(i.e., the level at which aircraft can begin to be heard by people with 
normal hearing, determined to be 8 decibels below the average A-
weighted natural ambient level at GCNP).
    The NPS received 19 comments in response to the Public Notice. 
Comments were received from industry associations (e.g., United States 
Air Tour Association, Helicopter Association International, National 
Air Transportation Association); environmental groups (e.g., Sierra 
Club, Grand Canyon Trust, Friends of Grand Canyon); air tour operators; 
representatives of tribal concerns; and the general public. The NPS 
considered all substantive comments.

DATES: The noise impact assessment methodology presented herein is 
effective immediately.


[[Page 38007]]


FOR FURTHER INFORMATION CONTACT: Tom Hale, National Park Service, Grand 
Canyon National Park Science Center, 2255 North Gemini Drive, Bldg. 3, 
Flagstaff, AZ 86001, Telephone (520) 556-7219.

Background

    In response to the comments received pursuant to the publication of 
the NPS Public Notice in the January 26, 1999 Federal Register (64 FR 
3969-3972), the NPS has attempted to clarify the reasons for and the 
expected effects of the proposed refinement in the methodology used to 
assess noise impacts below and in the Discussion of Comments to follow.

Reasons for the Proposed Change

    This Notice is one of several steps being taken by the Secretary of 
the Interior, through the NPS, and the Federal Aviation Administration 
(FAA) to fulfill the mandate established by Congress in Public Law 100-
91, the National Parks Overflights Act, to provide for the substantial 
restoration of natural quiet in the Grand Canyon National Park. Section 
3 of the Overflights Act mandated the Secretary of the Interior to 
submit to the Administrator of the FAA recommendations ``regarding 
actions necessary for the protection of resources in the Grand Canyon 
from adverse impacts associated with aircraft overflights.'' The 
express statutory goal for these recommendations is the ``substantial 
restoration of natural quiet and experience of the park and protection 
of public health and safety from adverse effects associated with 
aircraft overflight.'' The Overflights Act requires the Administrator 
to adopt the recommendations of the Secretary of the Interior ``without 
change unless the Administrator determines that implementing the 
recommendations would adversely affect aviation safety.''
    Congress did not define natural quiet or substantial restoration of 
natural quiet in the Overflights Act and, instead, delegated the 
interpretation of the statute to the Secretary. Under well established 
rules of statutory construction, the agency's interpretation is given 
deference so long as it is based on a reasonable construction of the 
statute. The D.C. Circuit Court of Appeals found that the NPS had 
reasonable justification for its interpretations of natural quiet and 
substantial restoration of natural quiet, as set forth in the 1995 
Report to Congress and the 1996 FAA Grand Canyon Special Flight Rules 
Area Final Rule. The court also deferred to the agencies' use of the 3 
decibels above natural ambient threshold to assess audibility as 
consistent with the Act. (See Grand Canyon Air Tour Coalition v. FAA, 
154 F.3d 455 (D.C. Cir. 1998)).
    In its Report to Congress on ``Effects of Aircraft Overflights on 
the National Park System'' that stemmed from this public law (published 
in 1995), the NPS stated:

    Before overflights began, natural quiet existed over most of the 
park, virtually all of the time. Aircraft sound intrusions are a 
significant source of mechanical noise that eliminate natural quiet. 
Since the legislative history of Public Law 100-91 indicates that 
flight-free zones are to be large areas where visitors can 
experience the park essentially free from aircraft sound intrusions, 
and where the sound from aircraft traveling adjacent to the flight-
free zone is not detectable from most locations within the zone, the 
primary measure of restoration is the percentage of time that 
aircraft are audible. Based on this definition from the legislative 
history, the policy decision of Grand Canyon National Park (GCNP) is 
that a substantial restoration requires that 50% or more of the park 
achieve ``natural quiet (i.e., no aircraft audible) for 75-100 
percent of the day.'' (page 182--emphasis added) 1

    From the outset, the consistent policy of the NPS has been that 
audibility is the basis for assessing progress toward the legislatively 
mandated goal of substantially restoring natural quiet to the Grand 
Canyon, and that the time period of interest is the day (i.e., the 
average 12 daylight hours).
    However, in preparation for the 1995 Report and modeling noise 
impacts, the NPS recognized that aircraft noise management in park 
environments was an emerging science. The NPS contracted with BBN 
Systems and Technologies to develop a model which could generate 
audibility-based metrics. That model became the National Park Service 
Overflights Decision Support System (NODSS).
    Another concern at the time was the reliability of information 
about the numbers of air tour operations over GCNP (there were no 
requirements to report such data until 1996). Given the limitations of 
the data, the NPS and its contractors made a decision to take a very 
conservative approach to ensure that noise impacts were not 
overestimated. For that reason, the NPS opted to model noise impacts 
using ``noticeability'' as the threshold of impact (for additional 
information see NPS Responses to Comment # 8, Audibility and 
Noticeability and Comment # 17, Proposal Conflicts with Definition of 
Substantial Restoration). However, the results of this modeling were 
interpreted with full knowledge that the definition of substantial 
restoration was based on audibility. Even using the less stringent 
standard of noticeability, the NPS determined in its 1995 Report that 
natural quiet had not been substantially restored to GCNP.
    The FAA continued this approach when it modified its own INM model 
for use in the 1996 Environmental Assessment supporting the rules 
developed for GCNP Special Flight Rules Area (SFRA). Because INM uses 
only A-weighted sound levels and has no frequency based calculation 
ability, the threshold used by the NPS and determined from actual Grand 
Canyon data was translated to an ambient plus 3 decibels threshold for 
use by the FAA. Again, using a conservative approach, the FAA found 
that the new rules did not result in the substantial restoration of 
natural quiet (61 FR 69302-69333).
    Since 1996 the agencies have continued to gather additional 
information and conduct additional research. The NPS and FAA have 
significantly improved their knowledge and understanding of air tour 
operations over GCNP as a direct result of the reporting requirements 
implemented by the 1996 FAA airspace regulation. The NPS has greatly 
improved its understanding of the natural ambient sound conditions 
across the Park. Through research, a greater number of ambient sound 
areas have been identified and the ambient sound levels in each area 
have been adjusted upwards between 3 and 16 decibels. In addition, NPS 
acoustic experts have concluded that the most accurate measurements of 
audibility for models based on A-weighted sound levels is 8 decibels 
below the average natural ambient. Consequently, the FAA and NPS have 
been better able to accurately assess the extent of aircraft noise in 
the park. Based on the more comprehensive data base and understanding 
of noise impacts on the park, the NPS has appropriately modified and 
improved its noise impact assessment methodology in GCNP.

Effects of the Proposed Change

    The NPS definitions of natural quiet and substantial restoration of 
natural quiet remain the same. Natural quiet is defined as the natural 
ambient sound conditions found in the Park. Substantial restoration of 
natural quiet at GCNP is defined as 50% or more of the Park achieving 
``natural quiet'' (i.e., no aircraft audible) for 75-100 percent of the 
day. The Notice only has the effect of changing the threshold for 
measuring the onset of noise impact for certain areas of the Park for 
noise modeling purposes.
    In Zone One, which includes developed areas of the park, the

[[Page 38008]]

threshold for measuring noise will continue to be based on 
noticeability, 3 decibels above the average natural ambient. NPS 
believes that using the noticeability threshold in Zone One is 
appropriate for the activities that occur and characteristics in this 
area of the park. In Zone Two, the threshold for measuring will be 
based on audibility, when people can hear aircraft in these areas. 
Again, because the FAA INM model uses only A-weighted sound levels and 
has no frequency based calculation, the threshold developed by NPS from 
actual GCNP data translates to approximately 8 decibels below the 
average natural ambient. In developing the audibility threshold, the 
Secretary has exercised his discretion to use the most appropriate 
measuring methodology which takes into account the resource 
characteristics in this zone. The thresholds for both Zones One and Two 
are affected by use of the recently updated average natural ambient 
sound levels in the respective zones.
    These measurements will collectively be used to assess whether 
natural quiet has been substantially restored to GCNP. NPS's 
interpretation of modeling results from both zones will take into 
account that the overall definition of substantial restoration of 
natural quiet is based on audibility.
    The measurements are not ``standards'' which would prohibit 
aircraft that generate noise louder than 8 decibels below the average 
natural ambient from conducting air tours. Neither ``audibility'' nor 
``noticeability'' are standards for aircraft to meet. They are 
thresholds for measuring the onset of noise impact.

Discussion of Comments

    About one-third of the commenters generally supported the noise 
impact assessment methodology. Other commenters expressed reservations 
regarding its validity, applicability, or workability. Still other 
commenters expressed some level of support for the concept, but 
rejected the specific threshold levels. Many commenters were critical 
of the NPS for not providing sufficient discussion of scientific 
methodology and technical information used in refining the impact 
assessment methodology. The NPS has provided additional technical 
information in response to the substantive issues raised by commenters 
below.
1. Approach and Resolution of Issue
    A few comments focused on using a conflict resolution approach to 
find an acceptable compromise. One commenter indicated that a more 
productive process would encourage open public deliberation to resolve 
the conflicting interests at stake. Another commenter urged NPS and FAA 
to stop the haphazard regulatory approach and seek closure through 
negotiated settlement.
    NPS Response: The FAA and NPS are committed to finding more 
effective approaches to problem solving in the Grand Canyon. As the NPS 
and FAA begin to develop a comprehensive noise management plan for 
GCNP, a forum will be re-established to facilitate discussion among all 
stakeholders to continue efforts toward consensus-building.
    The current action simply takes advantage of better data and 
experience to update noise impact assessment methodology, allowing more 
accurate assessment of the onset of impact as previously defined at 
GCNP. The effects of this action will be apparent in the Draft 
Supplemental Environmental Assessment for rulemaking actions at Grand 
Canyon available for public review soon. The FAA rulemaking actions are 
part of a phased approach to achieve the mandated goal of substantial 
restoration of natural quiet at GCNP by 2008. The NPS and the FAA 
welcome suggestions for improvements in ways to achieve that goal. 
While impact assessment methodology is not normally opened for public 
review as this action is, the NPS is taking that extra step in this 
case to ensure open public deliberation in the hope of resolving 
conflicting interests.
2. Concurrence With Two-Zone System
    Several commenters commended the new two-zone geographic system as 
an improvement over the current system. One commenter wrote that the 
two-zone geographic system with different noise thresholds appeared to 
be a long awaited answer to restoring a substantial amount of natural 
quiet in the Park. Another commenter applauded the NPS for recognizing 
that its current standard was inconsistent with the Overflights Act, 
1994 Report to Congress, and visitor experience. One commenter called 
the change a welcome, significant, and valuable improvement. Another 
commenter welcomed NPS' new flexible approach to analyzing noise 
impacts and agreed that the methodology should take into account the 
characteristics of specific areas of GCNP. Another also commended the 
NPS for recognizing that different areas and land uses required 
distinct standards for measuring noise intrusions and impacts.
    NPS Response: The NPS appreciates the positive acknowledgement. 
Although the NPS believes the proposed change in methodology is a much 
more realistic and flexible approach to assessing the onset of impacts 
to natural quiet at GCNP, there is in fact little new in the ``new'' 
approach. Audibility (i.e., can aircraft be heard by people with normal 
hearing) has always been the basis for the definition of substantial 
restoration of natural quiet and determining whether it has been 
achieved. Noticeability was used in previous noise modeling for GCNP 
due to limitations in operations data, ambient sound level data, and 
the ability of software to manage different impact thresholds. Since 
that time, software has been improved to address these issues. Results 
of previous modeling were interpreted with full knowledge that the 
definition of substantial restoration was based on audibility and that 
noise modeling used noticeability for impact assessment. Now, with 
better data available for natural ambient levels as well as when 
aircraft become audible at GCNP, it is possible to be more accurate and 
to incorporate the use of audibility for impact assessment. 
Acknowledging that impact assessment can and should vary across 
different parts of the Park better aligns park planning in the Grand 
Canyon with FAA regulatory and noise modeling approaches. In proposing 
these changes to the noise impact assessment methodology, the NPS 
attempted to integrate the best acoustic data available with park 
management policy and FAA noise modeling technology in light of the 
mandated goal of substantial restoration of natural quiet.
3. Disagreement With Two-Zone System
    Commenters claimed that noticeability and audibility could not 
simultaneously represent the concept of natural quiet and that NPS 
provided no reason why ``natural quiet'' should mean different things 
in different parts of the Park. One of the commenters asserted that NPS 
reasoning was inconsistent with the ``substantial restoration'' 
definition and claimed that the threshold in Zone Two would be 
exceptionally difficult to attain. The commenter further stated that 
much of the substantial restoration would likely be achieved in Zone 
One (noticeability threshold)--a backward result based on NPS' 
reasoning. The commenter proposed to abandon the two-zone approach and 
suggested that if there were some parts of the Park that were more 
noise sensitive (e.g., backcountry), it made sense to identify those 
critical areas and set standards for them, thereby allowing regulations 
to directly meet apparent need.

[[Page 38009]]

    NPS Response: The definitions of ``natural quiet'' and 
``substantial restoration of natural quiet'' remain unaffected by this 
action. They remain the same as previously defined, with no differences 
in any part of the Park.
    The concept being proposed simply indicates that the threshold for 
measuring the onset of impact will vary across the Park for the two 
zones as described. NPS has made a management decision to use the 
audibility threshold for measuring in Zone Two. This approach does not 
set standards for different parts of the Park that industry will be 
required to meet. For example, aircraft will not be required to meet a 
``standard'' of 8 decibels below the average natural ambient level in 
Zone 1.
    Following the 1996 and 1997 FAA rulemaking actions and 
environmental assessment, the NPS determined that using a threshold of 
3 decibels above the average A-weighted natural ambient level could 
result in a situation where aircraft could be heard below that 
threshold as much as 100% of the time, but the noise modeling would 
show no impact. This result would be possible because the data showed 
that aircraft can be heard an average of 8 decibels below natural 
ambient A-weighted sound levels at GCNP. Clearly, this problem was an 
artifact of the noise modeling threshold and other tools, and the use 
of single A-weighted decibel values for ambient and aircraft sound 
levels. This action corrects that potential problem, proposing a more 
flexible and realistic approach to impact assessment using available 
modeling tools for the Park as a whole.
    As indicated in the Public Notice, it is common to zone areas of 
National Parks differently for management purposes based on differences 
in natural and cultural resources, resource protection goals, visitor 
use, visitor experience goals and opportunities, etc. Because of the 
varying circumstances affecting these zones, they are managed 
differently, but for the same ultimate goal. In the context of new 
ambient sound levels used in modeling and a variety of management 
actions being proposed, the NPS believes that there will be progress 
toward the restoration of natural quiet in both zones.
    The NPS does not expect to achieve substantial restoration of 
natural quiet in Zone One, except where Zone One occurs within flight-
free zones. Flight-free zones remain the areas where substantial 
restoration is expected to be achieved. However, by definition, 
achievement of substantial restoration of natural quiet is calculated 
on a park-wide basis, not by zones.
4. Noise Threshold for Zone One
    Several commenters contended that the Sanup Plateau and Marble 
Canyon should not be included in Zone One. One commenter submitted that 
Zone One included many areas that were qualitatively different from one 
another and did not merit the same treatment. The commenter stated that 
the areas of the Sanup Plateau, Marble Canyon, Bright Angel Point, and 
stretches of the South Rim should not use the same noise threshold as 
the developed South Rim Area and that the NPS should apply the 
audibility threshold to 99% of the Park, instead of only 67%. In 
agreement with this point, another commenter asserted that there should 
be a distinct difference in noise thresholds for developed areas and 
backcountry/wilderness areas. Another commenter further stated that 
one-third of the Park (Zone One) should not receive a weaker standard, 
noting that the Sanup Plateau, Marble Canyon, and North Rim backcountry 
were all part of NPS' wilderness recommendation because of their wild 
and pristine qualities. The commenter contended that those areas should 
receive the highest level of protection. One commenter strongly 
objected to exempting one-third of the Park from the objective standard 
of audibility. The commenter claimed there was no scientific 
justification for this, because Marble Canyon and the Sanup Plateau are 
among the least developed areas of the Park and are therefore the most 
noise sensitive. This commenter asserted that use of the noticeability 
standard was inconsistent with the ``wild'' and ``primitive'' 
designations these areas have in the Backcountry Management Plan. 
Another commenter wrote that he was disappointed that the NPS was 
leaving one-third of the Park in a noise sacrifice zone and that 
inclusion of the Sanup Plateau and Marble Canyon in that zone rested on 
failures to properly correct existing air tour noise, not on their lack 
of natural character or pristine quality.
    NPS Response: Again, these are not standards to be achieved, but 
simply points from which the onset of impact will be modeled. They 
result partially from the need to use single A-weighted decibel values 
in the noise modeling for GCNP. They are not meant to directly reflect 
physical properties of the Park's natural environment or of the 
aircraft flying over the Park.
    In the Notice, the NPS stated that there are multiple reasons for 
including diverse areas within Zone One, and the NPS believes that 
these remain valid. The Sanup Plateau and Marble Canyon were included 
in Zone One after consultations with FAA regarding safety 
considerations. FAA has the sole authority to make safety 
determinations. The safety measures employed in these areas create the 
potential for greater noise and NPS has made a management decision to 
include the Sanup Plateau and Marble Canyon in Zone One. In the event 
FAA modifies any safety measures in these areas, NPS will revisit their 
inclusion in Zone One.
    Zones do not set standards, zones are in fact described in terms of 
indicators and standards. A difference in the threshold of impact does 
not turn Zone One into a noise sacrifice zone. The NPS definition of 
``substantial restoration of natural quiet'' allows for some level of 
impact in all parts of the Park, and the two-zone impact assessment 
proposal does not change that. As with most complex management issues, 
it is easy to generalize the apparent outcome of a given action. 
Ultimately, the end result relies on the interplay of many variable 
elements (e.g., impact assessment thresholds, location of air tour 
routes, natural ambient levels, air tour operation levels). Different 
combinations of these elements may or may not result in increased noise 
levels in particular portions of the Park. Substantial restoration is 
calculated on a park-wide basis, not by zone. Adjusting the thresholds 
for impact assessment in the two zones is appropriate and consistent 
with NPS management philosophy.
    Respondents will better understand the effect of the thresholds and 
zones when they see the Supplemental Environmental Assessment for FAA 
rulemaking actions to be published soon. The NPS and FAA are committed 
to an adaptive management approach and new comments or criticisms are 
welcomed at any point.
5. Noise Threshold for Zone Two
    One commenter disagreed that a lower threshold (audibility) should 
be used over areas where the numbers of visitors were very few. 
Instead, noise should be regulated to benefit the visitor experience. 
For this reason, the commenter claimed the threshold for Zone Two was 
illogical.
    NPS Response: The NPS does not and cannot regulate national parks 
solely on the basis of visitor attitudes, annoyance, or experience. The 
NPS is required by law to preserve park resources in an unimpaired 
condition for the benefit of present as well as future generations. 
Where and how to protect park resources and provide various visitor 
experience opportunities are addressed in park legislation and planning 
documents that address resource

[[Page 38010]]

protection and visitor services. In this context, the NPS manages park 
resources to minimize impacts and to ensure that future generations 
have similar or better opportunities to experience these national 
treasures. The proposal in this notice is consistent with Park 
legislation and management plans for the GCNP. The threshold for Zone 
Two is based upon the NPS definition of substantial restoration of 
natural quiet, which is based upon audibility (i.e., can a person with 
normal hearing hear aircraft).
6. Tribal Trust Resources
    Regarding the Sanup Plateau, one commenter questioned the Park 
Service's acceptance of noise generated by tour flights over the Sanup 
Plateau as inevitable and charged that the FAA and NPS were shirking 
their responsibility to consider all alternatives to restore natural 
quiet to the Park, the Grand Canyon, and to protect tribal trust 
resources.
    Related to the issue of zonation and appropriate threshold levels 
for modeling, the commenter requested standards that accurately 
reflected uses of tribal lands, similar to the zones and thresholds 
developed for the GCNP. The commenter asserted that the residential 
noise threshold of 65 dB(A) was not appropriate for most tribal land 
uses and that some areas of the Hualapai reservation deserved the 
stricter standard of 8-12 dB below ambient.
    NPS Response: The NPS has authority to establish policy regarding 
noise impact assessment methodology for Grand Canyon National Park 
under Section 3 of PL 100-91. For purposes of this notice, PL 100-91 
limits the application of the methodology to areas within the 
boundaries of Grand Canyon National Park. NPS may not exceed its 
delegated authority and establish similar thresholds for areas of the 
Hualapai Reservation or other neighboring lands. Regarding the 
inclusion of the Sanup Plateau in Zone One, please refer back to NPS 
Response to Comment #4, Noise Threshold for Zone One. As to the use of 
the residential noise standard, refer back to NPS Response to Comment 
#3, Disagreement with Two-zone System, which explains that this is not 
a standard, but rather a way of assessing impacts on GCNP.
7. Ambient Sound Levels
    One commenter stated that a fundamental component of the 
establishment of any threshold sound value keyed to ``ambient levels'' 
must be the completion of a scientifically unbiased and 
methodologically rigorous ambient sound measurement program within the 
Park. In addition, some commenters pointed out that current data show 
some areas of the Grand Canyon (and other parks) have ambient levels 
that are, at times, below the human threshold of hearing. Some 
commenters were of the opinion that typical daytime, not overall 
minimum, sound levels should be used to characterize ambient levels.
    Regarding the use of ambient levels, a commenter asserted that it 
was possible to develop effective regulations concerning aircraft 
overflights of GCNP without reference to ambient sound levels (e.g., 
routes should stay in areas that have low visitor usage or where 
natural sounds mask aircraft noise).
    NPS Response: The NPS agrees that the data show Grand Canyon to be 
a very quiet place, with A-weighted natural ambient levels near the 
threshold of human hearing in many places. In the presence of such 
quiet natural ambient levels, it is not surprising that aircraft can be 
heard at A-weighted decibel levels for the aircraft also near the 
threshold of human hearing. This occurs because the A-weighted decibel 
level collapses the frequency spectra into a single number. Aircraft 
noise levels in part of the frequency spectra may be well above the 
threshold of hearing (and well above the same part of the natural 
ambient environment's spectra) and still have an A-weighted decibel 
value at or near the threshold of human hearing. The NPS agrees to 
further explore the situation where aircraft are audible close to the 
threshold of human hearing, to see if any additional refinements to 
this methodology might be warranted in the future development of the 
comprehensive noise management plan.
    The NPS agrees that the best data available on natural ambient 
sound levels should be used in impact assessment, since hearing 
aircraft depends upon both the aircraft and the natural ambient sound 
levels throughout their entire frequency spectra. The NPS has recently 
updated natural ambient levels used in the noise modeling based upon a 
recent analysis of the best available data for GCNP. Natural ambient 
levels vary both over time and from place to place. Currently, the best 
available A-weighted data have been used to estimate the natural 
ambient levels in the Canyon.\1\ However, in the interest of improving 
currently available data, the NPS is developing a rigorous data 
collection program to refine these levels for future use in a 
comprehensive noise management plan.
    All natural ambient sound level values currently being used by GCNP 
for noise modeling purposes are derived from daytime acoustic 
measurements. This is consistent with its definition of the substantial 
restoration of natural quiet (i.e., aircraft not audible in 50 percent 
of the Park for 75 percent of the day).
    The resource of natural quiet is not meant to represent the 
``average'' natural ambient sound environment. In the 1994 Report to 
Congress, the NPS sets forth that ``The quiet to be preserved is the 
lower end of the ambient sound level range that occurs regularly 
between wind gusts, animal sounds, etc., not just the average sound 
level.'' (pg. 83). Further, it states: ``In considering natural quiet 
as a resource, the ability to hear clearly the delicate and quieter 
intermittent sounds of nature, the ability to experience interludes of 
extreme quiet for their own sake, and the opportunity to do so for 
extended periods of time is what natural quiet is all about.'' (pg. 78, 
emphasis added).
    In terms of protecting park resources, the NPS agrees that it is 
desirable to place air tour operations over areas with higher natural 
ambient levels to help minimize impacts of aircraft noise. However, 
given the NPS' audibility-based definition of the substantial 
restoration of natural quiet, it is not possible to restore natural 
quiet without establishing natural ambient levels. Detailed acoustic 
information on the natural ambient and aircraft noise levels is 
essential for accurate noise modeling.
8. Audibility and Noticeability
    Many commenters expressed opinions about the use of audibility of 
aircraft as a threshold for evaluating whether natural quiet is 
substantially restored. While some supported its use as the only 
``truly scientific standard,'' others were strongly opposed, stating 
that the audibility threshold was unreasonably low and that the use of 
noticeability was a more appropriate threshold. Several commenters 
preferred the use of noticeability, and one used data from an NPS 
report.
    NPS Response: The NPS has consistently used an audibility-based 
definition for the ``substantial restoration of natural quiet.'' While 
it is true that NPS noise modeling in the 1994 Report to Congress used 
a noticeability threshold (as 10 log d = 17), the modeling results were 
interpreted with full knowledge that the definition of substantial 
restoration was based on audibility (see NPS Responses to Comments # 2, 
Concurrence with Two-zone System and #17, Proposal

[[Page 38011]]

Conflicts with Definition of Substantial Restoration). This 
conservative approach was taken so that noise impacts, based on limited 
data, were not overestimated. The FAA followed this approach in its 
1996 and 1997 GCNP rulemaking. Now, with more accurate data available, 
including better information on natural ambient sound levels, numbers 
of air tour operations, etc., it is possible to make the modeling 
conform better to the NPS definition of substantial restoration of 
natural quiet by moving to the proposed two zone approach to noise 
impact assessment.
    Because of numerous misperceptions, the following information is 
provided to clarify the definitions of audibility and noticeability, 
how they are quantified, and how they relate to the concept of natural 
quiet. One misperception seems to be that NPS is trying not only to 
restore natural quiet, but to restore 8 decibels below natural quiet. 
This is not the case. Natural quiet remains the same as ``no aircraft 
audible'' in this context. Due to the nature of A-weighted decibel 
values, the data indicates that aircraft become audible at GCNP on 
average at 8 decibels below the ambient A-weighted levels, as explained 
below. In order to adequately model how much of the time aircraft are 
audible, the modeling must start measuring the impact of aircraft noise 
on natural quiet when it first becomes audible (i.e., 8 decibels below 
the ambient A-weighted level, on average) not at the much higher level 
called for by the use of noticeability.
    Audibility and Hearing Aircraft. In common usage, audibility (also 
called detectability) refers to the ability of a human, free of 
external distractions, to hear a specific sound in a particular 
setting.\2\ In this context, the question concerning audibility at GCNP 
is: Can a person with normal hearing hear aircraft in the presence of 
the natural ambient Park soundscape? Whether one can hear aircraft 
noise or not depends on the interplay of several variables including 
natural ambient level, volume of the sound (``loudness''), and 
frequency distribution of the sound (``pitch''). The importance of 
frequency is discussed below. Clearly there are degrees of audibility, 
from a condition where the sound cannot be heard at all, all the way to 
where the sound is deafening. For the purposes of this discussion, the 
sound to be heard will be termed the ``target'' sound.
    The two most important factors affecting audibility of the target 
sound are the frequency-based sound levels of both the target sound and 
the ambient or ``background'' sound. If the two sets of frequencies are 
similar and completely overlap, the target sound will not be heard. But 
the less these frequencies overlap, the more audible the target sound 
will be. With even a few non-overlapping frequencies, the target sound 
will become audible.
    Over the past 30 to 40 years, considerable research has been 
directed at understanding how humans hear or detect one sound in the 
presence of another. Listening tests, using a wide variety of 
specialized sounds have been employed to determine how people process 
the combined target and background sounds to do the best possible job 
of hearing and identifying (detecting) the target sound. Three 
important findings of these tests are: (1) Humans can listen to sound 
in narrow regions of pitch, called frequency bands, and determine the 
presence or absence of the target sound separately in each frequency 
band;\3\ (2) there is little variation in detection performance across 
a population of healthy young adults;\4\ and (3) the results of these 
tests can be reduced to a series of mathematical equations that 
describe the physiological detection process.\5\ Given moderately 
detailed knowledge of both the target and background sounds, the 
probability of detecting the target sound can be predicted from these 
equations.
    The ability of humans to listen to sound in frequency bands has 
significance in many activities. For example, during a concert if we 
listen for a high note on the piccolo, only the portion of the 
background sound (e.g., the rest of the orchestra) that is of nearly 
the same pitch can interfere with our ability to hear the piccolo. The 
base violins can play as loudly as they like without the piccolo 
becoming inaudible. On the other hand, loud high notes on the violins 
could interfere with hearing the piccolo as long as the violins were 
playing in the same frequency band as the piccolo. Hence, the relative 
overall ``loudness'' of the background and the target sounds is not the 
key factor in the detection process. For a target sound to be audible, 
it must contain more sound energy (be ``louder'' than) background 
sounds in the same frequency band.
    The equations that predict when a target sound is audible calculate 
a metric called ``acoustic detectability,'' abbreviated d (pronounced 
``dee-prime''). The d calculation is performed in each narrow frequency 
band (one-third Octave Bands), using both the target and background 
sounds. The d calculation yields a result in each band; the results 
from each band are then summed and averaged to yield a composite 
total.\6\ The band or bands with the highest d values are the ones that 
most influence the result and that are most likely to be audible. In 
the laboratory setting, a target sound is likely to become audible to 
trained listeners when the acoustical detectability, in decibels 
(computed as 10 log d) lies between 3 dB and 5dB.
    In the fall of 1989 and spring of 1990, tape recordings were made 
at 13 different sites in the Grand Canyon. During each recording 
session, an observer identified air tour aircraft and pressed a button 
whenever the aircraft could be heard. By using the tape recorded 
natural ambient sound level information just before each aircraft was 
audible, and the total level at the onset and offset of observed 
audibility, the acoustic detectability level (10 log d) was computed 
for 163 air tour aircraft overflights at 11 of the measurement sites. 
These computations yielded an average value of 7.3 dB. ``The grand 
average of 10 log d of 7.3 (rounded to 7) can therefore be taken as a 
typical 10 log d value for a vigilant observer, and thus provides a 
useable working definition of audibility under field conditions.'' \7\
    From this research and field work, aircraft overflights of the 
Grand Canyon are judged to be audible when, compared with the local 
ambient level, the detectability level equals 7 dB or greater. It is 
important to keep in mind that any determination of audibility depends 
upon the levels and frequency content of both the aircraft and ambient 
sounds present at the location under consideration. Hence, to fully 
determine audibility, both the sound levels of the target sound and of 
the ambient or background sound must be known as a function of 
frequency.
    Audibility and A-Weighted Sound Levels. In examining the effects of 
tour operations on `natural quiet' across a large area, computer 
modeling is necessary to judge how natural quiet might be substantially 
restored through changes in airspace use, increased use of quieter 
aircraft, etc. The most widely used program for analysis of the noise 
effects of changes in airspace or aircraft operations is the Integrated 
Noise Model (INM) developed and supported by the FAA. The INM currently 
uses only ``A-weighted'' sound levels to compute sound levels and 
display results.
    The A-weighted level of a sound is a single number determined by 
combining the sound levels in all frequencies. This combining de-
emphasizes the low and high frequencies in a manner similar to the 
sensitivities of human hearing. The A-weighted level is widely accepted 
as one of the best over-all sound level metrics for analysis of 
transportation noise. It has been shown to correlate

[[Page 38012]]

well with human assessment of the loudness or noisiness of a sound. It 
has not, however, been used as a measure for evaluating audibility.
    Because it uses only A-weighted sound levels, the INM does not 
currently compute audibility directly because it contains no frequency 
based calculation ability. It can, however, compute the time that 
aircraft sound levels exceed a specific A-weighted threshold. If the 
threshold can be chosen to be approximately equal to the level at which 
an aircraft becomes audible, then it can compute, to a reasonable 
approximation, the time that aircraft sound is predicted to be audible.
    This threshold of audibility depends not only upon the sound level 
of the aircraft, but on the level of the natural ambient as well, so 
adequate A-weighted approximations of aircraft and natural ambient 
sound levels must also be provided. Hence, each natural ambient level 
in the Park requires identification of a different A-weighted 
audibility threshold. The method used to determine these thresholds is 
through determining the typical difference between the natural ambient 
level and the aircraft level at the onset of aircraft audibility.
    Five different national park natural ambient spectra and eight 
different aircraft spectra were examined to determine the differences 
in the natural ambient and the aircraft A-weighted sound levels when 
the aircraft spectra were adjusted so that the acoustic detectability 
(10 log d') equaled 7.\8\ Average A-weighted differences between seven 
non-jet, tour type aircraft and three Grand Canyon ambient environments 
were computed from these data. (The 117.4 mile camp ambient was omitted 
because it included higher frequency noise, such as water noise, which 
affects the ambient A-weighted level, but does not affect aircraft 
audibility.) From these data, helicopter sound levels were computed to 
become audible (10 log d' = 7) on average when their A-weighted level 
was 7.2 dB below the ambient; propeller aircraft become audible when 
their A-weighted level was 9.5 dB below the ambient, on average. For 
the noise impact assessment method presented in the Federal Register 
notice, the average of these two of 8.4 dB (or 8 dB) was used as the 
typical difference between natural ambient and aircraft at onset of 
audibility.
    Noticeability. Several research efforts have addressed the premise 
that people who are engaged in some activity other than listening may 
not be aware that a new or intruding sound is present, though it is 
audible. Three different studies have examined the acoustic 
detectability of sounds when subjects engaged in a specific task first 
``notice'' the presence of a specific sound.\9\ Different types of 
background sounds were played, and different types of activities were 
used, such as reading, playing video games, or maintaining a specific 
speed in an automobile set up to run in place. In general, the target 
sound needed to be about 10 decibel units above the threshold of 
audibility for the people in these experiments to take a positive 
action (such as pushing a button) indicating they had heard the sound.
    As noted above, several commenters preferred the use of 
noticeability, and one used data from an NPS report \10\ to determine a 
noticeability threshold. This method used A-weighted sound level data 
measured at 13 different sites, and applied what is generally a 
technically reasonable method to derive a noticeability threshold. Two 
aspects of this method, however, are inapplicable to this situation and 
inconsistent with NPS policy. First, audibility (rather than 
noticeability) is the criterion upon which the definition of 
substantial restoration of natural quiet at GCNP is based: 
``substantial restoration requires that 50 percent or more of the Park 
achieve `natural quiet' (i.e. no aircraft audible) for 75-100 percent 
of the day.'' Second, the resulting threshold from the suggested method 
is an absolute level (i.e., 30 dB(A)), dependent upon the ambient 
levels at just the 13 sites, which are not necessarily representative 
of all ambient environments in the Canyon (see also the NPS Response to 
Comment #10 below). The NPS approach overcomes these issues by: (1) 
Using natural ambient levels characteristic of various areas of the 
Park; and (2) determining the difference between aircraft and ambient 
A-weighted levels at the onset of audibility.
9. Hearing Aircraft Below Ambient Levels
    Some commenters said that hearing an aircraft when its sound level 
is below the average A-weighted ambient level was not intuitive and 
made no sense to the lay person.
    NPS Response: Although perhaps not intuitive to the lay person, 
aircraft can be heard when their average A-weighted noise level is 
below average A-weighted values for the natural ambient. This is 
because one or more specific aircraft tones are, in fact, louder than 
ambient tones in the same frequency. This phenomenon is more easily 
understood using the piccolo example offered previously (see NPS 
Response to Comment #8, Audibility and Noticeability). This comment may 
arise from trying to understand audibility strictly through the use of 
average A-weighted sound levels for the comparison of aircraft and 
natural ambient sound levels. Since A-weighted levels are computed from 
all the individual levels at specific frequencies, important 
information concerning differences in the individual frequency bands is 
lost.
    For example, at the onset of audibility (detectability level = 7 
dB), the A-weighted level of a propeller plane measured at Point 
Imperial (Prop 1 \11\) is about 11 dB lower than the Point Imperial 
ambient A-weighted level. However, in the 125 Hz one-third Octave Band, 
the sound level of the propeller plane is about 4 dB louder than the 
ambient level in the band; hence the aircraft is clearly audible to 
visitors at Point Imperial even though the A-weighted levels would 
suggest that the aircraft noise is not audible.
    In general, for the target sound to be audible in the presence of 
background sound, the target will either exceed the background level in 
at least one one-third Octave Band, or be within one or two decibels of 
the background sound levels in several one-third Octave Bands.\12\
10. Threshold of Impact is 30 dB(A)
    Some commenters identified 30 dB(A) [A-weighted average of 30 
decibels] as an appropriate threshold for impacts because this level 
was the approximate average of the level at onset and offset of 
audibility, as reported in Report NPOA Report No. 93-1.\13\ One 
commenter proposed a method for deriving the threshold of noticeability 
from these data.
    NPS Response: The NPS-determined threshold for impact assessment is 
audibility or noticeability, depending upon the zone. Although thirty 
dB(A) may be noticeability for certain natural ambient sound areas, it 
is not an appropriate threshold of impact assessment for the whole 
Park. Nor is any other single value an appropriate acoustic threshold 
for the entire Park. Natural ambient values have been found to vary by 
vegetation communities, the presence of water-produced sounds from 
perpetually running water sources, and size and distance from water 
falls and rapids. Commenters may also have misinterpreted the data in 
NPOA Report No. 93-1. Specifically, the A-weighted levels reported in 
Table E-3 of that report are total sound levels, not those produced by 
aircraft alone. Because the aircraft are audible when their A-weighted 
level is below the A-weighted natural ambient levels, the onset and

[[Page 38013]]

offset levels reported are primarily a measure of the average ambient 
A-weighted levels, not the aircraft levels (as verified by the authors 
of that report).
11. HMMH Memorandum on A-Weighted Level Differences \14\
    In response to requests for additional information from several 
commenters, the NPS provided copies of a memorandum: ``A-weighted Level 
Differences Compared with Detectability,'' Memorandum to W. R. Henry, 
HMMH Job No. 294530.22, May 15, 1997. Several commenters saw no 
relevance in the information presented in this memorandum, stating that 
it provided no new information and that it gave only calculations using 
previous data. Some also commented that it identified thresholds of 
audibility that were below the human threshold of hearing.
    NPS Response: The purpose of the memorandum was to provide: (1) A-
weighted differences between measured park environment sound levels and 
measured aircraft overflight sound levels at the onset of audibility; 
(2) values of the acoustic detectability level for situations where the 
aircraft A-weighted sound level is 3 dB greater than the A-weighted 
park background level. This information was computed from available 
tape recordings of only park environment sounds (natural ambient) and 
of only aircraft overflight sounds. All recorded samples used were 
carefully checked for absence of other non-park, non-aircraft sounds, 
so that the frequency information analyzed was either pure park 
environment sound or pure overflight sound. Because the purpose was to 
determine differences only, only the differences in sound level by 
frequency band were of interest, not the absolute level of the sounds. 
Hence, the data of this memorandum should be used only for analysis of 
differences between sounds, not for determining absolute levels of 
either the background or of the aircraft.
    Because the INM uses A-weighted levels and cannot reproduce the 
standard calculations to determine onset of audibility, a method was 
needed to accommodate this limitation. The INM does calculate the time 
a specific threshold (as an A-weighted level) is exceeded by aircraft 
sound. The method chosen was to use existing data to compute a typical 
difference between representative A-weighted national park ambient 
sound environments and A-weighted aircraft sound levels at the onset of 
audibility (detectability level = 7 dB) and to compute detectability 
levels when aircraft A-weighted levels equal ambient A-weighted levels 
plus 3 dB.
    This memorandum used frequency band levels and A-weighted levels 
tape recorded for five national park environments and eight different 
aircraft overflights. It computed and provided first the A-weighted 
differences for all 40 combinations of background and aircraft levels 
by adjusting the aircraft spectra to yield the onset of audibility 
(detectability level = 7 dB). At onset of audibility, aircraft A-
weighted levels ranged from about 4dB below the ambient to about 22 dB 
below the ambient. Using just the three most relevant Grand Canyon 
ambients and the tour type aircraft, the aircraft A-weighted noise 
level averaged 8 dB below the average A-weighted ambient level (the 
117.4 mile camp ambient was omitted because it included higher 
frequency noises, such as water noise, which affects the ambient A-
weighted level, but does not affect aircraft audibility).
    Second, the memorandum sought to answer the question: What is the 
detectability level when aircraft sound is 3dB above the background? 
This question arose because the FAA's 1996 and 1997 Environmental 
Assessments used 3 dB above the ambient in the INM modeling as the 
threshold sound level that indicates when natural quiet is lost. The 
threshold of audibility occurs at a detectability level (10 log d') of 
7 dB; the memorandum shows that the detectability level, when A-
weighted aircraft sound is 3 dB greater than the background, averages 
about 18 dB or about 10 dB higher than the threshold of audibility. In 
other words, aircraft sound 3 dB above the ambient is approximately 
equivalent in detectability level to the detectability level needed for 
people to notice a target sound in the laboratory tests of 
noticeability.
    Finally, it should be emphasized that the information in the 
memorandum was derived from tape recordings made separately of ambient 
and of aircraft sounds. The goal, as stated, was to use ambient-only 
and aircraft-only spectra to conduct the calculations. Thus, there was 
no need to derive aircraft spectra from recordings made at the on-set 
of audibility; rather, the aircraft spectra were derived from portions 
of the recordings when only the aircraft were the dominant source of 
sound.
12. Use of Attentive Listeners
    Some commenters claimed that the use of technicians actively 
seeking to hear aircraft noise did not reasonably represent the 
disruption of natural quiet for park visitors and that listening for 
aircraft was not the same as enjoying natural quiet. Other commenters 
objected to the use of attentive listeners to determine at what point 
aircraft could be heard and said that this method was subjective and 
not associated with what would be considered credible research.
    NPS Response: Park visitors sitting quietly and enjoying the 
natural sounds of the Park are equivalent to technicians actively 
seeking to hear aircraft noise. The definition of substantial 
restoration is very clearly based on audibility, not noticeability (see 
earlier discussion in NPS Response to Comment #8, Audibility and 
Hearing Aircraft and Noticeability sections). Audibility is the ability 
of a human with normal hearing, free of external distractions, to hear 
a specific sound in a particular setting. Noticeability is much more 
difficult to measure, and much more variable and difficult to reproduce 
because of the variability of activities people might be engaged in. 
One of the activities people engage in at the Grand Canyon is sitting 
quietly and enjoying the natural ambient sound environment--this 
represents a more stable and reproducible measuring point because it 
depends on the ambient. Anyone with normal hearing will begin to hear 
aircraft at very close to the same audibility point (10 log d = 7) 
given the same ambient level.
13. Laboratory Tests
    One commenter suggested that laboratory tests did not necessarily 
reflect actual audibility in the Grand Canyon.
    NPS Response: As discussed in the NPS Response to Comment #8, 
Audibility and Noticeability, considerable scientific research has 
developed equations for predicting when a target sound is audible in 
the presence of background sounds. These equations that compute the 
audibility metric have been developed from laboratory tests, and relate 
to human physiology; that is, to basic properties of human hearing, and 
these properties vary little across a population of healthy young 
adults.
    These equations were then applied to field data, gathered in the 
Canyon, of some 163 tour aircraft overflights at onset and offset of 
audibility. The results yielded an average acoustic detectability level 
of 7 dB which ``* * * provides a useable working definition of 
audibility under field conditions.'' Hence, the scientific laboratory 
work determined the basic ability of human hearing to detect one sound 
in the presence of a second, and the field data provided the value of 
the

[[Page 38014]]

detectability level that applies to the audibility of tour aircraft at 
GCNP.
14. NPS Statutory Authority
    Some commenters asserted that the NPS' Notice was not authorized by 
the Overflights Act. One commenter summarized this sentiment as 
follows: ``Continued regulation is unnecessary and illegal if NPS is 
citing PL 100-91 as its authority to act.'' The commenter alleged that 
NPS had discharged its obligation under the act by submission of its 
original Report to Congress and with the completion of that 
requirement, NPS' authority under the act was expired.
    NPS Response: Section 3 of PL 100-91 authorizes the Secretary to 
provide continued advice and recommendations to the FAA regarding the 
interpretation of policy on noise impact assessment. The NPS may issue 
this guidance to assist the FAA in the development of its regulations. 
In reviewing the Special Flight Rules in the Vicinity of Grand Canyon 
National Park, 61 Fed. Reg. 69,302 issued by FAA in December, 1996, the 
Court of Appeals for the D.C. Circuit upheld NPS's and FAA's pursuit of 
additional measures to substantially restore natural quiet after NPS 
submitted its Report to Congress in 1994. (Grand Canyon Air Tour 
Coalition v. FAA, 154 F.3d 455 (D.C. Cir. 1998).
15. No Need for Further Action
    One commenter stated that NPS surveys showed that relatively small 
percentages of visitors (about 5% for the Grand Canyon) reported 
annoyance with aircraft noise. The commenter also stated that in 1995, 
30 of 5 million visitors complained of aircraft noise and this was 
attributable to the notion that the current SFAR 50-2 and noise 
thresholds were working.
    Many commenters expressed the opinion that there was no need for 
this or any other regulatory action, as the substantial restoration of 
natural quiet had already been achieved under SFAR 50-2 and that the 
NPS was addressing a problem that did not exist. One commenter wrote 
that Flight-free zones had already substantially restored natural quiet 
to the Park. Another commenter recommended that the Public Notice be 
retracted and that the air tour route structure in SFAR 50-2 be 
maintained, as it has already achieved substantial restoration.
    NPS Response: The NPS protects resources and provides visitor 
services on the basis of policy, legislation, and careful public 
planning. The purpose of SFAR 50-2 was to provide for the substantial 
restoration of natural quiet to Grand Canyon, not low annoyance levels 
among ground visitors. The NPS agrees that SFAR 50-2 has subsequently 
been responsible for some increase in the percent of substantial 
restoration at GCNP and that SFAR 50-2 has certainly benefited the 
Park. However, substantial restoration of natural quiet is not based on 
visitor annoyance. It is based on audibility of aircraft. As the Report 
to Congress clearly states: ``When visitors can hear the sound of 
aircraft, they cannot experience natural quiet.'' \15\ In addition, 
continuing industry growth results in a perpetual decline in the 
percent of substantial restoration achieved.
    Furthermore, using somewhat different procedures, INM and NODDS 
each independently indicate that the implementation of SFAR 50-2 has 
not resulted in substantial restoration of natural quiet (i.e., 50% or 
more of the Park 75-100% of the day no aircraft audible). Noise 
measurements confirming that tour aircraft are still clearly audible 
within the flight-free zones further bear this out. Achieving the 
mandated goal will require implementation of a variety additional 
management actions that will contribute to a reduction of air tour-
produced noise.
16. Focus on Visitor Experience
    One commenter suggested that the NPS had ignored actual visitor 
experience in developing the new thresholds and indicated that the 
Overflights Act required that NPS' recommendations to the FAA provide 
for ``substantial restoration of the natural quiet and experience of 
the park and protection of public health and safety from adverse 
affects associated with aircraft overflights.'' The commenter stated 
that it did not speak of ``substantial restoration of natural quiet'' 
in isolation and that the context clearly stated the purpose of 
substantial restoration was to reduce the effect of aircraft 
overflights on disturbing visitors' park experience.
    NPS Response: The NPS has not ignored visitor experience in 
developing its proposed policy. Visitor experience is just one of many 
factors involved in NPS policy and planning. The NPS manages national 
parks based on the NPS Organic Act, as amended by the Redwoods Act (16 
U.S.C. 1 to 1a-1) individual park legislation, and a variety of park 
management plans that have gone though extensive public review. These 
address the most salient resource protection and visitor services in 
the park.
    The commenter is correct that the NPS is required to consider both 
resources and visitor experiences in managing parks. However, the 
commenter incorrectly asserts that the phrase applies only to visitor 
experience; as with virtually everything else in parks, it applies both 
to resources (i.e., natural quiet) and visitor experience.
17. Proposal Conflicts With Definition of Substantial Restoration
    A commenter claimed that the two-zone noise threshold system 
conflicted with the current definition and interpretation of 
``substantial restoration.'' The commenter said that the new noise 
standard arbitrarily departed from its previously settled definition of 
``substantial restoration of natural quiet'' and asserted that natural 
quiet was defined as ``no more than 3 dB above ambient background 
noise'' in the 1994 Report to Congress.
    NPS Response: The two-zone proposal for assessing noise impacts 
does not conflict with or affect the current definition and 
interpretation of the ``substantial restoration of natural quiet;'' it 
still requires that 50% or more of the Park achieve natural quiet 
(i.e., no aircraft audible) for 75-100% of the day.\16\ The definition 
of substantial restoration has always been based on audibility, and 
this is not changed.
    However, early in the modeling process, the NPS took a fairly 
conservative approach in setting an impact threshold for noise modeling 
used in predicting the achievement of substantial restoration in the 
Park. This stemmed primarily from having coarse natural ambient levels 
and limited air tour operations data to work with and was also driven 
by the inability of INM to calculate audibility directly. The NPS used 
a conservative metric of 10 log d = 17 (approximating the threshold of 
noticeability) to make certain that even with limited data, it could be 
certain that noise impacts were not overestimated. This is the 
threshold that was used for the modeling in the Report to Congress. 
However, because INM uses only A-weighted sound levels and has no 
frequency based calculation ability, the 10 log d = 17 metric used by 
the NPS was roughly translated to an ambient plus 3 dB threshold for 
use in the FAA's 1996 Environmental Assessment. The rationale given in 
the FAA document (page 4-4) for use of the ambient plus 3 dB threshold 
was that ``an aircraft was audible if it increased the ambient noise 
level by three decibels, the smallest change perceptible to the human 
ear'' (see NPS Responses to Comment #8, Audibility and Noticeability 
and Comment #11, HMMH Memorandum on A-Weighted

[[Page 38015]]

Level Differences for additional information).
    Now that the GCNP is approaching its goal of substantial 
restoration of natural quiet and with the availability of more accurate 
natural ambient and air tour operations data, the NPS is proposing the 
two-zone approach to further refine its impact assessment methodology 
to more closely coincide with the audibility-based definition of 
substantial restoration. This will allow the NPS to be more accurate in 
determining the level of substantial restoration of natural quiet being 
achieved at GCNP.
18. Premature Release of Notice
    One commenter asserted that the proposal was premature with the 
ongoing efforts of the FAA to develop Special Flight Rules in the 
vicinity of GCNP. A commenter also stated that because the Notice did 
not present the scientific methodology used in deciding the new 
standard, the Notice should be rescinded or the comment period 
lengthened so the public would have a chance to review the methodology. 
Another commenter claimed that the NPS did not fulfill its obligation 
to ensure an adequate factual basis for rulemaking before implementing 
a new regulation.
    NPS Response: This action is not a regulation; it simply announces 
a change in impact assessment methodology related to noise modeling at 
GCNP.
    The FAA and NPS are committed to an adaptive management approach as 
they continue to work cooperatively to develop regulations in a phased 
manner that assist in meeting the mandated goal of substantial 
restoration of natural quiet at GCNP. At any appropriate point in this 
process the agencies must make adjustments to approaches and 
methodologies where new knowledge or better science make the changes 
warranted. This change in impact assessment methodology recognizes that 
different areas of the Park need to be treated differently in impact 
assessment and represents an effort to bring the current methodology 
more in line with the goal of substantial restoration, which is based 
on audibility. The new approach starts measuring aircraft noise and 
counting it against the substantial restoration goal as soon as 
aircraft are audible, as the definition requires. Using the former 
single impact assessment threshold, aircraft could theoretically be 
audible to ground visitors 100% of the time over 100% of the Park 
without being counted against the goal.
    While NPS did not provide all the details of field data collection, 
data reduction, and analyses used in the study referred to in the 
Notice, sufficient information was provided in a reasonably 
understandable format to adequately describe the purpose and 
justification for the action. In response to this and similar comments, 
however, the NPS is providing substantial additional technical detail 
in this disposition of comments to better portray the justification for 
changes in the noise impact assessment methodology. The NPS also 
provided copies of technical memoranda to anyone who requested such 
additional detailed information during the public comment period. See 
the NPS Response to Comment #8, Audibility and Hearing Aircraft, for a 
more detailed treatment of methods.
    FAA's 1996 EA used the best data and analysis approaches available 
at that time. However, some of the public comments criticized some of 
the approaches. Therefore, in consideration of the comments and when it 
became apparent that additional rulemaking actions and NEPA analysis 
would be needed to meet the mandate to substantially restore natural 
quiet to GCNP, the NPS cooperated with the FAA and asked HMMH, Inc. to 
determine if better approaches or data were currently available that 
should be used in subsequent analyses. The subsequent HMMH analysis 
found, using actual Grand Canyon data and the findings of the previous 
BBN work, that aircraft noise could be heard considerably below the A-
weighted ambient levels.\17\ Based upon this analysis, and the limits 
of using A-weighted ambient and aircraft values rather than values 
based upon frequency spectral characteristics, the NPS subsequently 
recommended to FAA that the best data available in 1998 indicated that 
the A-weighted ambient level minus 8dB was the best single A-weighted 
surrogate for 10 log d = 7 (detectability level at which aircraft can 
begin to be heard by people with normal hearing) in the Park.
19. Model Validation Study
    A few commenters asserted that the noise model validation study 
currently underway should be completed prior to any additional 
rulemaking. Supporting this, one commenter wrote that the Notice was 
prematurely issued and should be withdrawn until the model validation 
study is completed, as the acoustic modeling upon which it is based has 
been challenged as bad science.
    NPS Response: As noted in the NPS Response to Comment 
18, Premature Release of Notice, the FAA and NPS 
are committed to an adaptive management approach as they continue to 
work cooperatively to develop regulations that assist in meeting the 
mandated goal of a substantial restoration of natural quiet at GCNP. At 
appropriate points in this process the agencies will make adjustments 
to approaches and methodologies where new knowledge or better science 
make the changes warranted. This will also be true if the FAA-NPS noise 
model validation study points to the need for further adjustments. 
Calling it a ``validation'' study does not imply that the current 
modeling is somehow ``invalid.'' On the contrary, it is the best 
science available and seems to produce results quite consistent with 
available data. In this context, ``validation'' is equivalent to 
``improvement.'' As with all scientific modeling, it will improve over 
time with the addition of new information and research. It is prudent 
to constantly question methodologies in order to incrementally improve 
them. It is imprudent, however, to indefinitely delay management 
efforts with the expectation that methodologies may be perfected.
    The Notice proposes one step in the process of assessing the 
impacts of air tour-produced noise and measuring the relative progress 
toward the mandated goal of substantial restoration of natural quiet. 
The step establishes the acoustic thresholds at which air tour-produced 
noise begins to be counted against the goal using current INM noise 
modeling technology. These thresholds are critical as inputs used in 
the noise modeling process. The model validation study may be another 
important step in strengthening methodologies, to the extent that it 
may allow the FAA and NPS to more accurately predict noise impacts and 
monitor the level of substantial restoration in the Grand Canyon. 
However, since the thresholds proposed are used as inputs for modeling 
and have no bearing on the internal workings of the models (i.e., the 
algorithms and assumptions upon which the models are built) that are 
addressed in the validation study, there is no reasonable basis to 
postpone implementation of the changes to the assessment process.
20. Rulemaking Process and Public Comment
    Some commenters viewed the publication of the Notice as ignoring 
established rulemaking procedures. One commenter stated that the Notice 
circumvented any reasonable attempt to

[[Page 38016]]

work with the air tour industry and that it was a clear statement of 
the Park Service's intention to implement the new noise threshold 
immediately.
    Regarding public involvement, one commenter stated that the NPS has 
an obligation to explain and make available to the public the research 
behind the decision. The commenter continued to say that at the very 
least, the Notice itself should have offered to make this information 
available to commenters that may have wanted to review it. As 
previously noted, another commenter suggested that the new noise 
standard departs from the previously settled definition of 
``substantial restoration of natural quiet'' and that the Notice amends 
that key statutory definition without opportunity for notice or 
comment. Further, the commenter submitted that NPS actions require 
notice and comment rulemaking and reiterated that NPS' own regulations 
require that actions of a ``* * * highly controversial nature, shall be 
published as rulemaking in the Federal Register.''
    NPS Response: This Notice falls under the interpretive rule and 
policy statement provisions of the Administrative Procedure Act, 5 
U.S.C. 552(a)(D) and is a statement of agency policy. Although not 
required, the NPS is following the more rigorous notice and comment 
procedures under 5 U.S.C. 553 to encourage public participation in this 
Notice. The Notice is an attempt to notify concerned publics that the 
NPS is refining its methodologies for assessing aircraft noise impacts, 
an internal policy decision. As explained previously, the Notice does 
not change the definition of substantial restoration of natural quiet. 
The NPS has in fact provided notice and the opportunity for public 
comment, and has considered such comments in this Disposition of Public 
Comments. As mentioned previously, the effects of these thresholds will 
be readily apparent in the noise modeling conducted for the 
supplemental environmental assessment accompanying FAA rulemaking 
actions that will soon be available for public review. The NPS embraces 
an adaptive management approach and welcomes any new comments or 
criticism regarding this methodology.
21. Effect of Proposal on Air Tour Industry
    Many commenters from the air tour industry felt that the action was 
an aggressive move against air tour businesses and that it was not in 
keeping with the assurances offered by the NPS that it supported the 
air tour industry at Grand Canyon. One commenter said that implicit in 
this Notice was that air tours were unwelcome anywhere over the Grand 
Canyon. Another commented that the Notice was evidence that the public 
statements made by NPS/DOI in support of the air tour industry were not 
true. One commenter called the Notice an act of bad faith.
    Many commenters were also concerned that the proposed noise 
thresholds would have substantial impacts on the viability of area air 
tour businesses. One commenter claimed that if the new noise standard 
was adopted, commercial air tours in the Grand Canyon would cease. 
Another commented that imposing these new standards would further 
restrict already limited aircraft operations and would jeopardize the 
existence of the areas air tour businesses. Another commenter asserted 
that this new benchmark would effectively ground every air tour 
aircraft in service at the Grand Canyon today.
    NPS Response: A refinement in the NPS approach to noise impact 
assessment may have indirect effects on the commercial air tour 
industry. As was previously mentioned, specific effects rely on the 
complex interplay of several elements (e.g., impact assessment 
thresholds, location of air tour routes, sensitive cultural sites, 
natural ambient levels, air tour operation levels). Different 
combinations of these elements may make it easier or harder to reach 
natural quiet in particular areas of the Park. For example, desert 
scrub areas have been recently reevaluated as having a natural ambient 
level of 20 dB (an increase of 5 dB, from 15dB). Thus, there would be 
less noise impacts on desert scrub areas in Zone One (noticeability 
threshold) than before (i.e., the impact threshold would be 5 dB higher 
than before). Another simple example: Air tour flights over coniferous 
forest (with a reassessed natural ambient of 31 dB, from 26 dB) in Zone 
Two may have a greater impact than before (i.e., the impact threshold 
would be 6 dB lower than before). However, substantial restoration of 
natural quiet is calculated for the Park as a whole, not by zones or 
other discrete areas.
    It should be noted that even if increased noise impacts are 
indicated, the decision regarding what should be done would be 
addressed through subsequent policymaking. Future effects on the 
industry is dependent upon other policy decisions that the FAA will 
make in cooperation with the NPS. The NPS policy that commercial air 
tours are a part of visitor services at GCNP remains unchanged. The NPS 
cannot make good management decisions on GCNP noise management and 
recommend these to the FAA unless it accurately assesses the impact of 
aircraft noise on GCNP. That is the purpose of the methodological 
refinements.
    The proposal is consistent with recommendations the NPS made in the 
1994 Report to Congress and with NPS policies that require the agency 
to accurately assess impacts in the process of determining management 
actions. However, there is a difference between accurately assessing 
impacts and deciding what to do about those impacts. The Public Notice 
addressed how impacts were to be assessed. It did not set management 
standards or actions. This change in methodology does not require 
aircraft to be quieter than the noise impact thresholds in order to fly 
over the Park. It does not restrict air traffic in any way. The 
thresholds are simply the point at which aircraft noise will begin to 
be measured against the mandated goal of substantial restoration. This 
action reflects the obligation NPS has to provide the FAA with the most 
accurate information possible about impacts on park resources and with 
recommendations on how to best achieve substantial restoration of 
natural quiet. Impact assessment is only one of many factors considered 
in policymaking. The FAA, in cooperation with the NPS, will propose and 
implement management actions designed to achieve the mandated goal of 
substantial restoration of natural quiet and allow a safe air tour 
industry to operate within the context of that legislative mandate. 
Application of the methodological refinements may, in conjunction with 
FAA rulemaking, require reductions in operations, changes to routes or 
other measures to achieve the statutory goal.
22. Need for Economic Analysis
    A few commenters pointed out the need for the NPS to consider 
potential economic impacts on air tour operators as a result of the 
action. One commenter stated that the NPS is required by the 
Overflights Act and the Small Business Regulatory Flexibility Act to 
consider the economic impacts of this action on the small business 
entities that comprise the air tour industry.
    NPS Response: As noted above in the NPS Response to Comment #21, 
Effect of Proposal on Air Tour Industry, the NPS is providing notice of 
a policy related to refinements in its noise impact assessment. The 
Notice, in and of itself, does not prescribe mitigation or management 
actions that lead to impacts on air tour operators. Neither the 
Overflights Act, nor the Regulatory Flexibility Act require an economic 
impact analysis for this Notice.

[[Page 38017]]

Economic impacts on the small business entities that comprise the air 
tour industry will be conducted in any FAA rulemaking actions employing 
the methodology in this Notice.
23. FAA Authority and Role
    Several commenters asserted that through this and other actions the 
NPS was essentially usurping the FAA's authority to make decisions 
regarding the use of air space. Commenters contended that by 
establishing a noise threshold which prohibits aircraft from accessing 
certain air space the NPS was exercising de facto control over that air 
space, an authority provided solely to the FAA by Congress. They stated 
that land management agencies, including the NPS, do not have the 
jurisdiction, mandate, or expertise needed to safely regulate the use 
of our nation's air space. Commenters emphasized that the FAA must 
remain the leader in dealing with all air space issues.
    NPS Response: Federal law and Congressional policy mandate that the 
authority to control the use of our nation's airspace resides solely 
with the FAA, while the NPS is charged with the management of the 
natural and cultural resources and values associated with units of the 
National Park System. Part of the NPS management responsibility is to 
determine the nature and extent of impacts on parks from all uses of 
the parks. The noise thresholds proposed in the Public Notice are 
strictly related to NPS noise impact assessment; they are not directly 
related to FAA's management authority of the airspace.
    The FAA retains its full authority to manage the air space, and in 
cooperation with the NPS, will continue to consider how to best address 
these impacts through its rulemaking process. The FAA evaluates all NPS 
proposals related to GCNP noise management and mitigation, rejecting 
any that it considers unsafe. The NPS believes that through continued 
cooperation with the FAA, alternatives that are both safe and sensitive 
to noise impacts can be developed and implemented.
24. Safety
    One commenter was concerned that the action would effectively 
restrict where operators could fly, resulting in increased congestion 
and potential adverse impacts on safety. Another commenter expressed 
that safety should not be forsaken in an effort to reduce sound and 
that if the NPS changes route structures that result in additional 
safety risks, they should be prepared to accept responsibility.
    NPS Response: Safety issues are of paramount importance to both the 
FAA and the NPS. The NPS always defers to FAA on all issues associated 
with aviation safety. Rulemaking for GCNP can only proceed if FAA 
concludes it to be safe. (See NPS Response to Comment #23, FAA 
Authority and Role.)
25. Terminology Used in the Notice
    Commenters questioned the use of various terms throughout the 
Public Notice. One commenter suggested that a ``day'' be defined 
universally as 24 hours. Another commenter requested consistency in the 
use of terms to avoid confusion, e.g., what was the difference, if any, 
between the terms ``quiet to be preserved'' (as stated in the 1994 
Report to Congress) and ``average natural ambient'' as stated in this 
Notice. Finally, a few commenters claimed that the action was not a 
``refinement'' of the current methodology, but that it was a departure 
from ``noticeability'' to ``detectability.''
    NPS Response: In its definition of substantial restoration, the 
Report to Congress carefully used the phrase ``of the day'' rather than 
``of the time'' (an earlier version) to distinguish that it referred to 
the daylight hours when air tours were flying. For modeling purposes, 
this refers to the 12 hour time period from 7 AM to 7 PM. Air tour 
aircraft do not operate at night, so that is how the definition was 
framed. To change that to a 24 hour period would require a redefinition 
of ``substantial restoration of natural quiet.''
    In the 1994 Report to Congress ``quiet to be preserved'' refers to 
the lower end of the ambient sound level range that occurs regularly 
between wind gusts, animal sounds, etc., not just the average sound 
level (pg. 83). The ``average natural ambient'' in the Notice refers to 
the average background sound level for a given location, less any 
mechanical noise from aircraft or other sources. Again, the definitions 
are not changed by this action.
    As has been mentioned previously, these changes are to bring 
current impact assessment methods more in line with the goal of 
substantial restoration, which is based on audibility.
26. Refining the Definition of Substantial Restoration of Natural Quiet
    Several commenters suggested that the current definition of 
substantial restoration of natural quiet was not adequate to protect 
GCNP resources.
    NPS Response: Though appreciated, these comments do not directly 
apply to the refinement of noise impact assessment methodology 
presented in the Public Notice. The NPS does not currently have any 
plans to redefine the substantial restoration of natural quiet.
27. Restrictions at Higher Altitudes
    One commenter expressed concern that though the current action is 
focused on air tour operations, noise thresholds could ultimately lead 
to restrictions at higher altitudes.
    NPS Response: No such restrictions are being contemplated. In any 
case, the GCNP SFRA has an effective ceiling of 17,999 feet MSL.
28. Proposed Wilderness
    One commenter suggested that any future actions should speak to the 
possible enactment of a wilderness area at the Park.
    NPS Response: This point is an important one and will be addressed 
in the development of the comprehensive noise management plan, in light 
of the Park's General Management Plan and Wilderness and Colorado River 
Management Plan.
29. Future Studies
    A few commenters indicated that future acoustical studies at the 
Park should use defensible methods and employ peer review to ensure 
scientific validity.
    NPS Response: The NPS concurs. The generation of solid, reliable 
scientific data is essential to inform sound resource management 
decisions.

Summary of the Proposal

    In summary, the proposal is to refine the current noise impact 
assessment methodology to incorporate a two-zone geographic system with 
different noise thresholds applicable to the circumstances of each of 
the two zones. Zone One would be composed of (1) the developed areas of 
GCNP as generally identified in GCNP's 1995 General Management Plan 
(except Tuweep, Phantom Ranch, and the North Rim paved roads), 
encompassing, on the South Rim, the area from approximately Desert View 
to Hermits Rest, and, on the North Rim, the developed area on Bright 
Angel Point; (2) the area of the Park west of Whitmore Rapids, 
including the Sanup Flight-free Zone; and (3) the Marble Canyon Sector. 
Zone One comprises approximately one-third of the area of GCNP.
    Zone Two would encompass, in a large contiguous area in the center 
of GCNP, approximately the remaining two-thirds of the Park's area .
    Under this proposal, the noise threshold for Zone One is set at 3 
decibels above the average natural ambient A-weighted sound levels 
found to exist in those areas of the Park as determined by previous 
scientific acoustic measurement studies. This is the same as the single 
standard used in

[[Page 38018]]

previous assessments (i.e., noticeability). The threshold for Zone Two 
will be set at 8 decibels below the average natural ambient A-weighted 
sound levels (i.e., audibility). The noise thresholds will be used in 
noise modeling for impact assessment to determine the onset of impact 
to natural quiet at GCNP.

Conclusion

    As discussed above, the National Park Service has carefully 
considered and responded to the comments received on the Public Notice 
published in the Federal Register on January 26, 1999 (64 FR 3969-
3972), concerning a change in the noise impact assessment methodology 
for air tour operations over Grand Canyon National Park. Based on this 
consideration, and the additional data and experience explained above 
which led to the Public Notice, the NPS decision is to adopt the 
proposed noise impact assessment methodology, and to request the FAA to 
use it for current rulemaking related to GCNP air tour overflight 
management and mitigation.
    Future application of these refinements of the impact assessment 
methodology in FAA rulemaking measures are likely to make more 
challenging the agencies' efforts to achieve the substantial 
restoration of natural quiet. However, the use of the two noise 
thresholds and two geographic zones will better achieve the 
preservation of the GCNP resources and visitor experiences the NPS is 
charged to protect, and be more in line with the definition of 
substantial restoration of natural quiet.
Robert Stanton,
Director.

References

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Human Observers. Los Altos, CA: Peninsula Publishing Co.
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the Effectiveness of SFAR 50-2 in Restoring Natural Quiet to Grand 
Canyon National Park. BBN Report 7197, NPOA Report No. 93-1. p. 55.
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with Detectability, Memorandum to W. R. Henry, HMMH Job No. 
294530.22, May 15, 1997.
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the Effectiveness of SFAR 50-2 in Restoring Natural Quiet to Grand 
Canyon National Park. BBN Report 7197, NPOA Report No. 93-1. 
National Park Service, Denver Service Center.
    \11\ Miller, N.P., (1997) A-weighted Level Differences Compared 
with Detectability, Memorandum to W. R. Henry, HMMH Job No. 
294530.22, May 15, 1997.
    \12\ Small, A.M. and Tyler, R.S. (1978), Additive Masking 
Effects of Noise Bands of Different Levels. Journal of the 
Acoustical Society of America, 34: 108-113.
    \13\ Fidel, S., Pearsons, K., Sneddon, M. (1994) Evaluation of 
the Effectiveness of SFAR 50-2 in Restoring Natural Quiet to Grand 
Canyon National Park. BBN Report 7197, NPOA Report No. 93-1. 
National Park Service, Denver Service Center.
    \14\ Miller, N.P., (1997) A-weighted Level Differences Compared 
with Detectability, Memorandum to W. R. Henry, HMMH Job No. 
294530.22, May 15, 1997.
    \15\ National Park Service, (1995) Report on Effects of Aircraft 
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1995, pg. 182.
    \17\ Miller, N.P., (1997) A-weighted Level Differences Compared 
with Detectability, Memorandum to W. R. Henry, HMMH Job No. 
294530.22, May 15, 1997.

[FR Doc. 99-17907 Filed 7-9-99; 2:30 pm]
BILLING CODE 4310-70-P