[Federal Register Volume 68, Number 216 (Friday, November 7, 2003)]
[Notices]
[Pages 63131-63132]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 03-28114]



[[Page 63131]]

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DEPARTMENT OF THE INTERIOR

National Park Service


Notice of Recommendation From the Aircraft Noise Model Validation 
Study

AGENCY: National Park Service, Department of Interior.

ACTION: Notice.

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SUMMARY: Following the recommendation in the recently issued National 
Park Service Aircraft Noise Model Validation Study, released January 
23, 2003, this announcement provides notice that the NOISEMAP 
Simulation Model is the model of choice for calculating aircraft 
audibility at Grand Canyon National Park and other National Park 
Service units.

ADDRESSES: Copies of the National Park Service Aircraft Noise Model 
Validation Study report are available on computer discs (CDs) and may 
be requested from Grand Canyon National Park, or viewed on the Grand 
Canyon National Park Webpage at http://www.nps.gov/grca/overflights/index.htm.

FOR FURTHER INFORMATION CONTACT: Ken McMullen, Overflights and Natural 
Soundscape Program Manager, National Park Service, Grand Canyon 
National Park, 823 N San Francisco Street, Flagstaff, Arizona 86001. 
Telephone 928-779-2095; or by e-mail at [email protected].

SUPPLEMENTARY INFORMATION:

Background

    Public Law 100-91 (1987) tasked the National Park Service (NPS) and 
the Federal Aviation Administration (FAA) with developing a plan for 
aircraft use of Grand Canyon airspace that will succeed in 
``substantially restoring the natural quiet in the park''. In its 
``Report on Effects of Aircraft Overflights on the National Park 
System'' (1995), the NPS defined substantial restoration of natural 
quiet as occurring when ``50% or more of the park achieve[s] `natural 
quiet' (i.e., no aircraft audible) for 75-100 percent of the day''. 
Computer modeling was determined to be the most practical method to 
assess whether or not natural quiet had been substantially restored at 
Grand Canyon National Park.

Model Validation Study

    Although models that compute when aircraft are audible over large 
land areas have not been widely used, two models have been employed to 
calculate the percent of time aircraft are audible at Grand Canyon 
National Park. The National Park Service Overflight Decision Support 
System model (NODSS) was developed for the NPS to calculate aircraft-
produced noise in backcountry settings; NODSS was designed to account 
for park terrain features, its calculations are based on one-third 
octave band acoustic spectra information, and it calculates audibility 
directly. The results from NODSS have been used by the NPS to calculate 
the percent of substantial restoration of natural quiet achieved by 
various airspace and operations alternatives at Grand Canyon National 
Park. A second model, the Integrated Noise Model (INM), version 5.1, is 
the FAA-developed, aircraft noise computation model used 
internationally to calculate aircraft-produced noise in airport 
environments. INM bases its computations on A-weighted aircraft sound 
levels and accounts for differences in site elevation but does not 
account for shielding due to terrain. Results from INM have been 
presented in environmental assessments associated with FAA draft and 
final rules on Grand Canyon National Park airspace regulations. The two 
models, using Grand Canyon operations data but based on different 
metrics, produced somewhat different results. To comply with the 
National Environmental Protection Act's requirement to use ``the best 
available science'', a model validation study was designed to compare 
computer model results with measurements made on-site at the Grand 
Canyon. A third model, NOISEMAP Simulation Model (NMSIM) developed by 
Wyle Laboratories, the U.S. Air Force, and the National Aeronautics and 
Space Administration, was included in this study as was a second 
version of INM (Research Version). NMSIM, like NODSS, uses spectral 
information, accounts for park terrain, and computes aircraft 
audibility. In addition to these capabilities, NMSIM also simulates 
aircraft flying in the time sequence in which they occurred and 
includes the directivity of each aircraft type. The Research Version of 
INM uses spectral, rather than A-weighted, information but is in other 
major ways similar to INM.
    The goal of the study was to: ``Determine the degrees of accuracy 
and precision that existing computer models provide, in comparison with 
field measurement, in the calculation of the percent of time tour 
aircraft are audible in the Canyon. * * *'' In this study, determining 
``accuracy and precision'' is termed ``validation''.
    The NPS Aircraft Noise Model Validation Study was designed through 
a cooperative process involving the NPS, the FAA, the Volpe National 
Transportation Systems Center, Wyle Laboratories, and Harris Miller 
Miller & Hansen Inc. After a draft research approach had been 
developed, a Technical Review Committee (TRC) consisting of 
internationally recognized experts reviewed and commented on the plan. 
Suggestions made by TRC members were incorporated into the study 
design. As results were produced the full team, including TRC members, 
were involved in review and comment. The full team has reviewed and 
commented on drafts of the study report. Their comments were 
incorporated extensively.
    Acoustic data for the NPS Aircraft Noise Model Validation Study 
were collected from some 39 sites at Grand Canyon over a four-day 
period in September 1999. The collected data were reduced to provide 
hourly information for modeling tour aircraft audibility and sound 
levels for each hour of operations, and for then analyzing the results. 
Each of the four models (NODSS, INM in two versions, and NMSIM) were 
exercised with the same set of input data. The models were run to 
produce for each site the hourly values of both the percent of time 
tour aircraft were audible and the tour aircraft hourly equivalent 
sound level, Leq. These values were then compared directly 
with measured values, site-by-site, hour-by-hour.
    In August, 2002, as the NPS Aircraft Noise Model Validation Study 
report was nearing completion, the United States Court of Appeals, 
District of Columbia Circuit ((in United States Air Tour Association v. 
Federal Aviation Administration (Grand Canyon Trust, Intervenors)) 
declared that the FAA's practice of including only air tour aircraft-
produced noise in the calculation of substantial restoration of natural 
quiet at GCNP should be remanded back to that agency for 
reconsideration. The Court indicated that noise from all aircraft 
overflying the park should be included in the noise calculations. 
Although the NPS Aircraft Noise Model Validation Study was based on 
data from tour aircraft conducting operations over Grand Canyon 
National Park, the inclusion of noise from other aircraft sources will 
not invalidate the results of this study. Similarly, as the models 
respond to the principles of acoustics and physics, the results of the 
NPS Aircraft Noise Model Validation Study are applicable to other 
National Park units.

Model Validation Study Recommendation

    The study concluded, ``We consider NMSIM to be the model most 
suited for immediate use in computing percent of

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the time tour aircraft are audible'' (p. 131).

Conclusion

    Audibility is a fundamental component in the definition and 
measurement of natural quiet and natural sounds at Grand Canyon 
National Park and other NPS units. The NPS Aircraft Noise Model 
Validation study found NMSIM to be the model best suited for computing 
audibility. Further, the National Environmental Protection Act's 
requirement for the use of the ``best available science'' is met with 
the selection of NMSIM. Therefore, the NPS announces that the NOISEMAP 
Simulation Model (NMSIM) is the model of choice for calculating 
aircraft audibility at Grand Canyon National Park and other NPS units.

    Dated: June 3, 2003.
Michael D. Sunder,
Acting Regional Director, Intermountain Region.
[FR Doc. 03-28114 Filed 11-6-03; 8:45 am]
BILLING CODE 4312-ED-P