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

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

\1\ Miller, N.P., (1999) Addendum: Grand Canyon Ambient

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February 5, 1999.

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Swets, J.A. (1964,1988). Signal Detection and Recognition by

Human Observers. Los Altos, CA: Peninsula Publishing Co.

\3\ Green, D.M., and Swets, J.A. (1966, 1988). Signal Detection

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\6\ 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.

\7\ 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. p. 55.

\8\ Miller, N.P., (1997) A-weighted Level Differences Compared

with Detectability, Memorandum to W. R. Henry, HMMH Job No.

294530.22, May 15, 1997.

\9\ Fidell, S., (1978). Effectiveness of Audible Warning Signals

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\10\ 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.

\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. 84.

\16\ National Park Service, (1995) Report on Effects of Aircraft

Overflights on the National Park System, Report to Congress, July

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