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Noise calculations supporting the Seasonal Relief QT 

Incentive for Grand Canyon National Park

February 24, 2015

On November 10th, 2014 the Federal Aviation Administration (FAA) and the National Park Service (NPS)

published a Federal Register Notice (FR 79(217): 66763) proposing a Quiet Aircraft Technology (QT) 

incentive for commercial air tour operators at Grand Canyon National Park (GRCA) to meet the 

requirements of the Moving Ahead for Progress in the 21st Century Act (MAP-21, P.L 112-141),

consistent with section 804(c) of the National Parks Air Tour Management Act of 2000 (NPATMA. P.L. 

106-181). To meet the statutory conditions in MAP-21 and NPATMA, the FAA and the NPS must ensure 

that the cumulative impact of QT operations relieved from allocations in accordance with the seasonal 

relief QT incentive does not increase noise at the park.

For the proposed seasonal relief incentive, the annual noise from both QT and non-QT commercial air 

tour flights conducted in the Dragon and Zuni Point corridors must not exceed the annual noise level of 

commercial air tour flights in these corridors under the current allocation system. The FAA and the NPS 

have modeled the annual noise of commercial air tour allocations in the Dragon and Zuni Point corridors 

as flown with the 2012 commercial air tour fleet mix and route structure—resulting in a noise baseline 

of LEQ12 58.1 decibels (dB).

1

 Noise will be determined to increase if the annual modeled Leq12 of 

commercial air tour operations conducted in the Dragon and Zuni Point corridors exceeds LEQ12 58.1 dB 

in the park. This document provides an expanded technical description of the methodology and 

calculations that resulted in the LEQ12 58.1 dB reference criterion.

The FAA and NPS agreed that the cumulative noise impact should be evaluated in terms of the sum of

commercial air tour noise energy2occurring inside park boundaries over an entire year. This summation 

is intended to produce a reference criterion for judging changes to the noise environment based on the 

cumulative impact of operations. This reference criterion is not intended to represent a value which 

may be experienced by visitors to GRCA on any particular day.

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To calculate commercial air tour noise energy occurring inside park boundaries, the FAA and NPS used a 

grid of 1224 points with 2 km spacing across the park. The agencies determined that the best way to 

ensure that the seasonal relief incentive for QT operations in the Dragon and Zuni Corridors (DZ) does 

not increase noise in the park is to compute noise at all grid points at the park, not just at grid points in 

the DZ. Technical experts from both agencies conferred on a methodology to work with computed 

noise values at 1224 grid points across the park to determine whether the cumulative impact increases

noise. They determined that a single number would provide the most straightforward means of 

measuring whether there is an increase in noise. This number is calculated by summing the noise 

energy calculated for each year’s air tour data at each of the individual grid points and averaging it over 

 

1

LEQ12 stands for Equivalent Sound Level for 12 hours, which is a cumulative measure of the noise exposure of Aweighted sound levels over a 12-hour period. LEQ12 is one of several metrics used to evaluate air tour noise in 

GRCA and other national parks. A 12-hour LEQ is used since air tour operations occur during the day, rather than 

over a 24-hour period. ANSI standard symbols would be LA,12h or LAeq,12h (ANSI Standard S1.1-1994, Acoustical 

Terminology, section 4.14).

2

In this document, “noise energy” refers to sums of squared, A-weighted sound pressure values divided by the 

square of the reference sound pressure level of 20 μPa.

3

Cumulative noise impact is not the same criterion as the substantial restoration of natural quiet (SRNQ). SRNQ is 

calculated and determined on the peak day of air tour operations using the percent time audible metric.
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the whole park. This number, calculated annually and averaged over the park, results in a single LEQ12

value. 

The Integrated Noise Model Version 6.2 (INM 6.2)4 was used to quantify the amount of noise energy 

produced by a single flight of each type of aircraft on each air tour route used in 2012. A program 

written in the R statistical environment5 was used to multiply the INM model results by the amount of 

annual traffic allocated on each aircraft-route combination, to sum up the noise energy from DZ 

operations across the entire park. The spatial summation was based on noise levels modeled on a grid 

of the 1224 locations with 2 km spacing throughout the park. The 44,901 annual DZ allocations6 were 

apportioned to the DZ corridor according to the reported 2012 fleet mix and route distribution by 

operator. In other words, the allocated numbers were used for each of the operators using the 2012 

reported fleet mix to distribute across the routes.7

The first step in calculating the LEQ12 was to sum the noise energy from all annual DZ aircraft operations 

spatially, across points throughout the park. This energy sum was converted to decibels (taking the base 

10 logarithm of summed noise energy and multiplying by 10), resulting in a sound exposure level (LAE) 

that compressed all the noise in the entire park from a year of DZ air tours into one second of exposure. 

For DZ allocations, this LAE equaled 135.4 dB.

This spatial summation of the cumulative energy produced by the total annual DZ allocations was 

converted to the LEQ12 by subtracting two values to produce an average noise value across a 12 hour 

day. The first value divided the noise spatially across all 1224 modeled points in the park:

10*log10(1224) = 30.9 dB. The second value averaged the noise across a 12 hour day: 

10*log10(12*3600) = 46.4 dB. The resulting final LEQ12 value was 135.4 - 30.9 - 46.4 = 58.1 dB. This 

LEQ12 value is the average noise level across the entire park if a year’s worth of DZ flight allocations took 

place in twelve hours on a single day.

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In order to accurately and reliably ensure compliance with the statutory mandate that the cumulative 

impact of operations under this QT incentive not increase noise at the park , the same calculation 

described above will be applied to each year’s air tour data. The technical choice of a particular set of 

scaling factors does not affect the outcome since all future annual calculations will be conducted with 

the same procedures used to generate the LEQ12 value to compare with the reference criterion of 58.1 

dB.

NPATMA also provides that QT incentives must not diminish achievement of substantial restoration of 

natural quiet (SRNQ). In addition to the calculation and determination regarding noise increase 

 

4

In February 2005, the Federal Interagency Committee on Aviation Noise (FICAN) recommended INM 6.2 as the 

best practice modeling methodology currently available to evaluate aircraft noise in national parks. FICAN 

members include FAA, NPS, NASA, DOD. INM is used to evaluate aircraft noise in GRCA and other national parks.

5

R Development Core Team (2012). R: A language and environment for statistical computing. R Foundation for 

Statistical Computing, Vienna, Austria. ISBN 3-900051-07-0, URL http://www.R-project.org/.

6

An air tour operator may conduct the number of air tours annually that equate to the number of annual 

allocations held by the operator. The proposed QT incentive would allow certain specified seasonal relief from 

annual allocations.

7

Note that 40,971 of the 44,901 DZ allocations were flown in 2012, so this calculation resulted in a noise level that 

was 10*log10(44901/44971) = 0.4 dB greater than was generated by commercial air tours flown in DZ in 2012.

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This LEQ12 value is not an average day noise level. To produce a noise level representing an average day at an 

average location in the park, the LEQ12 58.1 dB value would be further reduced according to the number of days in 

the year. Since 2012 was a leap year, the calculation would be: 10*log10(366) = 25.6 dB, yielding an LEQ12 average 

day of 581. – 25.6 = 32.5 dB.
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described in this document, the NPS and the FAA will ensure that SRNQ is not diminished in 

conformance with the definition of SRNQ as modified by MAP-21, based on the peak day of air tour 

operations and using the percent time audible metric.