778 

14 CFR Ch. I (1–1–19 Edition) 

Pt. 36, App. A 

to compute equivalent weighted sound at-
tenuations in each one-third octave band; 
the FAA will determine whether a sufficient 
number of layered sections have been used. 
For each measurement, where multiple 
layering is not required, equivalent sound at-
tenuations in each one-third octave band 
must be determined by averaging the atmos-
pheric absorption coefficients for each such 
band at 33 ft (10 m) above ground level, and 
at the flight level of the airplane at the time 
of PNLTM, for each measurement; 

(e) Average wind velocity 33 ft (10 m) above 

ground may not exceed 12 knots and the 
crosswind velocity for the airplane may not 
exceed 7 knots. The average wind velocity 
must be determined using a 30-second aver-
aging period spanning the 10 dB-down time 
interval. Maximum wind velocity 33 ft (10 m) 
above ground is not to exceed 15 knots and 
the crosswind velocity is not to exceed 10 
knots during the 10 dB-down time interval; 

(f) No anomalous meteorological or wind 

conditions that would significantly affect 
the measured noise levels when the noise is 
recorded at the measuring points specified 
by the FAA; and 

(g) Meteorological measurements must be 

obtained within 30 minutes of each noise test 
measurement; meteorological data must be 
interpolated to actual times of each noise 
measurement. 

A36.2.2.3 When a multiple layering calcula-

tion is required by section A36.2.2.2(c) or 
A36.2.2.2(d) the atmosphere between the air-
plane and 33 ft (10 m) above the ground must 
be divided into layers of equal depth. The 
depth of the layers must be set to not more 
than the depth of the narrowest layer across 
which the variation in the atmospheric ab-
sorption coefficient of the 3150 Hz one-third 
octave band is not greater than 

±

1.6 dB/1000 

ft (

±

0.5 dB/100m), with a minimum layer 

depth of 100 ft (30 m). This requirement must 
be met for the propagation path at PNLTM. 
The mean of the values of the atmospheric 
absorption coefficients at the top and bot-
tom of each layer may be used to charac-
terize the absorption properties of each 
layer. 

A36.2.2.4 The airport control tower or an-

other facility must be aproved by the FAA 
for use as the central location at which 
measurements of atmospheric parameters 
are representative of those conditions exist-
ing over the geographical area in which noise 
measurements are made. 

A36.2.3 

Flight path measurement. 

A36.2.3.1 The airplane height and lateral 

position relative to the flight track must be 
determined by a method independent of nor-
mal flight instrumentation such as radar 
tracking, theodolite triangulation, or photo-
graphic scaling techniques, to be approved 
by the FAA. 

A36.2.3.2 The airplane position along the 

flight path must be related to the noise re-

corded at the noise measurement locations 
by means of synchronizing signals over a dis-
tance sufficient to assure adequate data dur-
ing the period that the noise is within 10 dB 
of the maximum value of PNLT. 

A36.2.3.3 Position and performance data re-

quired to make the adjustments referred to 
in section A36.9 of this appendix must be 
automatically recorded at an approved sam-
pling rate. Measuring equipment must be ap-
proved by the FAA. 

Section A36.3 Measurement of Airplane Noise 

Received on the Ground 

A36.3.1 

Definitions. 

For the purposes of section A36.3 the fol-

lowing definitions apply: 

A36.3.1.1 

Measurement system 

means the 

combination of instruments used for the 
measurement of sound pressure levels, in-
cluding a sound calibrator, windscreen, 
microphone system, signal recording and 
conditioning devices, and one-third octave 
band analysis system. 

N

OTE

: Practical installations may include 

a number of microphone systems, the out-
puts from which are recorded simultaneously 
by a multi-channel recording/analysis device 
via signal conditioners, as appropriate. For 
the purpose of this section, each complete 
measurement channel is considered to be a 
measurement system to which the require-
ments apply accordingly. 

A36.3.1.2 

Microphone system 

means the com-

ponents of the measurement system which 
produce an electrical output signal in re-
sponse to a sound pressure input signal, and 
which generally include a microphone, a pre-
amplifier, extension cables, and other de-
vices as necessary. 

A36.3.1.3 

Sound incidence angle 

means in de-

grees, an angle between the principal axis of 
the microphone, as defined in IEC 61094–3 and 
IEC 61094–4, as amended and a line from the 
sound source to the center of the diaphragm 
of the microphone (incorporated by ref-
erence, see § 36.6). 

N

OTE

: When the sound incidence angle is 

0

°

, the sound is said to be received at the 

microphone at ‘‘normal (perpendicular) inci-
dence;’’ when the sound incidence angle is 
90

°

, the sound is said to be received at ‘‘graz-

ing incidence.’’ 

A36.3.1.4 

Reference direction 

means, in de-

grees, the direction of sound incidence speci-
fied by the manufacturer of the microphone, 
relative to a sound incidence angle of 0

°

, for 

which the free-field sensitivity level of the 
microphone system is within specified toler-
ance limits. 

A36.3.1.5 

Free-field sensitivity of a micro-

phone system 

means, in volts per Pascal, for 

a sinusoidal plane progressive sound wave of 
specified frequency, at a specified sound inci-
dence angle, the quotient of the root mean 
square voltage at the output of a microphone 

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