779 

Federal Aviation Administration, DOT 

Pt. 36, App. A 

system and the root mean square sound pres-
sure that would exist at the position of the 
microphone in its absence. 

A36.3.1.6 

Free-field sensitivity level of a micro-

phone system 

means, in decibels, twenty 

times the logarithm to the base ten of the 
ratio of the free-field sensitivity of a micro-
phone system and the reference sensitivity 
of one volt per Pascal. 

N

OTE

: The free-field sensitivity level of a 

microphone system may be determined by 
subtracting the sound pressure level (in deci-
bels re 20 

µ

Pa) of the sound incident on the 

microphone from the voltage level (in deci-
bels re 1 V) at the output of the microphone 
system, and adding 93.98 dB to the result. 

A36.3.1.7 

Time-average band sound pressure 

level 

means in decibels, ten times the loga-

rithm to the base ten, of the ratio of the 
time mean square of the instantaneous 
sound pressure during a stated time interval 
and in a specified one-third octave band, to 
the square of the reference sound pressure of 
20 

µ

Pa. 

A36.3.1.8 

Level range 

means, in decibels, an 

operating range determined by the setting of 
the controls that are provided in a measure-
ment system for the recording and one-third 
octave band analysis of a sound pressure sig-
nal. The upper boundary associated with any 
particular level range must be rounded to 
the nearest decibel. 

A36.3.1.9 

Calibration sound pressure level 

means, in decibels, the sound pressure level 
produced, under reference environmental 
conditions, in the cavity of the coupler of 
the sound calibrator that is used to deter-
mine the overall acoustical sensitivity of a 
measurement system. 

A36.3.1.10 

Reference level range 

means, in 

decibels, the level range for determining the 
acoustical sensitivity of the measurement 
system and containing the calibration sound 
pressure level. 

A36.3.1.11 

Calibration check frequency 

means, in hertz, the nominal frequency of 
the sinusoidal sound pressure signal pro-
duced by the sound calibrator. 

A36.3.1.12 

Level difference 

means, in deci-

bels, for any nominal one-third octave 
midband frequency, the output signal level 
measured on any level range minus the level 
of the corresponding electrical input signal. 

A36.3.1.13 

Reference level difference 

means, 

in decibels, for a stated frequency, the level 
difference measured on a level range for an 
electrical input signal corresponding to the 
calibration sound pressure level, adjusted as 
appropriate, for the level range. 

A36.3.1.14 

Level non-linearity 

means, in deci-

bels, the level difference measured on any 
level range, at a stated one-third octave 
nominal midband frequency, minus the cor-
responding reference level difference, all 
input and output signals being relative to 
the same reference quantity. 

A36.3.1.15 

Linear operating range 

means, in 

decibels, for a stated level range and fre-
quency, the range of levels of steady sinus-
oidal electrical signals applied to the input 
of the entire measurement system, exclusive 
of the microphone but including the micro-
phone preamplifier and any other signal-con-
ditioning elements that are considered to be 
part of the microphone system, extending 
from a lower to an upper boundary, over 
which the level non-linearity is within speci-
fied tolerance limits. 

N

OTE

: Microphone extension cables as con-

figured in the field need not be included for 
the linear operating range determination. 

A36.3.1.16 

Windscreen insertion loss 

means, 

in decibels, at a stated nominal one-third oc-
tave midband frequency, and for a stated 
sound incidence angle on the inserted micro-
phone, the indicated sound pressure level 
without the windscreen installed around the 
microphone minus the sound pressure level 
with the windscreen installed. 

A36.3.2 

Reference environmental conditions. 

A36.3.2.1 The reference environmental con-

ditions for specifying the performance of a 
measurement system are: 

(a) Air temperature 73.4 

°

F (23 

°

C); 

(b) Static air pressure 101.325 kPa; and 
(c) Relative humidity 50%. 
A36.3.3. 

General. 

N

OTE

: Measurements of aircraft noise that 

are made using instruments that conform to 
the specifications of this section will yield 
one-third octave band sound pressure levels 
as a function of time. These one-third octave 
band levels are to be used for the calculation 
of effective perceived noise level as described 
in section A36.4. 

A36.3.3.1 The measurement system must 

consist of equipment approved by the FAA 
and equivalent to the following: 

(a) A windscreen (See A36.3.4.); 
(b) A microphone system (See A36.3.5): 
(c) A recording and reproducing system to 

store the measured aircraft noise signals for 
subsequent analysis (see A36.3.6); 

(d) A one-third octave band analysis sys-

tem (see A36.3.7); and 

(e) Calibration systems to maintain the 

acoustical sensitivity of the above systems 
within specified tolerance limits (see 
A36.3.8). 

A36.3.3.2. For any component of the meas-

urement system that converts an analog sig-
nal to digital form, such conversion must be 
performed so that the levels of any possible 
aliases or artifacts of the digitization proc-
ess will be less than the upper boundary of 
the linear operating range by at least 50 dB 
at any frequency less than 12.5 kHz. The 
sampling rate must be at least 28 kHz. An 
anti-aliasing filter must be included before 
the digitization process. 

A36.3.4 

Windscreen. 

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