Code of Federal Regulations - Title 14

922

14 CFR Ch. I (1–1–14 Edition)

Pt. 36, App. A

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.

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.

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.

A36.3.4.1 In the absence of wind and for si-

nusoidal sounds at grazing incidence, the in-
sertion loss caused by the windscreen of a
stated type installed around the microphone
must not exceed

1.5 dB at nominal one-third

octave midband frequencies from 50 Hz to 10
kHz inclusive.

A36.3.5

Microphone system.

A36.3.5.1 The microphone system must

meet the specifications in sections A36.3.5.2
to A36.3.5.4. Various microphone systems
may be approved by the FAA on the basis of
demonstrated equivalent overall
electroacoustical performance. Where two or
more microphone systems of the same type
are used, demonstration that at least one
system conforms to the specifications in full
is sufficient to demonstrate conformance.

OTE

: An applicant must still calibrate

and check each system as required in section
A36.3.9.

A36.3.5.2 The microphone must be mounted

with the sensing element 4 ft (1.2 m) above
the local ground surface and must be ori-
ented for grazing incidence,

i.e., with the

sensing element substantially in the plane
defined by the predicted reference flight path
of the aircraft and the measuring station.
The microphone mounting arrangement
must minimize the interference of the sup-
ports with the sound to be measured. Figure
A36–1 illustrates sound incidence angles on a
microphone.

A36.3.5.3 The free-field sensitivity level of

the microphone and preamplifier in the ref-
erence direction, at frequencies over at least
the range of one-third-octave nominal
midband frequencies from 50 Hz to 5 kHz in-
clusive, must be within

1.0 dB of that at the

calibration check frequency, and within

2.0

dB for nominal midband frequencies of 6.3
kHz, 8 kHz and 10 kHz.

A36.3.5.4 For sinusoidal sound waves at

each one-third octave nominal midband fre-
quency over the range from 50 Hz to 10 kHz
inclusive, the free-field sensitivity levels of
the microphone system at sound incidence
angles of 30

, 60

, 90

, 120

and 150

, must not

differ from the free-field sensitivity level at
a sound incidence angle of 0

(‘‘normal inci-

dence’’) by more than the values shown in
Table A36–1. The free-field sensitivity level

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