Pt. 36, App. F

14 CFR Ch. I (1-1-19 Edition)

wind and head wind must be made in equal
numbers. If the measurement site is within 1
n.m. of an airport anemometer, the airport
reported wind may be used.
(5) There may be no temperature inversion
or anomalous wind conditions that would
significantly alter the noise level of the airplane when the noise is recorded at the required measuring point.
(6) The flight test procedures, measuring
equipment, and noise measurement procedures must be approved by the FAA.
(7) Sound pressure level data for noise evaluation purposes must be obtained with
acoustical equipment that complies with
section F36.103 of this appendix.
Sec. F36.103 Acoustical measurement system.
The acoustical measurement system must
consist of approved equipment equivalent to
the following:
(a) A microphone system with frequency
response compatible with measurement and
analysis system accuracy as prescribed in
section F36.105 of this appendix.
(b) Tripods or similar microphone mountings that minimize interference with the
sound being measured.
(c) Recording and reproducing equipment
characteristics, frequency response, and dynamic range compatible with the response
and accuracy requirements of section F36.105
of this appendix.
(d) Acoustic calibrators using sine wave or
broadband noise of known sound pressure
level. If broadband noise is used, the signal
must be described in terms of its average and
maximum root-mean-square (rms) value for
nonoverload signal level.

spaschal on DSK3GDR082PROD with CFR

Sec. F36.105 Sensing, recording, and
reproducing equipment.
(a) The noise produced by the airplane
must be recorded. A magnetic tape recorder
is acceptable.
(b) The characteristics of the system must
comply with the recommendations in IEC 179
(incorporated by reference, see Section 36.6).
(c) The response of the complete system to
a sensibly plane progressive sinusoidal wave
of constant amplitude must lie within the
tolerance limits specified in IEC Publication
No. 179, dated 1973, over the frequency range
45 to 11,200 Hz.
(d) If limitations of the dynamic range of
the equipment make it necessary, high frequency pre-emphasis must be added to the
recording channel with the converse de-emphasis on playback. The pre-emphasis must
be applied such that the instantaneous recorded sound pressure level of the noise signal between 800 and 11,200 Hz does not vary
more than 20 dB between the maximum and
minimum one-third octave bands.
(e) If requested by the Administrator, the
recorded noise signal must be read through
an  - A -  filter with dynamic characteristics

designated  - slow, -  as defined in IEC Publication No. 179, dated 1973. The output signal
from the filter must be fed to a rectifying
circuit with square law rectification, integrated with time constants for charge and
discharge of about 1 second or 800 milliseconds.
(f) The equipment must be acoustically
calibrated using facilities for acoustic
freefield calibration and if analysis of the
tape recording is requested by the Administrator, the analysis equipment shall be electronically calibrated by a method approved
by the FAA.
(g) A windscreen must be employed with
microphone during all measurements of aircraft noise when the wind speed is in excess
of 6 knots.
Sec. F36.107 Noise measurement procedures.
(a) The microphones must be oriented in a
known direction so that the maximum sound
received arrives as nearly as possible in the
direction for which the microphones are calibrated. The microphone sensing elements
must be approximately 4- above ground.
(b) Immediately prior to and after each
test; a recorded acoustic calibration of the
system must be made in the field with an
acoustic calibrator for the two purposes of
checking system sensitivity and providing
an acoustic reference level for the analysis
of the sound level data.
(c) The ambient noise, including both
acoustical background and electrical noise of
the measurement systems, must be recorded
and determined in the test area with the system gain set at levels that will be used for
aircraft noise measurements. If aircraft
sound pressure levels do not exceed the background sound pressure levels by at least 10
dB(A), approved corrections for the contribution of background sound pressure level to
the observed sound pressure level must be
applied.
Sec. F36.109 Data recording, reporting, and
approval.
(a) Data representing physical measurements or corrections to measured data must
be recorded in permanent form and appended
to the record except that corrections to
measurements for normal equipment response deviations need not be reported. All
other corrections must be approved. Estimates must be made of the individual errors
inherent in each of the operations employed
in obtaining the final data.
(b) Measured and corrected sound pressure
levels obtained with equipment conforming
to the specifications described in section
F36.105 of this appendix must be reported.
(c) The type of equipment used for measurement and analysis of all acoustic, airplane performance, and meteorological data
must be reported.

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