810 

14 CFR Ch. I (1–1–19 Edition) 

Pt. 36, App. F 

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 air-
plane when the noise is recorded at the re-
quired measuring point. 

(6) The flight test procedures, measuring 

equipment, and noise measurement proce-
dures must be approved by the FAA. 

(7) Sound pressure level data for noise eval-

uation 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 mount-

ings that minimize interference with the 
sound being measured. 

(c) Recording and reproducing equipment 

characteristics, frequency response, and dy-
namic 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. 

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 § 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 fre-
quency pre-emphasis must be added to the 
recording channel with the converse de-em-
phasis on playback. The pre-emphasis must 
be applied such that the instantaneous re-
corded sound pressure level of the noise sig-
nal 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 Publi-
cation No. 179, dated 1973. The output signal 
from the filter must be fed to a rectifying 
circuit with square law rectification, inte-
grated with time constants for charge and 
discharge of about 1 second or 800 milli-
seconds. 

(f) The equipment must be acoustically 

calibrated using facilities for acoustic 
freefield calibration and if analysis of the 
tape recording is requested by the Adminis-
trator, the analysis equipment shall be elec-
tronically calibrated by a method approved 
by the FAA. 

(g) A windscreen must be employed with 

microphone during all measurements of air-
craft 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 cali-
brated. 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 sys-
tem gain set at levels that will be used for 
aircraft noise measurements. If aircraft 
sound pressure levels do not exceed the back-
ground sound pressure levels by at least 10 
dB(A), approved corrections for the contribu-
tion 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 measure-

ments or corrections to measured data must 
be recorded in permanent form and appended 
to the record except that corrections to 
measurements for normal equipment re-
sponse deviations need not be reported. All 
other corrections must be approved. Esti-
mates 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 meas-

urement and analysis of all acoustic, air-
plane performance, and meteorological data 
must be reported. 

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