834 

14 CFR Ch. I (1–1–19 Edition) 

Pt. 36, App. J 

response and accuracy requirements of para-
graph (d) of this section; and 

(4) The calibration and checking of meas-

urement systems must use the procedures 
described in Section A36.3.9. 

(d) 

Sensing, recording, and reproducing 

equipment. 

(1) The noise levels measured from 

helicopter flyovers under this appendix may 
be determined directly by an integrating 
sound level meter, or the A-weighted sound 
level time history may be written onto a 
graphic level recorder set at ‘‘slow’’ response 
from which the SEL value may be deter-
mined. With the approval of the FAA, the 
noise signal may be tape recorded for subse-
quent analysis. 

(i) The SEL values from each flyover test 

may be directly determined from an inte-
grating sound level meter complying with 
the standards of IEC 804 (Incorporated by ref-
erence, see § 36.6) for a Type 1 instrument set 
at ‘‘slow’’ response. 

(ii) The acoustic signal from the heli-

copter, along with the calibration signals 
specified under paragraph (e) of this section 
and the background noise signal required 
under paragraph (f) of this section, may be 
recorded on a magnetic tape recorder for 
subsequent analysis for an integrating sound 
level meter identified in paragraph (d)(1)(i) 
of this section. The record/playback system 
(including the audio tape) of the tape re-
corder must conform to the requirements 
prescribed in section A36.3.6 of appendix A to 
this part. The tape recorder shall comply 
with the specifications of IEC 561 (Incor-
porated by reference, see § 36.6). 

(iii) The characteristics of the complete 

system shall comply with the recommenda-
tions given in IEC 651 (Incorporated by ref-
erence, see § 36.6) with regard to the speci-
fications concerning microphone, amplifier, 
and indicating instrument characteristics. 

(iv) The response of the complete system 

to a sensibly plane progressive wave of con-
stant amplitude shall lie within the toler-
ance limits specified in Table IV and Table V 
for Type 1 instruments in IEC 651 for 
weighting curve ‘‘A’’ over the frequency 
range of 45 Hz to 11500 Hz. 

(2) [Reserved] 
(v) A windscreen must be used with the 

microphone during each measurement of the 
helicopter flyover noise. Correction for any 
insertion loss produced by the windscreen, as 
a function of the frequency of the acoustic 
calibration required under paragraph (e) of 
this section, must be applied to the meas-
ured data and any correction applied must be 
reported. 

(e) 

Calibrations. 

(1) If the helicopter acous-

tic signal is tape recorded for subsequent 
analysis, the measuring system and compo-
nents of the recording system must be cali-
brated as prescribed under section A36.3.6 of 
appendix A of this part. 

(2) If the helicopter acoustic signal is di-

rectly measured by an integrating sound 
level meter: 

(i) The overall sensitivity of the measuring 

system shall be checked before and after the 
series of flyover tests and at intervals (not 
exceeding one-hour duration) during the fly-
over tests using an acoustic calibrator using 
sine wave noise generating a known sound 
pressure level at a known frequency. 

(ii) The performance of equipment in the 

system will be considered satisfactory if, 
during each day’s testing, the variation in 
the calibration value does not exceed 0.5 dB. 
The SEL data collected during the flyover 
tests shall be adjusted to account for any 
variation in the calibration value. 

(iii) A performance calibration analysis of 

each piece of calibration equipment, includ-
ing acoustic calibrators, reference micro-
phones, and voltage insertion devices, must 
have been made during the six calendar 
months proceeding the beginning of the heli-
copter flyover series. Each calibration shall 
be traceable to the National Institute of 
Standards and Technology. 

(f) 

Noise measurement procedures. 

(1) The 

microphone shall be of the pressure-sensitive 
capacitive type designed for nearly uniform 
grazing incidence response. The microphone 
shall be mounted with the center of the sens-
ing element 4 feet (1.2 meters) above the 
local ground surface and shall be oriented for 
grazing incidence such that the sensing ele-
ment, the diaphragm, is substantially in the 
plane defined by the nominal flight path of 
the helicopter and the noise measurement 
station. 

(2) If a tape recorder is used, the frequency 

response of the electrical system must be de-
termined at a level within 10 dB of the full- 
scale reading used during the test, utilizing 
pink or pseudorandom noise. 

(3) The ambient noise, including both 

acoustical background and electrical noise of 
the measurement systems shall be deter-
mined in the test area and the system gain 
set at levels which will be used for helicopter 
noise measurements. If helicopter sound lev-
els do not exceed the background sound lev-
els by at least 15 dB(A), flyovers at an FAA- 
approved lower height may be used and the 
results adjusted to the reference measure-
ment point by an FAA-approved method. 

(4) If an integrating sound level meter is 

used to measure the helicopter noise, the in-
strument operator shall monitor the contin-
uous A-weighted (slow response) noise levels 
throughout each flyover to ensure that the 
SEL integration process includes, at min-
imum, all of the noise signal between the 
maximum A-weighted sound level (L

AMAX

) 

and the 10 dB down points in the flyover 
time history. The instrument operator shall 
note the actual db(A) levels at the start and 
stop of the SEL integration interval and doc-
ument these levels along with the value of 

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