976
14 CFR Ch. I (1–1–14 Edition)
Pt. 36, App. J
§ 36.6 of this part, 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 re-
corded on a magnetic tape recorder for sub-
sequent analysis by 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 of
this part. The tape recorder shall comply
with specifications of IEC Publication No.
561, ‘‘Electro-acoustical Measuring Equip-
ment for Aircraft Noise Certification,’’ as in-
corporated by reference under § 36.6 of this
part.
(iii) The characteristics of the complete
system shall comply with the recommenda-
tions given in IEC Publication No. 651,
‘‘Sound Level Meters,’’ as incorporated by
reference under § 36.6 of this part, with re-
gard to the specifications concerning micro-
phone, 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 Publication
No. 651, ‘‘Sound Level Meters,’’ as incor-
porated by reference under § 36.6 of this part,
for weighting curve ‘‘A’’ over the frequency
range of 45 Hz to 11500 Hz.
(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
L
AMAX
and the integration interval (in sec-
onds) for inclusion in the noise data sub-
mitted as part of the reporting requirements
under section J36.111(b) of this appendix.
Section J36.111 Reporting Requirements.
(a)
General. Data representing physical
measurements, and corrections to measured
data, including corrections to measurements
for equipment response deviations, must be
recorded in permanent form and appended to
the record. Each correction is subject to
FAA approval.
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