spaschal on DSK3GDR082PROD with CFR

Pt. 36, App. J

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

response and accuracy requirements of paragraph (d) of this section; and
(4) The calibration and checking of measurement 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 determined. With the approval of the FAA, the
noise signal may be tape recorded for subsequent analysis.
(i) The SEL values from each flyover test
may be directly determined from an integrating sound level meter complying with
the standards of IEC 804 (Incorporated by reference, see Section 36.6) for a Type 1 instrument set
at  - slow -  response.
(ii) The acoustic signal from the helicopter, 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 recorder 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 (Incorporated by reference, see Section 36.6).
(iii) The characteristics of the complete
system shall comply with the recommendations given in IEC 651 (Incorporated by reference, see Section 36.6) with regard to the specifications concerning microphone, amplifier,
and indicating instrument characteristics.
(iv) The response of the complete system
to a sensibly plane progressive wave of constant amplitude shall lie within the tolerance 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 measured data and any correction applied must be
reported.
(e) Calibrations. (1) If the helicopter acoustic signal is tape recorded for subsequent
analysis, the measuring system and components of the recording system must be calibrated as prescribed under section A36.3.6 of
appendix A of this part.

(2) If the helicopter acoustic signal is directly 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 flyover 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, including acoustic calibrators, reference microphones, and voltage insertion devices, must
have been made during the six calendar
months proceeding the beginning of the helicopter 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 sensing element 4 feet (1.2 meters) above the
local ground surface and shall be oriented for
grazing incidence such that the sensing element, 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 determined at a level within 10 dB of the fullscale 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 determined in the test area and the system gain
set at levels which will be used for helicopter
noise measurements. If helicopter sound levels do not exceed the background sound levels by at least 15 dB(A), flyovers at an FAAapproved lower height may be used and the
results adjusted to the reference measurement point by an FAA-approved method.
(4) If an integrating sound level meter is
used to measure the helicopter noise, the instrument operator shall monitor the continuous A-weighted (slow response) noise levels
throughout each flyover to ensure that the
SEL integration process includes, at minimum, all of the noise signal between the
maximum A-weighted sound level (LAMAX)
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 document these levels along with the value of

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