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Federal Register / Vol. 86, No. 164 / Friday, August 27, 2021 / Notices
Where LAt is the time varying Aweighted sound level.
d The integration time t2t1 in practice must not be less than the time interval during which LAt first rises to within 10 dBA of its maximum value LAmax and last falls below 10 dBA of its maximum value.
19 Measurement of Aircraft Noise Received on the GroundMeasurement System Reference part 36, appendix J, section J36.109c, as modified:
a Acoustical measurement system instrumentation must be equivalent to the following and approved by the FAA:
i. A microphone system with frequency response that is compatible with the measurement and analysis system accuracy prescribed in paragraph 20 of this rule;
ii. Tripods or similar microphone mountings that minimize interference with the sound energy being measured;
and iii. Recording and reproducing equipment with characteristics, frequency response, and dynamic range that are compatible with the response and accuracy requirements of paragraph 20 of this rule.
b The calibration and checking of measurement systems must be accomplished in accordance with the procedures described in part 36, appendix A, section A36.3.9.
20 Measurement of Aircraft Noise Received on the GroundSensing, recording, and reproducing equipment Reference part 36, appendix J, section J36.109d, as modified:
a The sound pressure time-history audio signals obtained from aircraft flyovers under this paragraph must be recorded digitally at a minimum sample rate of 44 kilohertz kHz for a minimum bandwidth of 20 hertz Hz to 20 kHz, and encoded using a minimum of 16 bit linear PCM or equivalent during analog to digital conversion. Digital audio recording must also meet the additional requirements specified in part 36, appendix A, section A36.3.6
Recording and Reproducing Systems.
b The LAE value from each flyover and A-weighed Leq LAeq values from each hover test flight condition may be determined directly from an integrating sound level meter that meets the specifications of International Electrotechnical Commission IEC
Standard 616721 2013 for a Class 1
instrument set at slow response.
c The acoustic signal from the aircraft, along with the calibration
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signals specified in paragraph 21 and the background noise signal required by paragraph 22 of this rule, must be recorded in a digital audio format as specified in paragraph 20a of this rule for subsequent analysis for an integrating sound level meter identified in paragraph 20b of this rule. The record/playback system must conform to the requirements prescribed in part 36, appendix A, section A36.3.6
Recording and Reproducing Systems.
The recorder must comply with the specifications of IEC standard 61265
2nd edition 2018.
d The characteristics of the complete system must meet the specifications of IEC standard 616721 for the microphone, amplifier, and indicating instrument characteristics.
e The response of the complete system to a plane, progressive wave of constant amplitude must lie within the tolerance limits specified for Class 1
instruments in IEC standard 616721 for weighting curve A over the frequency range of 45 Hz to 20 kHz.
f A windscreen must be used with the microphone during each measurement of the aircraft flyover noise. Correction for any insertion loss produced by the windscreen, as a function of the frequency of the acoustic calibration required by paragraph 21 of this rule, must be applied to the measured data, and each correction applied must be included in the test report.
21 Measurement of Aircraft Noise Received on the GroundCalibrations Reference part 36, appendix J, section J36.109e, as modified:
a For the aircraft acoustic signal recorded for subsequent analysis, the measuring system and components of the recording system must be calibrated as prescribed in Title 14 CFR, part 36, appendix A.
b If the aircraft acoustic signal is measured directly using an integrating sound level meter:
i. The overall sensitivity of the measuring system must be checked before and after the series of flyover tests and at intervals not exceeding a two-hour duration during the flyover tests using an acoustic calibrator generating a sinusoidal signal at a known sound pressure level and at a known frequency.
ii. The performance of equipment in the system is considered satisfactory if, during each days testing, the variation in the measured value for the acoustic
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calibrator does not exceed 0.5 dB. The LAE data collected during the flyover tests must 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 preceding the beginning of the aircraft flyover series.
Each calibration must be traceable to the National Institute of Standards and Technology.
22 Measurement of Aircraft Noise Received on the GroundNoise measurement procedures Reference part 36, appendix J, section J36.109f, as modified:
a The microphone must be of a pressure-sensitive capacitive type designed for nearly uniform grazing incidence response. The microphone must be mounted with the center of the sensing element 4 feet 1.2 meters above the local ground surface and must be oriented for grazing incidence such that the sensing element diaphragm is substantially in the plane defined by the nominal flight path of the aircraft and the noise measurement station. A
microphone that satisfies the requirements of this paragraph must be used when determining compliance with the noise limit prescribed in paragraph 29 of this rule.
b For each aircraft acoustic signal recorded for subsequent analysis, the frequency response of the electrical system must be determined at a level within 10 dB of the full-scale reading used during the test.
c The background noise, including both ambient acoustical sound present at the microphone site and electrical noise of the measurement systems, must be determined in the test area and the system gain set at levels which will be used for aircraft noise measurements. If aircraft sound levels do not exceed the background sound levels by at least 15
dBA, flyovers at an FAA-approved lower height may be used; the results must be adjusted to the reference measurement point by an FAAapproved method.
d When an integrating sound level meter is used to measure the aircraft noise, the instrument operator must monitor the continuous A-weighted slow response noise levels throughout each flyover to ensure that the Aweighted sound exposure level LAE
integration process includes, at
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