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Federal Register / Vol. 86, No. 164 / Friday, August 27, 2021 / Notices minimal influence of wind on the noise recording.
d Hover condition 2 sound elevation angle at 45 degrees: The aircraft maintains a hover condition at a lateral distance of 20 feet to the microphone position and at 20 feet AGL. Test when the conditions are optimal for minimal influence of wind on the noise recording.
e Hover condition 3 overhead, or sound elevation angle at 90 degrees:
The aircraft maintains a hover condition at 20 feet AGL and hold centered within
270 degrees relative to the microphone position for hover conditions described in paragraphs 16c and d of this rule, stabilize the aircraft in hover and record the sound in accordance with paragraph 16f of this rule.
c Hover condition 1 sound elevation angle at zero degrees: The aircraft maintains a hover condition at a lateral distance of 20 feet to the microphone and at 4 feet AGL rotors in the same plane as the microphone. Test when the conditions are optimal for
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a one foot radial over the microphone location.
f For the noise measurements at each hover condition, record the value of the equivalent sound level Leq and SPL in 13 octave bands for a minimum of 30
seconds for each of the test conditions paragraphs 16c through e of this rule.
g The tolerance of the hover height or lateral distance is within 1 ft., and the tolerance of the headings is within 5 degrees.

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Sketch of supplemental hover test conditions. D = 20 feet.
b Acoustic equipment that meets the specifications prescribed in paragraphs 19 and 20 of this rule; and c The calibration and measurement procedures prescribed in paragraphs 21 and 22 of this rule.
18 Measurement of aircraft noise received on the groundNoise unit definition Reference part 36, appendix J, section J36.109b, as modified:
a The sound exposure level, as expressed in LAE, is defined as the level,
L Where T0 is the reference integration time of one second and t2t1 is the integration time interval.

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c The integral equation of paragraph 18b can also be expressed as:

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in decibels, of the time integral of squared A-weighted sound pressure PA over a given time period or event, with reference to the square of the standard reference sound pressure P0
of 20 micropascals and a reference duration of one second.
b The sound exposure level in units of decibels dB is defined by the expression:

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17 Measurement of aircraft noise received on the groundGeneral Reference: Part 36, appendix J, section J36.109a, as modified: Aircraft noise measurements made for the purpose of noise certification in accordance with the requirements of this regulation must be obtained using:
a The noise evaluation metric prescribed in paragraph 18 of this rule;