Federal Aviation Administration, DOT

Pt. 36, App. J

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(a) General. (1) The helicopter noise measured under this appendix for noise certification purposes must be obtained with FAAapproved acoustical equipment and measurement practices.
(2) Paragraph (b) of this section identifies
and prescribes the specifications for the

L AE = 10 Log10

1
T0

2

t2 

 - t

1

PA (t ) 

 dt dB
 P0 

Where TO is the reference integration time of
one second and (t2-t1) is the integration
time interval.
(3) The integral equation of paragraph
(b)(2) of this section can also be expressed as:

L AE = 10 Log10

1
T0

t2

 - t

10

0 .1 LA ( t )

Where LA(t) is the time varying A-weighted
sound level.
(4) The integration time (t2-t1) in practice
shall not be less than the time interval during which LA(t) first rises to within 10 dB(A)
of its maximum value (LAMAX) and last falls
below 10 dB(A) of its maximum value.
(5) The SEL may be approximated by the
following expression:
LAE = LAMAX + <delta>A
where <delta>A is the duration allowance
given by:
<delta>A = 10 log10 (T)
where T = (t2-t1)/2 and LAMAX is defined as the
maximum level, in decibels, of the Aweighted sound pressure (slow response)
with reference to the square of the standard reference sound pressure (P0).
(c) Measurement system. The acoustical
measurement system must consist of FAAapproved equipment equivalent to the following:
(1) A microphone system with frequency
response that is compatible with the measurement and analysis system accuracy prescribed in paragraph (d) of this section;
(2) Tripods or similar microphone mountings that minimize interference with the
sound energy being measured;
(3) Recording and reproducing equipment
with characteristics, frequency response, and
dynamic range that are compatible with the

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EC28SE91.119</MATH>

Section J36.107 [Reserved]
Section J36.109 Measurement of helicopter noise
received on the ground.

noise evaluation measurements required
under this appendix. Paragraphs (c) and (d)
of this section prescribe the required acoustical equipment specifications. Paragraphs
(e) and (f) of this section prescribe the calibration and measurement procedures required under this appendix.
(b) Noise unit definition. (1) The value of
sound exposure level (SEL, or as denoted by
symbol, LAE), is defined as the level, 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 (PO) of 20 micropascals and a reference
duration of one second.
(2) This unit is defined by the expression:

EC28SE91.118</MATH>

the number of level flights made with a tailwind component over the noise measurement
station:
(1) In level flight and in cruise configuration;
(2) At a height of 492 feet Section50 feet (150 Section15
meters) above the ground level at the noise
measuring station; and
(3) Within Section10 degrees from the zenith.
(c) Each flyover noise test must be conducted:
(1) At the reference airspeed specified in
section J36.3(c) of this appendix, with such
airspeed adjusted as necessary to produce
the same advancing blade tip Mach number
as associated with the reference conditions;
(i) Advancing blade tip Mach number (MAT)
is defined as the ratio of the arithmetic sum
of blade tip rotational speed (VR) and the
helicopter true air speed (VT) over the speed
of sound (c) at 77 degrees Fahrenheit (1135.6
ft/sec or 346.13 m/sec) such that MAT = (VR +
VT)/c; and
(ii) The airspeed shall not vary from the
adjusted reference airspeed by more than Section3
knots (Section5 km/hr) or an equivalent FAA-approved variation from the reference advancing blade tip Mach number. The adjusted reference airspeed shall be maintained throughout the measured portion of the flyover.
(2) At rotor speed stabilized at the power
on maximum normal operating rotor RPM
(Section1 percent); and
(3) With the power stabilized during the period when the measured helicopter noise
level is within 10 dB of the maximum Aweighted sound level (LAMAX).
(d) The helicopter test weight for each flyover test must be within plus 5 percent or
minus 10 percent of the maximum takeoff
weight for which certification under this
part is requested.
(e) The requirements of paragraph (b)(2) of
this section notwithstanding, flyovers at an
FAA-approved lower height may be used and
the results adjusted to the reference measurement point by an FAA-approved method
if the ambient noise in the test area, measured in accordance with the requirements
prescribed in section J36.109 of this appendix,
is found to be within 15 dB(A) of the maximum A-weighted helicopter noise level
(LAMAX) measured at the noise measurement
station in accordance with section J36.109 of
this appendix.