975
Federal Aviation Administration, DOT
Pt. 36, App. J
knots (
±
5 km/hr) or an equivalent FAA-ap-
proved variation from the reference advanc-
ing blade tip Mach number. The adjusted ref-
erence airspeed shall be maintained through-
out the measured portion of the flyover.
(2) At rotor speed stabilized at the power
on maximum normal operating rotor RPM
(
±
1 percent); and
(3) With the power stabilized during the pe-
riod when the measured helicopter noise
level is within 10 dB of the maximum A-
weighted sound level (L
AMAX
).
(d) The helicopter test weight for each fly-
over 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 meas-
urement point by an FAA-approved method
if the ambient noise in the test area, meas-
ured in accordance with the requirements
prescribed in section J36.109 of this appendix,
is found to be within 15 dB(A) of the max-
imum A-weighted helicopter noise level
(L
AMAX
) measured at the noise measurement
station in accordance with section J36.109 of
this appendix.
Section J36.107 [Reserved]
Section J36.109 Measurement of helicopter noise
received on the ground.
(a)
General. (1) The helicopter noise meas-
ured under this appendix for noise certifi-
cation purposes must be obtained with FAA-
approved acoustical equipment and measure-
ment practices.
(2) Paragraph (b) of this section identifies
and prescribes the specifications for the
noise evaluation measurements required
under this appendix. Paragraphs (c) and (d)
of this section prescribe the required acous-
tical equipment specifications. Paragraphs
(e) and (f) of this section prescribe the cali-
bration and measurement procedures re-
quired under this appendix.
(b)
Noise unit definition. (1) The value of
sound exposure level (SEL, or as denoted by
symbol, L
AE
), is defined as the level, in deci-
bels, of the time integral of squared ‘A’-
weighted sound pressure (P
A
) over a given
time period or event, with reference to the
square of the standard reference sound pres-
sure (P
O
) of 20 micropascals and a reference
duration of one second.
(2) This unit is defined by the expression:
L
Log
T
P t
P
dt dB
AE
A
t
t
=
⎛
⎝⎜
⎞
⎠⎟
∫
10
1
10
0
0
2
1
2
( )
Where T
O
is the reference integration time of
one second and (t
2
-t
1
) is the integration time
interval.
(3) The integral equation of paragraph
(b)(2) of this section can also be expressed as:
L
Log
T
dt dB
AE
L
t
t
t
=
∫
10
1
10
10
0
0 1
1
2
.
( )
A
Where L
A
(t) is the time varying A-weighted
sound level.
(4) The integration time (t
2
-t
1
) in practice
shall not be less than the time interval dur-
ing which L
A
(t) first rises to within 10 dB(A)
of its maximum value (L
AMAX
) and last falls
below 10 dB(A) of its maximum value.
(5) The SEL may be approximated by the
following expression:
L
AE
=L
AMAX
+ <delta>A
where <delta>A is the duration allowance
given by:
<delta>A=10 log
10
(T)
where T=(t
2
-t
1
)/2 and L
AMAX
is defined as
the maximum level, in decibels, of the A-
weighted sound pressure (slow response) with
reference to the square of the standard ref-
erence sound pressure (P
0
).
(c)
Measurement system. The acoustical
measurement system must consist of FAA-
approved equipment equivalent to the fol-
lowing:
(1) A microphone system with frequency
response that is compatible with the meas-
urement and analysis system accuracy pre-
scribed in paragraph (d) of this section;
(2) Tripods or similar microphone mount-
ings 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
response and accuracy requirements of para-
graph (d) of this section; and
(4) The calibration and checking of meas-
urement 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 deter-
mined. With the approval of the FAA, the
noise signal may be tape recorded for subse-
quent analysis.
(i) The SEL values from each flyover test
may be directly determined from an inte-
grating sound level meter complying with
the Standards of the International Electro-
technical Commission (IEC) Publication No.
804, ‘‘Integrating-averaging Sound Level Me-
ters,’’ as incorporated by reference under
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