925
Federal Aviation Administration, DOT
Pt. 36, App. A
and 1s. At subsequent times the sum of the
rising and falling responses must be ¥7.5 dB
or less. This equates to an exponential aver-
aging process (slow time-weighting) with a
nominal 1s time constant (
i.e., 2s averaging
time).
A36.3.7.5 When the one-third octave band
sound pressure levels are determined from
the output of the analyzer without slow
time-weighting, slow time-weighting must
be simulated in the subsequent processing.
Simulated slow time-weighted sound pres-
sure levels can be obtained using a contin-
uous exponential averaging process by the
following equation:
L
s
(i,k)=10 log [(0.60653) 10
0.1
Ls[i,
(
k¥1
)
]
+
(0.39347) 10
0.1 L
(
i, k
)
]
where L
s
(i,k) is the simulated slow time-
weighted sound pressure level and L(i,k)
is the as-measured 0.5s time average
sound pressure level determined from the
output of the analyzer for the k-th in-
stant of time and i-th one-third octave
band. For k=1, the slow time-weighted
sound pressure L
s
[i, (k¥1=0)] on the right
hand side should be set to 0 dB. An ap-
proximation of the continuous expo-
nential averaging is represented by the
following equation for a four sample
averaging process for k
≥
4:
L
s
(i,k)=10 log [(0.13) 10
0.1 L[i,
(
k¥3
)
]
+ (0.21) 10
0.1
L[i,
(
k¥2
)
]
+ (0.27) 10
0.1 L[i,
(
k¥1
)
]
+ (0.39) 10
0.1 L[i,
k]
]
where L
s
(i, k) is the simulated slow time-
weighted sound pressure level and L (i, k)
is the as measured 0.5s time average
sound pressure level determined from the
output of the analyzer for the k-th in-
stant of time and the i-th one-third oc-
tave band.
The sum of the weighting factors is 1.0 in
the two equations. Sound pressure levels cal-
culated by means of either equation are valid
for the sixth and subsequent 0.5s data sam-
ples, or for times greater than 2.5s after ini-
tiation of data analysis.
N
OTE
: The coefficients in the two equa-
tions were calculated for use in determining
equivalent slow time-weighted sound pres-
sure levels from samples of 0.5s time average
sound pressure levels. The equations do not
work with data samples where the averaging
time differs from 0.5s.
A36.3.7.6 The instant in time by which a
slow time-weighted sound pressure level is
characterized must be 0.75s earlier than the
actual readout time.
N
OTE
: The definition of this instant in
time is needed to correlate the recorded
noise with the aircraft position when the
noise was emitted and takes into account
the averaging period of the slow time-
weighting. For each 0.5 second data record
this instant in time may also be identified as
1.25 seconds after the start of the associated
2 second averaging period.
A36.3.7.7 The resolution of the sound pres-
sure levels, both displayed and stored, must
be 0.1 dB or finer.
A36.3.8
Calibration systems.
A36.3.8.1 The acoustical sensitivity of the
measurement system must be determined
using a sound calibrator generating a known
sound pressure level at a known frequency.
The minimum standard for the sound cali-
brator is the class 1L requirements of IEC
60942 as amended.
A36.3.9
Calibration and checking of system.
A36.3.9.1 Calibration and checking of the
measurement system and its constituent
components must be carried out to the satis-
faction of the FAA by the methods specified
in sections A36.3.9.2 through A36.3.9.10. The
calibration adjustments, including those for
environmental effects on sound calibrator
output level, must be reported to the FAA
and applied to the measured one-third-oc-
tave sound pressure levels determined from
the output of the analyzer. Data collected
during an overload indication are invalid and
may not be used. If the overload condition
occurred during recording, the associated
test data are invalid, whereas if the overload
occurred during analysis, the analysis must
be repeated with reduced sensitivity to
eliminate the overload.
A36.3.9.2 The free-field frequency response
of the microphone system may be deter-
mined by use of an electrostatic actuator in
combination with manufacturer’s data or by
tests in an anechoic free-field facility. The
correction for frequency response must be
determined within 90 days of each test series.
The correction for non-uniform frequency re-
sponse of the microphone system must be re-
ported to the FAA and applied to the meas-
ured one-third octave band sound pressure
levels determined from the output of the an-
alyzer.
A36.3.9.3 When the angles of incidence of
sound emitted from the aircraft are within
±
30
°
of grazing incidence at the microphone
(see Figure A36–1), a single set of free-field
corrections based on grazing incidence is
considered sufficient for correction of direc-
tional response effects. For other cases, the
angle of incidence for each 0.5 second sample
must be determined and applied for the cor-
rection of incidence effects.
A36.3.9.4 For analog magnetic tape record-
ers, each reel of magnetic tape must carry at
least 30 seconds of pink random or pseudo-
random noise at its beginning and end. Data
obtained from analog tape-recorded signals
will be accepted as reliable only if level dif-
ferences in the 10 kHz one-third-octave-band
are not more than 0.75 dB for the signals re-
corded at the beginning and end.
A36.3.9.5 The frequency response of the en-
tire measurement system while deployed in
the field during the test series, exclusive of
VerDate Mar<15>2010
10:12 Mar 18, 2014
Jkt 232046
PO 00000
Frm 00935
Fmt 8010
Sfmt 8002
Y:\SGML\232046.XXX
232046
pmangrum on DSK3VPTVN1PROD with CFR