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mounting arrangement shall minimize the interference
of the supports with the sound to be measured. Figure A2-1
illustrates sound incidence angles on a microphone.
3.5.3 The free-field sensitivity level of the microphone
and preamplifier in the reference direction, at frequencies over
at least the range of one-third octave nominal midband
frequencies from 50 Hz to 5 kHz inclusive, shall be within
*1.0 dB of that at the calibration check frequency, and within
*2.0 dB for nominal midband frequencies of 6.3 kHz, 8 kHz
and 10 kHz.
3.5.4 For sinusoidal sound waves at each one-third octave
nominal midband frequency over the range from 50 Hz to
10 kHz inclusive, the free-field sensitivity levels of the microphone
system at sound incidence angles of 30°, 60°, 90°, 120"
and 150" shall not differ from the free-field sensitivity level at
a sound incidence angle of 0" ("normal incidence") by more
than the values shown in Table A2-1. The free-field sensitivity
level differences at sound incidence angles between any two
adjacent sound incidence angles in Table A2-1 shall not
exceed the tolerance limit for thc grcatcr angle.
3.6 Recording and
reproducing systems
3.6.1 A recording and reproducing system, such as a
digital or analogue magnetic tape recorder, a computer-based
system or other permanent data storage device, shall be used
to store sound pressure signals for subsequent analysis. The
sound produced by the aircraft shall be recorded in such a way
that a record of the complete acoustical signal is retained. The
recording and reproducing systems shall conform to the
specifications in 3.6.2 to 3.6.9 at the recording speeds and/or
data sampling rates used for the noise certification tests.
Conformance shall be demonstrated for the frequency bandwidths
and recording channels selected for the tests.
3.6.2 The recording and reproducing systems shall be
calibrated as described in 3.9.
Note.- For aircraft noise signals for which the high
frequency spectral levels decrease rapidly with increasing
frequency, appropriate pre-emphasis and complementary de-
~mphaslr n~tworks may be inchded in the measurement
system. Ifpre-emphasis is included, over the range of nominal
one-third octave midbandfrequencies from 800 Hz to 10 kHz
inclusive, the electrical gain provided by the pre-emphasis
network shall not exceed 20 dB relative to the gain at 800 Hz.
3.6.3 For steady sinusoidal electrical signals applied to
the input of the entire measurement system exclusive of the
microphone system, but including the microphone preamplifier,
and any other signal-conditionir~g elements that are
Figure A2-1. Illustration of sound incidence angles on a microphone
1 90" Grazing incidence
APP 2-5
I
Annex 16 - Environmental Protection Volume I
Table A2-1. Microphone directional response requirements
Maximum difference between the free-field sensitivity level of a microphone system
at normal incidence and the free-field sensitivity level at specified sound
incidence angles (dB)
Nominal midband
frequency kHz Sound incidence angle degrees
considered to be part of the microphone system, at a selected
signal level within 5 dB of that corresponding to the calibration
sound pressure level on the reference level range, the time
average signal level indicated by the readout device at any
one-third octave nominal midband frequency from 50 Hz to
10 kHz inclusive shall be within rt1.5 dB of that at the
calibration check frequency. The frequency response of a
measurement system, which includes components that convert
analogue signals to digital form, shall be within *0.3 dB of the
response at 10 kHz over the frequency range from 10 kHz to
11.2 kHz.
Note.- It is not necessary to include microphone extension
cables as configured in the field.
3.6.4 For analogue tape recordings, the amplitude fluctuations
of a 1 kHz sinusoidal signal recorded within 5 dB of the
level corresponding to the calibration sound pressure level shall
not vary by more than *0.5 dB throughout any reel of the type
of magnetic tape utilized. Conformance to this requirement
shall be demonstrated using a device which has time-averaging
properties equivalent to those of the spectrum analyser.
3.6.5 For all appropriate level ranges and for steady sinusoidal
electrical signals applied to the input of the measurement
system exclusive of the microphone system, but including the
Note 1.- Level linearity of measurement system components
should be tested according to the methods described in
IEC 61265~a s amended.
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