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Reference level range. In decibels, the level range for determining
the acoustical sensitivity of the measurement system
and containing the calibration sound pressure level.
Sound incidence angle. In degrees, an angle between the
principal axis of the microphone, as defined in IEC
61094-3' and IEC 61094-4; as amended and a line from
the sound source to the centre of the diaphragm of the
microphone.
Note.- When the sound incidence angle is 0°, the sound is
said to be received at the microphone ut "normal (perpendicular)
incident "; when the sound incidence angle is 90°, the
sound is said to be received at "grazing incidence ".
Tune-average band soundpressure IeveL In decibels, ten times
the logarithm to the base ten, of the ratio of the time meansquare
of the instantaneous sound pressure during a stated
time interval and in a specified one-third octave band, to the
square of the reference sound pressure of 20 FP~.
Wndscreen insertion loss. In decibels, at a stated nominal
one-third octave midband frequency, and for a stated sound
incidence angle on the inserted microphone, the indicated
sound pressure level without the windscreen installed
around the microphone minus the sound pressure level with
the windscreen installed.
- air temperature 23OC
- static air pressure 101.325 kPa
- relative humidity 50%.
3.3 General
Note.- Measurements of aircraff noise that utilize instruments
that conform to the specijcations of this section yield
one-third octave band sound pressure levels as a jitnction oj
time, for the calculation of the efSective perceived noise level
as described in Section 4.
3.3.1 The measurement system shall consist of equipment
approved by the certificating authority and equivalent to the
following:
a) a windscreen (see 3.4);
b) a microphone system (see 3.5);
C) a recording and reproducing system to store the measured
aircraft noise signals for subsequent analysis (see 3.6);
d) a one-third octave band analysis system (see 3.7); and
e) calibration systems to maintain the acoustical sensitivity
of the above systems within specified tolerance limits
(see 3.8).
3.3.2 For any component of the measurement system that
converts an analogue signal to digital form, such conversion
shall be performed so that the levels of any possible aliases or
artefacts of the digitization process will be less than the upper
bounday of the linear operating range by at least 50 dB at any
frequency less than 12.5 kHz. The sampling rate shall be at
least 28 kHz. An anti-aliasing filter shall be included before
the digitization process.
3.2 Reference environmental conditions
3.4 Windscreen
The reference environmental conditions for specifying the performance
of a measurement system are: In the absence of wind and for sinusoidal sounds at grazing
incidence, the insertion loss caused by the windscreen of a
stated type installed around the microphone shall not exceed
*1.5 dB at nominal one-third octave midband frequencies
from 50 Hz to 10 kHz inclusive.
1. IEC 61094-3:1995 entitled "Measurement microphones - Part 3:
Primary method for free-field calibration of laboratory standard
microphones by the reciprocity technique". This IEC publication
3.5 Microphone system
mElaeyc trboet ecohbntiackael d Cfroommm tihsseio cne, nt3r alr uoef fidcee oVf ntrheern bIne,t erGnaetnioenvaal, 3.5.1 The microphone system shall conform to the speci-
Switzerland. fications in 3.5.2 to 3.5.4. Various microphone systems may be
2, IEC 61094-4:1995 entitled "Measurement microphones- part 4: approved the certificating authority on the basis of demonspecifications
for working standard microphonesn. hi^ IEC strated equivalent overall electroacoustical performance.
publication may be obtained from the Central Ofice of the Inter- Where two or more microphone systems of the same type are
national Electrotechnical Commission, 3 rue de VarembB, Geneva, used, demonstration that at least one system conforms to the
Switzerland. specifications in full is sufficient to demonstrate conformance.
24/11/05 APP 2-4
Appendix 2 Annex 16 - Environmental Protection
Note.- This demonstration of equivalent performance does
not eliminate the need to calibrate and check each system as
deJned in 3.9.
3.5.2 The rnicrophonc shall be mounted with the sensing
element 1.2 m (4 ft) above the local ground surface and shall
be oriented for grazing incidence, i.e. with the sensing element
substantially in the plane defined by the predicted reference
flight path of the aircraft and the measuring station. The microphone
　
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