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considered satisfactory if the variation over the period immediately
prior to and immediately following each test series within
a given test day is not greater than 0.5 dB.
Note.- A pistonphone operating at a nominal 124 dB and
250 Hz is generally u~efdo r this purpose.
4.3.6 A wind screen should be employed with the microphone
during all measurements of helicopter noise. Its characteristics
should be such that when it is used, the complete
system including the wind screen will meet the specifications
above. Its insertion loss at the frequencies of the pistonphone
should also be known and included in the acoustic reference
level for analysis of the measurements.
4.4 Noise measurement procedures
4.4.1 The microphone shall be of the pressure-sensitive
type designed tor nearly uniform grazing incidence response.
4.4.2 The microphone shall be mounted with the centre of
the sensing element 1.2 m 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 nominal
flight path of the helicopter and the measuring station. The
microphone mounting arrangement shall minimize the interference
of the supports with the sound to be measured.
4.4.3 If the noise signal is tape-recorded, the frequency
response of the electrical system shall be determined, during
each test series, at a level within 10 dB of the full-scale reading
used during the tests, utilizing random or pseudo-random
pink noise. The output of the noise generator shall have been
checked by an approved standards laboratory within six
months of the test series, and tolerable changes in the relative
output at each one-third octave band shall be not more than
0.2 dB. Sufficient determinations shall be made to ensure that
the overall calibration of the system is known for each test.
4.4.4 Where a magnetic tape recorder forms part of the
measuring chain, each reel of magnetic tape shall carry 30 s of
this electrical calibration signal at its beginning and end for
this purpose. In addition, data obtained from tape-recorded
signals shall be accepted as reliable only if the level difference
in the 10 kHz one-third octave band filtered levels of the two
signals is not more than 0.75 dB.
4.4.5 Background noise, including ambient noise and
electrical noise of the measuremerit systems, shall be determined
in the test area with the system gain set at levels which
will be used for helicopter noise measurements. If helicopter
sound pressure levels do not exceed background sound pressure
levels by at least 15 dB(A), flyovers at an approved lower
height may be used and the results adjusted to the reference
measurement point by an approved method.
5. ADJUSTMENT TO
TEST RESULTS
5.1 When certification test conditions differ from the reference
conditions, appropriate adjustments shall be made to
the measured noise data by the methods of this section.
5.2 Corrections and adjustments
1. Available from the Central Ofice of the International Electrotech- 5.2.1 'The adjustments may be limited to the effects of difnical
Commission, 3 rue de Varembk, Geneva, Switzerland. ferences in spherical spreading between the helicopter test
APP 4-3 2411 1/05
Annex 16 - Environmental Protection Volume 1
flight path and the reference flight path (and between reference
and adjusted reference airspeed). No adjustment for the
differences in atmospheric attenuation between the test and
reference meteorological conditions and between the helicopter
test and reference ground speeds need be applied.
5.2.2 The adjustments for spherical spreading and duration
may be approximated from:
where H is the height, in metres, of the test helicopter when
directly over the noise measurement point.
5.2.3 The adjustment for the difference between reference
airspeed and adjusted reference airspeed is calculated from:
A2 = 10 log pr)d~
lo Vr
where A2 is the quantity in decibels that must be algebraically
added to the measured SEL noise level to correct for the
influence of the adjustment of the reference airspeed on
the duration of the measured flyover event as perceived at the
noise measurement station. Vr is the reference airspeed as
prescribed under Part 11, Chapter 11, 11.5.2, and Var is the
adjusted reference airspeed as prescribed in 2.4.1 of this
appendix.
6. REPORTING OF DATA TO
THE CERTIFICATING AUTHORITY
AND VALIDITY OF RESULTS
6.1 Data reporting
6.1.1 Measured and corrected sound Dressure levels
obtained with equipment conforming to the specifications
described in Section 4 of this appendix shall be reported.
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