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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.
4.4.3 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.
4.3.4 The noise signal shall be passed through an "A"
filter as defined in IEC Publication No. 651. 4.4.4 Background noise, including ambient noise and
electrical noise of the measurement systems, shall be deter-
4.3.5 The overall sensitivity of the measuring system mined in the test area with the system gain set at levels which
shall be checked before tests start and at intervals during will be used for aeroplane noise measurements. If aeroplane
testing using an acoustic calibrator generating a known sound peak sound pressure levels do not exceed background sound
pressure level at a known frequency. pressure levels by at least 10 dB(A), a take-uff measurement
point nearer to start of roll shall be used and the results
Note.- A pistonphone operating at a nominal 124 dB and adjusted to thc rcfcrcnce measurement point by an approved
250 Hz is generally used for this purpose. method.
4.3.6 When a tape recording is used, the maximum
A-weighted noise level Lh, may be determined using a
graphic level recorder or digital equivalent. 5. ADJUSTMENT TO
TESTRESULTS
Note.- The maximum noise level LAmax could also be
determined using an approved sound level meter 5.1 When certification test conditions differ from the
reference conditions, appropriate adiustments shall be made to .. -
the measured noise data by the methods of this section.
5.2 Corrections
I . Available from the Central Office of the International and adjustments
Electrotechnical Commission, 3 rue de VarembC, Geneva,
Switzerland. 5.2.1 The adjustments take account of the effects of:
APP 6-3 24/11/05
Annex 16 - Environmental Protection Volume I
a) differences in atmospheric absorption between meteorological
test conditions and reference conditions;
b) differences in the noise path length between the actual
aeroplane flight path and the reference flight path;
c) the change in the helical tip Mach number between test
and reference conditions; and
d) the change in engine power between test and reference
conditions.
5.2.2 The noise level under reference conditions (Lh,)
REF is obtained by adding increments for each of the above
effects to the test day noise level (Lb,) TEST.
(L,) REF = (Lb,) TEST + A(M)
+ A1 + AZ + A3
where
A(M) is the adjustment for the change in atmospheric
absorption between test and reference conditions;
Al is the adjustment for noise path lengths;
AZ is the adjustment for helical tip Mach number; and
A3 is the adjustment for engine power.
a) When the test conditions are within those specified in
Figure A6-2, no adjustments for differences in atmospheric
absorption need be applied, i.e. A(M) = 0. If
conditions are outside those specified in Figure A6-2
then adjustments must be applied by an approved
procedure or by adding an increment A(M) to the test
day noise levels where,
and where HT is the height in metres of the test
aeroplane when directly over the noise measurement
point. HR is the reference height of the aeroplane above
the noise measurement point, and a is the rate of absorption
at 500 Hz specified in Tables A1-5 to A1-16 of
Appendix 1.
b) Measured noise levels should be adjusted to the height
of the aeroplane over the noise measuring point on a
reference day by algebraically adding increment
equal to dl. When test day conditions are within those
specified in Figure A6-2:
When test day conditions are outside those specified in
Figure A6-2:
where HT is the height of the aeroplane when directly
over the noise measurement point, and HR is the reference
height of the aeroplane over the measurement
point.
Relative
humidity
Yo
0-
5 10 15 20 25 30
Temperature "C
Figure A6-2. Measurement window for
no absorption correction
APP 6-4
Appendix 6 Annex 16 - Environmental Protection
c) No adjustments for helical tip Mach number variations
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