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of sound (c) at 25OC such that:
Mat =
(Vt + Vr)
C
3. NOISE UNIT DEFINITION
3.1 The value of sound exposure level LAE is defined as
the level, in decibels, of the time integral of squared
A-weighted sound pressure (PA) over a given time period or
event, with reference to the square of the standard reference
sound pressure (Po) or 20 micropascals and a reference
duration of one second.
3.2 This unit is defined by the expression:
LAE = 10 log - l;(F?* dt
where To is the reference integration time of one second and
(t2 - tl) is the integration time interval.
3.3 The above integral can also be expressed as:
(1)
LA, = 10 log - 1: 1OLA~dr
To
where LA(t) is the time varying A-weighted sound level.
3.4 The integration time (t2 - tl) in practice shall not be
less than the time interval during which LA(t) first rises to
within 10 dB(A) of its maximum value (LAmax) and last falls
below 10 &(A) of its maximum value.
3.5 The SEL may be approximated by the following
cxprcssion:
where AA is the duration allowance given by
where r = (t2 - t1)/2.
LArnax is defined as the maximum level, in decibels, of the
A-weighted sound pressure (slow response) with reference to
the square of the standard reference sound pressure Po.
4. MEASUREMENT OF
HELICOPTER NOISE RECEIVED
ONTHEGROUND
4.1 General
4.1.1 All measuring equipment shall be approved by the
certificating authority.
4.1.2 Sound oressure level data for noise evaluation
purposes shall be obtained with acoustical equipment and
measurement practices that conform to the specifications given
4.2 Measurement system
The acoustical measurement system shall consist of approved
equipment equivalent to the following:
a) a microphone system with frequency response compatible
with measurement and analysis system accuracy
as stated in 4.3;
b) tripods or similar microphone mountings that minimize
interference with the sound being measured;
c) recording and reproducing equipment characteristics,
frequency response, and dynamic range compatible with
the response and accuracy requirements of 4.3; and
d) acoustic calibrators using sine wave or broadband noise
of known sound pressure level. If broadband noise is
used, the signal shall be described in terms of its average
and maximum root-mean-square (rms) value for nonoverload
signal level.
APP 4-2
Annex 16 - Environmental Protection
4.3 Sensing, recording and
reproducing equipment
4.3.1 With the approval of the certificating authority the
sound level produced by the helicopter may be stored on a
magnetic tape recorder for later evaluation. Alternatively, the
A-weighted sound level time history may be written onto a
graphic level recorder set at "slow" response from which the
SEL value may be determined or the SEL may be directly
determined from an integrating sound level meter complying
with the Standards of International Electrotechnical Commission
(IEC) Publication No. 804' for a Type 1 instrument set
at "slow" response. The SEL value may also be calculated
from one-third octave band data obtained from measurements
made in conformity with Section 3 of Appendix 2. In this case
each one-third octave band sound pressure level shall be
weighted in accordance with the A-weighting values given in
IEC Publication No. 651.'
4.3.2 The characteristics of the complete system shall
comply with the recommendations given in IEC Publication
No. 651' with regard to the sections concerning microphone,
amplifier and indicating instrument characteristics. The text
and specifications of IEC Publication No. 65 1 entitled "Sound
Level Meters" are incorporated by reference into this section
and are made a part hereof.
4.3.3 If a tape recording is used, the tape recorder shall
comply with IEC Recommendation 561.'
4.3.4 The response of the complete system to a sensibly
plane progressive sinusoidal wave of constant amplitude shall
lie within the tolerance limits specified in Table IV and
Table V for Type I instruments in IEC Publication No. 65 1, for
weighting curve A over the frequency range 45 to 11 500 Hz.
4.3.5 The overall sensitivity of the measuring system shall
be checked before tests start and at intervals during testing
using an acoustic calibrator generating a known sound pressure
level at a known frequency. The output of the acoustic calibrator
shall have been checked by a standardizing laboratory
within 6 months of the test series; tolerable changes in output
shall be not more than 0.2 dB. The equipment shall be
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