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(see Figure A5-1) should apply. An approximation, to the
nearest decibel, of the inverse of the 40 noy contour relative to
the value at 1 000 Hz, is given in Table A5-1. The relative
frequency response of the weighting element of the equipment
should be maintained within a tolerance of *0.5 dB. When
such a weighting network is incorporated in a direct reading
instrument, the relation between the acoustical input to the
microphone and the meter reading should follow the inverse of
the 40 noy contour with the same tolerances as those specified
for weighting curve C in IEC Publication No. 179.'
Measurements obtained by means of the instrumentation
described above provide, after adding 7 dB, values which are
approximations to the perceived noise levels in PNdB.
3.3 An alternative method of determining approximations
to the perceived noise levels can be obtained from measuring
the noise using a sound-level meter incorporating the
A-weighting network2 and adding a correction K normally
between 9 and 14 dB dependent on the frequency spectrum of
the noise. The value of K and the method used by the
measuring authorities for determination of that value should be
specified when reporting results.
3.4 The field installation of microphones for aircraft noise
monitoring purposes should provide for suitable protection of
the microphones from rain, snow and other adverse weather
conditions. Adequate correction for any insertion loss, as a
function of frequency and weather conditions, produced by
wind screens or other protective enclosures should be applied
to the measured data.
3. MEASUREMENT EQUIPMENT 1. This publication was first issued in 1965 by the Central Ofice of
the International Electrotechnical Commission, 3 rue de Varembe,
3.1 The measurement equipment should consist of Geneva, Switzerland.
either portable recording apparatus capable of direct reading, 2. The A-weighting network is described in IEC Publication No. 179.
ANNEX 16 - VOLUME 1 APP 5-1 2411 1/05
Annex 16 - Environmerrtal Proleetion Volume I
Frequency, Hz
Figure AS-1. Contours of perceived noisines
APP 5-2
Annex 16 - Environmental Protection
Table A5-1. Approximation to the nearest decibel of the inverse of
the 40 qoy contour relative to the value >t 1 000 Hz
Note.- Where a record of the noise as ~function of time is
required this can be obtained by recording the noise signal on
a magnetic tape, a graphic level recorder or other suitable
equipment.
3.5 The recording and indicating equipment should
comply with IEC Publication No. 17g3 regarding the dynamic
characteristics of the indicating instrument designated as
"slow".
4. FIELD EQUIPMENT INSTAL1,ATION
4.1 Microphones used for monitoring noise levels from
aircraft operations should be installed at appropriate locations
with the axis of maximum sensitivity of each microphone oriented
in a direction such that the highest sensitivity to sound
waves is achieved. The microphone position should be
selected so that no obstruction which influences the sound
field produced by an aircraft exists above a horizontal plane
passing through the active centre of the microphone.
Note.- Ifthe anticipated duration of the noise signal is less
than 5 s, the dynamic characteristics designated as 'Ifast " may Note I.- Monitoring microphones may need to be placed
be used
For the pulpose of this note, the duration is described as
the length of the significant time history during which the
recorded signal, passed through a weighting network having
an amplitude characteristic equal to the inverse 40 noy
contoul; remains within I0 dB of its maximum value.
3.6 The microphone system should have been originally
calibrated at a laboratory equipped for free-field calibration
and its calibration should be rechecked at least every six
months.
3.7 The complete measurement system prior to field
installation and at periodic intervals thereafter should be
calibrated in a laboratory to ensure that the frequency response
and dynamic range requirements of the system comply with
the specifications described in this document.
Note.- Direct reading measuring systems that yield
approximate values of perceived noise levels other than those
defined above are not meant to be excluded fiom use in
monitoring.
in locations having substantial background noise levels caused
by motor vehicle trafJic, children playing, etc. In these
instances it is ojten expedient lo locate the microphone on a
rooftop, telephone pole or other structure rising above the
ground. Consequently, it is necessary to determine the background
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