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3.
MEASUREMENT OF THE NOISE UNDER INVESTIGATION
| 3.1 |
General
The
Authority should measure the noise under investigation
in accordance with the procedures outlined in
Section 3.2. Corrections may need to be applied
to the Measured Noise Level (MNL) to account for
certain noticeable characteristics of the noise
and these shall be made in accordance with Section
3.3 to determine the Corrected Noise Level (CNL).
Where
the NSR is considered by the Authority to be materially
affected by one or more other noise sources falling
within the scope of this Technical Memorandum,
the Authority shall assess the noise under investigation
in a manner which the Authority considers appropriate
in the circumstances, having regard to standard
acoustical principles and practices.
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| 3.2 |
Determination of the Measured Noise Level (MNL)
The
MNL of the noise under investigation shall be
measured over a Sample Time Period (STP) by the
Authority in accordance with the calibration and
measurement procedures detailed in the Annex.
An adjustment may be made where appropriate to
allow for the influence of the background noise,
in accordance with standard acoustical principles
and practices.
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| 3.3
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Determination
of the Corrected Noise Level (CNL)
| 3.3.1 |
General
Where
the noise under investigation is considered by
the Authority to have tonal, impulsive or intermittent
characteristics, appropriate corrections shall
be made to the MNL to obtain the CNL, as detailed
below.
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| 3.3.2 |
Correction for Tonality
A
correction for tonality shall be applied if, between
31.5 Hz and 16 kHz, any one-third octave band
or any pair of adjacent one-third octave bands
of the A-weighted spectrum of the noise under
investigation satisfies all of the following conditions:
| (a) |
the level of the one-third octave band under
consideration, or, in the case of a pair of
bands, the level of the highest band in that
pair, is not more than 15.0 dB below the level
of the highest one-third octave band; |
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| (b) |
the level of the one-third octave band under
consideration, or, in the case of a pair of
bands, the arithmetic average of the levels
of the two bands, is more than 1.0 dB higher
than the level of each of the adjacent bands
on either side of the band or pair of bands
under consideration; and |
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| (c) |
the level difference, known as the tonality
factor, ftone, between the level
of the one-third octave band under consideration,
or, in the case of a pair of bands, the
arithmetic average of the levels of the
two bands, and the arithmetic average of
the levels of the adjacent bands on either
side of the band or pair of bands under
consideration is 3.0 dB or more.
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The
analysis to determine if a tonal correction is
necessary shall be carried out over such a time
period or periods as the Authority considers appropriate
within the STP, so as to be representative of
the tonal characteristics of the noise under investigation.
Where
the noise under investigation is assessed to have
a tonal characteristic with a tonality factor,
ftone, the correction, ctone,
to the MNL shall be as shown in Table 3.
Table
3 - Tonal Correction
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ftone(dB)
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Ctone(dB(A))
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| in
cases where the frequency of any band under
consideration is below 250 Hz |
in
cases where the frequency of each band under
consideration is higher than or equal to 250
Hz |
| greater
than or equal to 3.0 and less than 6.0 |
0
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3
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| greater
than or equal to 6.0 and less than 9.0 |
3
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6
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| greater
than or equal to 9.0 |
6
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6
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| 3.3.3 |
Correction
for Impulsiveness
If
the noise under investigation is considered by
the Authority to be impulsive in character a positive
correction, Cimp, of not more than
3 dB(A) may be applied to the MNL.
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| 3.3.4 |
Correction for Intermittency
A
correction for intermittency shall be applied
to the MNL, for the night-time period (2300 to
0700 hours) only, when the A-weighted sound pressure
level of the noise under investigation is subject
to rapid changes in level of 5.0 dB(A) or more
occurring with a degree of regularity within the
STP, the typical magnitude of these changes in
level being known as the intermittency factor,
fint.
Where
the noise under investigation is assessed to have
an intermittent characteristic with an intermittency
factor, fint, the correction, Cint,
to the MNL shall be as shown in Table 4.
Table
4 - Intermittency Correction
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fint
(dB(A))
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Cint
(dB(A))
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| greater
than or equal to 5.0 and less than 10.0 |
3
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| greater
than or equal to 10.0 |
6
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| 3.3.5 |
Calculation of Corrected Noise Level (CNL)
The
CNL shall be calculated by applying the appropriate
corrections to the MNL in accordance with the
following formula:
CNL
= MNL + Ctone + Cimp + Cint dB(A)
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