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 |
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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