MSC 69/20/4
2 Calculation
Lneed needs to have enough sound pressure level so
that the sound at D1 has the sound pressure level of Lmin
or more after the sound is decreased the sound pressure
level of ) L for propagating in air from D0 to D1.
Therefore Lneed, Lmin and ) L have a relation as below:
Lneed = Lmin + )L (1)
where the amount of decrease of the sound
pressure level for propagating from D0 to D1 could be shown
as follows:
△L = 10 log (D1/D0)2 (2)
Substitution of 1m into D1 and of 926 m (0.5 miles) into
D1 respectively, Equation (2) becomes
△L≒60[dB] (3)
From Equation (3), it becomes
clear that we have to take into account that the sounds needs
at least 60 dB for propagating in air from D0 to D1.
On the other hand, Lnoise is
prescribed in Annex III 1 (c) dependent on the cantered
frequencies. Average background noise level in the octave
band cantered on 250Hz and 500Hz is 68dB, 63dB. Moreover
average background noise level in the octave band cantered on
800Hz is estimated as 60dB by linear Interpolation above.
L'noise to each frequencies is provided
respectively as follows.
1 where a background noise level
is 68dB in the octave band cantered on 250Hz:
| L'noise |
= L noise 10 log (△f[Hz]/1[Hz])
=Lnoise - 10 log{f0(2 1/2 - 2
-1/2)}
=68 - 10 log (177)
=45.5 [dB]
|
2 where a background noise level
is 63dB in the octave band cantered on 500Hz:
| |
=L'noise = 63 - 10 log (354)
=38[dB] |
3 where a background noise level
is 60dB in the octave band cantered on 800Hz:
| |
= L'noise = 60 - 10 log (566)
= 32.5 [dB] |
Therefore Lmin is provided by applying above
(1)-(3) to Graph 1-2 as follows:
Graph 1-2
So, a Lneed for each frequencies,
acquired by substitution of aL on (3) and Lmin on (4)
into (1), corresponds to the Lneed in the table of draft
amendments to 1(c) of the annex 3 of the COLREGs.
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I:/MSC/69/20-4
