(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 dL for propagating in air from D0 to D1. Therefore Lneed, Lmin and dL have a relation as below:
Lneed = Lmin + dL (1)
where the amount of decrease of the sound pressure level for propagating from D0 to D1 could be shown as follows:
dL = 10 log (D1/D0)^2 (2)
Substitution of 1m into D0 and of 926m (0.5 miles) into D1 respectively, Equation (2) becomes
dL = 60[dB] (3)
From Equation (3), it becomes clear that we have to take into account that the sounds need at least 60dB for propagating in air from D0 to D1.
On the other hand, Lnoise is prescribed in Annex III 1 (c) dependent on the centered frequencies. Average background noise level in the octave band centered on 250Hz and 500Hz is 68dB, 63dB. Moreover, average background noise level in the octave band centered 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 centered on 250Hz:
L'noise = Lnoise - 10 log (df [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 centered on 500Hz:
L'noise = 63 - 10 log (354)
= 38 [dB]
(3) where a background noise level is 60dB in the octave band centered on 800Hz:
L'noise = 60 - 10 log (566)
= 32.5 [dB]
Therefore Lmin is provided by applying above (1)-(3) to graph I-2 as follows:
