Problem
The equation I = Pav / 4?r2 is predicated on the assumption that the transmitting medium does not absorb any energy.
It is known that absorption of sound by dry air results in a decrease of intensity of approximately 8 dB/km.
The intensity of sound at a distance of 120 m from a jet engine is 130 dB.
Find the intensity at 2.4 km from the jet engine (a) assuming no absorption of sound by air, and (b) assuming a diminution of 8 dB/km. (Assume that the sound radiates uniformly in all directions.)
(a) ?SL = 10 log(I1/I0)-10 log (I2/I0)= 10 log(I1/I2) = 10log(r22/r12)=20log(r2/r1)
20 log (20) = 26; SL at 2.4 km = (130 - 26) dB = 104 dB
(b) Subtract 2.28 x 8 dB from result of (a)
SL= (104 - 18.2) dB = 85.8 dB