VRIJE UNIVERSITEIT BRUSSEL Acoustics - the Dept. of ...

1.7. SOURCE POWER 21with W 0 = 10 −12 W the reference level.One have to pay attention to the terms we use because all those differentlevels have the same units (dB).1.7.2 The sound field produced by a point sourceFor a spherical source the source power is W = I4πr 2 , with I the intensityaccording to the radius of this sphere on a distance r from the source. In adistant sound field r ≫ λ we can write I = p2 effρcand thus :W = p2 eff 4πr2ρc(1.38)from which one can get an expression for the effective sound pressure, measuredat a distance r from the source :√Wρcp eff =(1.39)4πr 2With known source acoustic power the sound pressure can be calculatedin a given point. The effective sound pressure decreases with increasingdistance 1 . One may rewrite Equation 1.39 as a function of values in dB.r√412Wp eff = ρc = 400 for air4πr 2⇒ p 2 eff = 412W4πr 2⇒p2 eff 412W=400∗10 −12 4πr 2 400∗10 −12⇒ L p = L W −10log(4πr 2 )For r = 1 one has approximately :L p = L W −10 (1.40)