Audio Engineering

Measurement 45

2.6.2 Acoustic Power Level, L W

The total acoustic power can also be expressed as a level ( LW ):

L

Total acoustic watts

W ^10 log 10  (^12) W. (2.24)
2.6.3 Acoustic Pressure Level, L P
To identify each of these parameters more clearly, consider a sphere with a radius of
0.282 m. (Since the surface area of a sphere equals 4 πr^2 , this yields a sphere with a
surface area of 1 m^2 .) An omnidirectional point source radiating one acoustic watt is
placed into the center of this sphere. Thus we have, by defi nition, an acoustic intensity at
the surface of the sphere of 1 W/m^2. From this we can calculate the Prms :
PWcrms (^10) a ρ (2.25)
where Wa is the total acoustic power in watts and ρc equals 406 RAYLS and is called the
characteristic acoustic resistance.
Knowing the acoustic watts, Prms is easy to fi nd:
PWrms a


10 406

20 15.Pa.

Thus the LP must be

Lp



20

20 15

20

120

log

.Pa

Pa

dB.

μ

and the acoustic power level in LW must be

LW 



(^10) 

1

10

120

log 12

W

W

dB.

Thus the LP , LI , and LW at 0.282 m are the same numerical value if the source is
omnidirectional (see Figure 2.4 ).