Acoustic Intensity Level (LI), Acoustic Power Level (LW), and Acoustic Pressure Level (LP)
Acoustic Intensity Level, LI
The acoustic intensity Ia (the acoustic power per unit of area—usually in W/m2 or W/cm2) is found by
Acoustic Power Level, LW
The total acoustic power can also be expressed as a level (LW):
Acoustic Pressure Level, LP
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 4πr2, this yields a sphere with a surface area of 1 m2.) An omnidirectional point source radiating one acoustic watt is placed into the center of this sphere. Thus we have, by definition, an acoustic intensity at the surface of the sphere of 1 W/m2. From this we can calculate the Prms:
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 find:
Thus the LP, LI, and LW at 0.282 m are the same numerical value if the source is omnidirectional (see Figure 2.4).
