8. Sound Intensity Level

Another quality described by a decibel level is sound intensity, which is the rate of energy flow across a unit area. The reference for measuring sound intensity level is Io = 10-12 Watt/m2, and the sound intensity level (IL or Li,) is defined as :

 

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For a free progressive wave in air (e.g., a plane wave travelling down a tube or a spherical wave travelling outward from a source), sound pressure level and sound intensity level are nearly equal () This is not true in general, however, because sound waves from many directions contribute to sound pressure at a point. When we speak of simply sound level, we nearly always mean sound pressure level, Lp, since that is what is indicated by our sound-measuring instruments.