Lecture 14B Intensity

Lecture 14B Intensity - EEL 4930 Audio Engineering Lecture...

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EEL 4930 Audio Engineering Lecture 14B Page 1 of 2 Intensity Intensity for a plane wave is the power per unit area carried by the wave, in watts per square meter. Since the wave impedance z of the medium is essentially real, the energy transmitted by a sinusoidal plane wave of pressure amplitude p and particle velocity v is I = 1 2 pv = 1 2 p 2 / z = 1 2 v 2 z per unit area, the factor ½ arising because p is an amplitude rather than an RMS value. On a log scale we may define an intensity level L ( I ) = 10log 10 ( I / I ref ) dB The reference intensity I ref is 10 -12 W/m 2 , which is the approximately the intensity of a wave with RMS pressure of p ref = 20 μ Pa. Thus for a plane wave in air the intensity level is approximately equal numerically to the sound pressure level. As an example consider a person speaking with an acoustic power output of one milliwatt, a typical value. If this intensity is measured at a distance of r = 2 m, then this power is spread over the area of a sphere, 4 π r 2 = 50.2 m 2 . This gives an intensity of 2 x 10 -5 W/m 2 . This translates to
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This note was uploaded on 02/08/2011 for the course EEL 4930 taught by Professor Staff during the Spring '08 term at University of Florida.

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Lecture 14B Intensity - EEL 4930 Audio Engineering Lecture...

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