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Physical Audio Signal Processing
    Elementary Physics, Mechanics, and Acoustics
       Properties of Gases
          Air Absorption

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Air Absorption

This section provides some further details regarding acoustic air absorption [326]. For a plane wave, the decline of acoustic intensity as a function of propagation distance $ x$ is given by

$\displaystyle I(x) = I_1 e^{-x/\xi}, $

where

\begin{eqnarray*} I(x) &=& \hbox{intensity $x$\ meters from the source (\sref {... ...n frequency, temperature, humidity}\\ & & \hbox{and pressure).} \end{eqnarray*}

Tables F.1 and F.2 (adapted from [322]) give some typical values for air.


Table: Attenuation constant $ m = 1/\xi $ (in inverse meters) at 20tex2html_wrap_inline^&cir#circ; C and standard atmospheric pressure
Relative Frequency in Hz
Humidity 1000 2000 3000 4000
40 0.0013 0.0037 0.0069 0.0242
50 0.0013 0.0027 0.0060 0.0207
60 0.0013 0.0027 0.0055 0.0169
70 0.0013 0.0027 0.0050 0.0145



Table: Attenuation in dB per kilometer at 20tex2html_wrap_inline^&cir#circ;C and standard atmospheric pressure.
Relative Frequency in Hz
Humidity 1000 2000 3000 4000
40 5.6 16 30 105
50 5.6 12 26 90
60 5.6 12 24 73
70 5.6 12 22 63


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written by Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.