Air Absorption

The intensity of a plane wave is observed to decay exponentially according to

$$I(x) = I_0\, e^{-x/\xi}$$

where

$$\begin{eqnarray*} I_0 &=& \hbox{intensity at the plane source (\textit{e.g.}, a vibrating wall)}\\ I(x) &=& \hbox{intensity $x$\ meters from the plane-source}\\ \xi &=& \hbox{intensity decay constant ($1/e$\ distance in meters)}\\ & & \hbox{(depends on frequency, temperature, humidity}\\ & & \hbox{and pressure)} \end{eqnarray*}$$

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

$\parbox{5in}{\emph{Attenuation} in dB per kilometer at 20\mbox{${}^{\circ}$}C and standard atmospheric pressure.}$