Formula for sound dissipation in air?

Is there an equation that deals with the dissipation of sound waves in air? As in, how much they die out with distance, because we know they must, otherwise you could hear me talking now!
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 The intensity is inversely proportional to the radius squared. That is, if a certain source has a power P and it emits pressure waves spherically, the intensity I you'll hear is $$I = \frac{P}{4\pi r^2}$$
 Recognitions: Gold Member Science Advisor There is also some absorption, which depends on frequency (proportional to frequency squared) and humidity. This produces an attenuation which is proportional to distance (as opposed to inverse square law). Both effects occur at the same time. See this link.

 Tags dissipation, medium, physics, sound, waves