I'm not sure I understand what you mean. Are you talking about the sound transmission properties of free air at a given frequency over a long distance?

Do you mean how much air is compressed measured in pascals? Check the [[url=http://[/URL]="http://en.wikipedia…"]wikipedia on SPL[/]="http://en.wikipedia…"]wikipedia on SPL[/]

To me it sounds like you are suggesting that air acts as a signal level compressor.

I'm interested in how when you move a mic away from an instrument you hear less transient detail. This must mean that the transient part of the wave has been dampened by the air. I was wondering if this effect has been researched. Exactly which frequencies are less prone to this damping effect? How many ms does the damping occur last at different frequencies? What other things does it depend on?

What you are looking for are models of damping, not compression. Try searching for terms like "frequency dependent damping." There are some standard modifications to the wave equation that exhibit this, but I'm not sure how accurate they are. A quick search shows you a lot of research papers in the area. (Most of them on solids, however.) All of the models I am aware of exhibit a monotone increase of damping with frequency (and usually the dependence is pretty simple).

Sound travels better in a humid environment compared to a dry environment. The reverb in church is longer in the humid summertime and shorter in the dryer winter months. So humidity is a factor.

Sound travels faster through water & concrete than it does through air.

Recording bricks breathing
Ms. Remy Ann David

_>>>Exactly which frequencies are less prone to this damping effect?

A snippet--->

"The amount of absorption depends on the frequency of the sound. A high frequency sound has many cycles in a second, and the particles in the medium are therefore vibrating very rapidly. Just as when you rub your hands together very rapidly, this produces more heat than if you rub your hands together slowly. Since the molecules get their energy to vibrate from the sound wave, the sound wave will run out of energy sooner when it is a high frequency sound. This means that, under the same conditions, a high frequency sound won't travel as far as a low frequency sound."
http://www.dosits.org/science/sndmoves/2b.htm

Great Topic By the way!

I think I get it.

A salt water bass-trap would be better then a fresh water bass-trap!

Another, much more basic explanation of why transient detail decreases with more distant mic placement is the logarithmic decay of sound energy. Remy is right that sound travels faster in humid air, but that applies at any given temperature. Given the same humidity level, sound travels faster in cold air than warm air since the air molecules are more densely concentrated. Airplanes get more lift in cold air for the same reason.

jonyoung wrote: more distant mic placement is the logarithmic decay of sound energy.

Isn't it related to 1/d^2, not logrithmic (log(d))?

Another reason is that the convolution of the echo's create loss of detail of the transients.