Craig,
Sound intensity decreases with the square of the distance. For example, going from 3 feet away to 6 feet away causes the sound power to be reduced by 6 dB. dB is a log scale, each change of 3dB represents twice the power (+3 dB) or half the power (-3dB). The chart linked to below show the relative change in dB relative to distance.
SOUND PRESSURE LEVELS vs DISTANCE
http://www.warehousesound.co.uk/Hire...versusdist.htm
Humans hear with their own log scale. To a person, something sounds twice/half as loud when there is a change of 10 dB. In addition, the perceive intensity of sound in humans changes both with frequency and volume. This was empirically measured by Bell labs.
Fletcher-Munson Curves
http://www.webervst.com/fm.htm
Hope this was what you were looking for.
Regards,
Jim