4159 (RP-755) -- Sound Pressure Level vs. Distance from Sources in Rooms PDF

Sound fields in furnished rooms are not diffuse and so do not conform to diffuse-field theory. In an earlier ASHRAE research project, RP-339, sound pressure levels in a variety of normally furnished rooms were found to be reduced by 3 decibels each time the distance from the source was doubled. No dependence on room absorption was found. Under the conditions of the project reported here, RP-755, the reduction in level with increasing distance for a source in the room was found to depend on the inverse of the room reverberation time. The approximate relationship found was attenuation = 0.9/RT +0.5 dB/distance doubling for 250 Hz and above. However, for furnished rooms, which are usually found to have reverberation times around 0.5 second, the attenuation is still very close to 3 dB per doubling of distance from the source, the value found in RP-339. When sources were in the plenum above the ceiling, the sound fields in the room below were, for practical purposes, uniform. Some attenuation was seen at 2 and 4 kHz, but it was small.

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