Volume’s effect on volume

I agree with Erica Ryherd, who stated in her Quick Study on acoustical design (Physics Today, August 2008, page 68 ) that small spaces can result in high sound levels that detract from the spaces’ proper use. However, Ryherd repeats the mistaken belief held by many acoustical design practitioners that the volume of a space “directly influences” the overall sound energy in the space.

Ryherd uses classical diffuse field theory as the basis for the metrics—most notably reverberation time—in her discussion. That theory says that the sound energy in an enclosure is influenced only by the power output of the source and the total sound absorption of the enclosure. She mentions that same absorption as an independent variable in her definition of reverberation time. The volume independence occurs essentially because the absorption of sound energy takes place only at the enclosure’s boundaries.

Simply put, the two ways to reduce the sound level in a space are to either turn down the volume of the source or add absorption. The room’s dimensions have nothing to do with it.