In close-fitting enclosures, noise reduction may be degraded by yet another resonance effect.
At frequencies where the average air spacing between a vibrating machine surface and enclosure
wall is an integral multiple of half wavelengths of sound, strong coupling will occur between the
vibrating machine surface and the enclosure wall, resulting in a marked decrease in the
enclosure wall transmission loss.
The effect of inadequate absorption in enclosures is very noticeable. Table 10.2 shows the
reduction in performance of an ideal enclosure with varying degrees of internal sound
absorption. The sound power of the source is assumed constant and unaffected by the enclosure.
"Percent" refers to the fraction of internal surface area which is treated.