The efficiency of a sound-absorbing material can be affected by its distribution and location in a room. 25 panels of sound-absorbing material, for example, each 2′×2′, will absorb more sound energy per panel when spaced in a “checkerboard” pattern on a 200 sq ft plaster ceiling than a uniform coverage of the same material.
This increase in efficiency (called the area effect) is due to the diffraction of sound energy around the perimeters of the spaced sound-absorbing panels and to the additional sound absorption provided by the exposed panels’ edges. The efficiency of sound-absorbing panels increases as the ratio of the perimeter to surface area increases.
The 25 spaced absorbers have a ratio of perimeter to surface area 5 times the ratio for the 25 uniform-coverage absorbers. Sound energy reflected from the hard-surfaced plaster adjacent to the absorbent edges in the checkerboard configuration tends to spill over onto the sound-absorbent panels.
Therefore, the spaced absorbing material absorbs more sound energy than would be accounted for by its area. This kind of surface treatment also can be used to achieve a diffuse sound field, which is desired in music practice rooms. Note that the total absorption contributed by spaced absorbers in this example will be only slightly less than the absorption provided by coverage of the entire 200 sq ft ceiling.
References
T.W. Bartel. “Effect of Absorber Geometry on Apparent Absorption Coefficients as Measured in a Reverberation Chamber”, Journal of the Acoustical Society of America, April 1981.
Reprinted from the 1988 edition of Architectural Acoustics with the kind permission of Author, David Egan.
Note: Assuming a 1″ thickness of the sound-absorbing panels in the foregoing example, the sound absorbing area of the panel edges contribute an additional 23% to the 100 sq ft surface area shown.
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