Page 272 - Master Handbook of Acoustics

For example, consider a piece of 1/4-in plywood spaced out from the wall on 2 × 4 studs on edge,
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which yields an airspace of about 3-3/4 in behind. The surface density of 1/4-in plywood, 0.74 lb/ft ,
can be measured or found in references. Substituting, we obtain a resonant frequency of about 102 Hz.

Figure 12-22 shows panel resonant frequencies as a function of cavity depth in inches, and surface
density in ounces per square foot. Knowing the thickness of the plywood and the depth of the space
behind the plywood, the frequency of resonance can be read off the diagonal lines. Equation (12-6)
applies to membranes and diaphragms of materials other than plywood such as Masonite, fiberboard,
or even Kraft paper. For other than plywood, the surface density must be determined. The surface
density is easily found by weighing a piece of the material of known area. The surface area of a panel

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absorber should be at least 5 ft .