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The effect of perforated facings of a small percent open area on the specific acoustic resistance of backing materials of appreciable thickness was investigated for several backing materials. The resistance was found to increase by factors of 2 to 7 depending on the percent open area of the facing and the resistance of the backing material. The increase was considerably less in all cases than 1/p, where p is the open area ratio of the facing. It was found that the specific acoustic resistance of a resonant absorber can be varied considerably above and below the resistance of the uncovered backing by means of an air space between the facing and the porous backing.
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Callaway et al. (1952) studied this question.
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