ABSORPTION COEFFICIENT

The fraction of energy which is absorbed on striking any surface. It therefore takes values between 0 and 1, and is usually frequency dependent. In room ACOUSTICS, this coefficient, given the symbol a, is expressed as a fraction of the perfect ABSORPTION at an open window of equal area. A table such as the following might be consulted for choosing suitable materials for SOUND INSULATION in a room or building.

Chart of Absorption Coefficients

 Material 128 Hz 256 Hz 512 Hz 1,024 Hz 2,048 Hz 4,096 Hz Author Draperies hung straight, in contact with wall, cotton fabric, 10 oz. per square yard 0.04 0.05 0.11 0.18 0.30 0.44 P.S. The same, velour, 18 oz. per square yard 0.05 0.12 0.35 0.45 0.40 0.44 P.S. Same as above, hung 4 inches from wall 0.09 0.33 0.45 0.52 0.50 0.44 P.S. Felt, all hair, contact with wall 0.13 0.41 0.56 0.69 0.65 0.49 P.S. Rock wool (1 inch) 0.35 0.49 0.63 0.80 0.83 ------ V.K. Carpet on concrete (0.4 inch) 0.09 0.08 0.21 0.26 0.27 0.37 B.R. Carpet, on 1/8 inch felt, on concrete (0.4 inch) 0.11 0.14 0.37 0.43 0.27 0.27 B.R. Concrete, unpainted 0.010 0.012 0.016 0.019 0.023 0.035 V.K. Wood sheeting, pine (0.8 inch) 0.10 0.11 0.10 0.08 0.08 0.11 W.S. Brick wall, painted 0.012 0.013 0.017 0.020 0.023 0.025 W.S. Plaster, lime on wood studs, rough finish (1/2 inch) 0.039 0.056 0.061 0.089 0.054 0.070 P.S.

P.S.: P.E. Sabine; V.K.: V.0. Knudsen; B. R.: Building Research Station, England; W.S.: Wallace Sabine.

Absorption coefficients for various building and insulation materials as a function of frequency (after Olson, Music, Physics and Engineering, Dover, 1967, p. 271, used by permission).

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