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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 α, 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.
Compare: Noise Reduction Coefficient, Noise Reduction Factor, Sound Transmission Coefficient. See also: Reflection Coefficient, Reverberation, Soundproof.
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Material |
128 Hz |
256 Hz |
512 Hz |
1,024 Hz |
2,048 Hz |
4,096 Hz |
Author |
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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. |
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The same, velour, 18 oz. per square yard |
0.05 |
0.12 |
0.35 |
0.45 |
0.40 |
0.44 |
P.S. |
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Same as above, hung 4 inches from wall |
0.09 |
0.33 |
0.45 |
0.52 |
0.50 |
0.44 |
P.S. |
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Felt, all hair, contact with wall |
0.13 |
0.41 |
0.56 |
0.69 |
0.65 |
0.49 |
P.S. |
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Rock wool (1 inch) |
0.35 |
0.49 |
0.63 |
0.80 |
0.83 |
------ |
V.K. |
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Carpet on concrete (0.4 inch) |
0.09 |
0.08 |
0.21 |
0.26 |
0.27 |
0.37 |
B.R. |
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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. |
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Concrete, unpainted |
0.010 |
0.012 |
0.016 |
0.019 |
0.023 |
0.035 |
V.K. |
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Wood sheeting, pine (0.8 inch) |
0.10 |
0.11 |
0.10 |
0.08 |
0.08 |
0.11 |
W.S. |
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Brick wall, painted |
0.012 |
0.013 |
0.017 |
0.020 |
0.023 |
0.025 |
W.S. |
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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).