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Normal incidence sound absorption and reflection coefficients
The normal incidence sound absorption coefficient indicates the part of the acoustical energy of the incident wave that is absorbed by the tested sample in a given configuration (ex: backed by a rigid end, backed by an air cavity, backed by an anechoic termination); what is not absorbed is reflected back to the source side. The reflection coefficient is the counter part of the absorption coefficient. Both are a useful acoustical indicators to evaluate the acoustical performance of a material to minimize reflections.
Equipment: Impedance tube
Standard: ASTM E 1050, ISO10534–2
Method: Impedance tube and the transfer-function method [1,2]
Property: α - normal incidence sound absorption coefficient; R - complex reflection coefficient; |R|˛ - energetic reflection coefficient
Tested frequency range
- Small impedance tube (29 mm diameter) : 500 to 6 000 Hz
- Medium impedance tube (44.5 mm diameter): 300 to 4 100 Hz
- Large impedance tube (100 mm diameter): 50 to 1 600 Hz.
Comments
The link between the complex reflection coefficient and the sound absorption coefficient is α=1-|R|˛, where |R|˛ is the energetic reflection coefficient.
Impedance tubes
Sound absorption curves