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Absorption

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|² – energetic reflection coefficient

Tested Frequency Range

  • Small impedance tube (29 mm diameter): 200 to 6600 Hz
  • Medium impedance tube (44.44 mm diameter): 100 to 4300 Hz
  • Large impedance tube (100 mm diameter): 35 to 1900 Hz

Comments

The link between the complex reflection coefficient and the sound absorption coefficient is α=1-|R|², where |R|² is the energetic reflection coefficient.

 

                                                                    
                    
                                                                                          Impedance tube                                       Sound absorption curves


                               
[1] J.Y. Chung and D.A. Blaser, “Transfer function method of measuring in-duct acoustic properties: I. Theory,” J. Acoust. Soc. Am. 68(3), 907-913 (1980).
[2] J.Y. Chung and D.A. Blaser, “Transfer function method of measuring in-duct acoustic properties: II. Experiment,” J. Acoust. Soc. Am. 68(3), 907-913 (1980).