The Bell System Technical Journal 



January, 1928 



The Measurement of Acoustic Impedance and the 

 Absorption Coefficient of Porous Materials 



By E. C. WENTE and E. H. BEDELL 



Synopsis: Various ways of determining the acoustic impedance and the 

 absorption coefficient of porous materials from measurements on the stand- 

 ing waves in tubes are discussed. In all cases the material under investiga- 

 tion is placed at one end of the tube and the sound is introduced at the other 

 end. Values of the coefficient of absorption of a number of commonly used 

 damping materials as obtained by one of the methods are given. Several 

 types of built-up structures are shown to have a greater absorption coefficient 

 for low frequency sound waves than is conveniently obtainable by a single 

 layer of material. 



THE most commonly used method of determining the sound ab- 

 sorption coefficient of a material is that devised by the late 

 Professor W. C. Sabine. In this method the reverberation time of a 

 room is measured before and after the introduction of a definite amount 

 of the material. This method has the great merit that the values so 

 determined usually apply to the materials precisely as they are 

 ordinarily used in rooms for damping purposes. However, it is tedious 

 and requires a very quiet room and large samples of the materials. 

 A simpler scheme has been devised by H. O. Taylor/ in which the 

 absorbing material is placed at one end of a tube. The coefficient of 

 absorption is determined from a measurement of the ratio of maximum 

 to minimum pressures of the standing waves within the tube when 

 sound is introduced at the open end. Thus only a small sample of the 

 material is required and with suitable apparatus the measurements can 

 be made with great facility. In this paper several modifications of 

 Taylor's tube method are discussed; in addition, it is shown that by a 

 similar method it is possible to determine not only the absorption co- 

 efficient but also the acoustic impedance, a quantity which is playing 

 an important part in present day applied acoustics. 



General Theory 



Consider a tube of length /, which is filled with a medium having a 

 propagation constant P = a + «jS and a characteristic acoustic im- 



^Phys. Rev., II, 1913, p. 270. 



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