MEASUREMENT OF ABSORPTION COEFFICIENT 275 



beration time. Sabine used an organ pipe as a source of sound, the ear 

 as a detector and a stop watch for measuring the time. Since that time 

 various types of chronographs, reverberation bridges, commutators, relays, 

 etc., have been developed to measure the reverberation time of an 

 enclosure. 



The high speed level recorder. Fig. 11.7, and the high speed level indi- 

 cator. Fig. 11.8, have been found to be the most useful means for mea- 

 suring reverberation time because the trace of the entire decay of the sounds 

 may be examined. From the rate at which the abscissa advances and 

 the magnitude of the ordinate the rate of decay may be computed. 



11.9. Measurement of Absorption Coefficient 40,41,42,43,44, _ xhe 

 acoustic absorption coefficient of a surface is the ratio of the rate of sound 

 energy absorbed by the surface to the incident rate of flow. All directions of 

 incidence are assumed to be equally probable. The sabin is a unit of 

 equivalent absorption and is equal to the equivalent absorption of one 

 square foot of a surface of unit absorptivity, that is, one square foot of 

 surface which absorbs all incident sound energy. 



The total absorption in a room may be obtained from equations 12.2 

 or 12.3, if the reverberation time and the dimensions of the room are known. 

 This method of obtaining the absorption coefficient of materials has been 

 considered to yield the most reliable results. 



Specialists in the measurement of absorption coefficients have used 

 large chambers (volume of 4000 to 20,000 cubic feet) for determining the 

 absorption coefficient of materials from the reverberation time. The 

 reverberation time of these chambers, when empty, is from five to ten 

 seconds. In chambers of this kind the absorption coefficients of very small 

 samples may be determined. The absorption coefficients of representative 

 materials obtained under the above conditions by various investigators 

 are shown in Table 12.1, Sec. 12.2yf. 



In this connection it is interesting to note that there is considerable dis- 

 crepancy between the values of absorption coefficients obtained in rever- 

 berant chambers and those obtained in actual use in rooms, studios and 

 theatres. For this reason the values given in Table 12.1, Sec. 12.2^, must 



4° Watson, F. R., "Acoustics of Buildings," John Wiley and Sons, New York, 1923. 



41 Bagenal and Wood, " Planning for Good Acoustics," Methuen, 1931. 



42 Knudsen, V. O., " Architectural Acoustics," John Wiley and Sons, New York, 

 1932. 



43 Sabine, P. E., " Acoustics and Architecture," McGraw Hill Book Co., New 

 York, 1932. 



44 Olson and Massa, " Applied Acoustics," 2nd Ed., P. Blakiston's Son and Co., 

 Philadelphia, 1939. 



