280 ARCHITECTURAL ACOUSTICS 



millionth of its original intensity is 



V 

 T = .050 - 12.2 



A 



where T = time, in seconds, 



V = volume, in cubic feet, and 



A = total absorption, in sabins. 

 Later work ^ has shown that equation 12.2 is unsatisfactory for large 

 rooms or rooms with very large absorption. The equation developed by 

 Eyring is 



■OSF 



— S loge(l — aav) 



12.3 



where F = volume, in cubic feet, 



6' = total area, in square feet, and 

 aav = average absorption per square foot, in sabins. 



A tabulation of sound absorption coefficients for various building 

 materials and objects is shown in Table 12.1. The coefficients in this 

 table were obtained upon small samples in chambers having a long rever- 

 beration time. In general, these measurements do not agree with those 

 obtained under actual conditions in practice. That is, field measurements 

 yield smaller values than laboratory measurements. However, the values 

 of Table 12.1 show the relative absorption coefficients of the various 

 materials. For a complete resume of this subject see the Anniversary 

 issue of the Journal of the Acoustical Society of Atnerica, Vol. 11, No. 1, 

 Part 1, July, 1939. 



There are a number of methods available for measuring the decay of 

 sound in a room. Sabine and others have used an organ pipe and stop 

 watch and have determined by ear the time required for the sound to decay 

 to one millionth of its original intensity. At least two dozen instrumental 

 methods have been developed for the measuring of the reverberation time 

 of a room. At the present time, high speed level indicators and recorders 

 appear to be the most suitable means for obtaining the reverberation time 

 of a room. See Sec. 11.3^^2. 



The articulation^ (see Sec. 11.12) of unamplified speech in auditoriums 

 of various sizes as a function of the reverberation time is shown in Fig. 12.1. 



8 Eyring, C. F., Jour. Acous. Soc. Amer., Vol. 1, No. 2, p. 217, 1930. 



9 Knudsen, V. O., Jour. Acous. Soc. Amer., Vol. 9, No. 3, p. 175, 1938. 



