STEADY STATE LOUD SPEAKER MEASUREMENTS 139 



where a is the absorbing power of the room obtained from the sum of the 

 products of the areas of the absorbing surfaces in the room and their 

 respective absorption coefficients. From (7) and (8) the acoustic 

 power delivered by the loud speaker could then be expressed in terms 

 of the excess r.m.s. pressure throughout a large room as follows: 



and the efficiency of the loud speaker would therefore be 





ill 



4r 



(10) 



K' I \ \ pHv 



Response. — By determining the mean square pressure at all points 

 in the measuring room or at all points on the surface of a large sphere 

 in an infinite medium as discussed above, it is therefore possible to 

 measure the "efficiency" of a loud speaker with a pressure indicating 

 device. Such a method for determining the merits of a loud speaker at 

 all frequencies of usual interest, however, would obviously be quite 

 impracticable. Furthermore, unless the radiation from the loud 

 speaker is uniform over a spherical surface it is not of particular interest 

 to know the magnitude of the total acoustic power or the value of the 

 quantity rj since the configuration of the sound field about a loud 

 speaker may change decidedly with frequency, with the result that 

 variations in sound loudness at different frequencies in a particular 

 region may be large even though the total power output from the loud 

 speaker may be constant. In order then that the measured charac- 

 teristic shall convey a true idea of the performance as it might be 

 observed by the ear, the square of the pressure at one representative 

 listening position or the average of the squares of the pressures in a 

 small region wherein an observer might normally be located may be 

 considered instead of the average throughout the room. In this 

 manner a sort of specific efficiency measure would be obtained in that 

 it is a measure of the efficiency with respect to the acoustic power 

 transmitted through the specified position or region. Throughout the 

 remainder of this paper, this specific measure of the efficiency is called 



