136 BELL SYSTEM TECHNICAL JOURNAL 



supply. Since in power transmission systems the purpose of a machine 

 is to draw a limited amount of power from a relatively unlimited source 

 and to deliver this power to a load with a minimum loss in the machine 

 itself, this ratio constitutes a useful measure of the performance. If it 

 were of interest a similar quantity could likewise be used as a measure 

 of the performance of a loud speaker. In this latter case however, the 

 function is not in general to draw a limited amount but as much power 

 as possible from a supply source and to radiate maximum power to the 

 air or load. A measure of the efficiency would therefore have to 

 involve the ability of the loud speaker to take maximum power from 

 the supply and might be defined as the ratio of acoustic power Pa 

 radiated to the maximum electrical power Pe which the supply circuit 

 is capable of delivering under optimum impedance conditions. Thus 

 the efficiency ?? at a specified frequency would be defined by the ratio 



Assuming the impedance of the electrical supply source for a loud 

 speaker essentially non-reactive (as is almost invariably the case) 

 and of a constant magnitude r suitable to the requirements of the loud 

 speaker, then the maximum power which the supply circuit is capable 

 of delivering under optimum impedance conditions with an open 

 circuit supply voltage e would be 



P':=ir- (2) 



These quantities are all readily measureable. The determination of 

 the quantity Pa however is more difficult. 



For measuring the acoustic energy or power stored in or transmitted 

 through the medium adjacent to a loud speaker, the condenser trans- 

 mitter is probably the most suitable free space acoustic measuring 

 device. The ruggedness of this transmitter for an instrument of this 

 type and the straightforward manner in which it can be used recom- 

 mend it for practical loud speaker measurements. The condenser 

 transmitter is not, however, an acoustic power indicating device but 

 is a device having a high impedance compared to the impedance of the 

 acoustic system in which it is used. It is therefore an acoustic 

 measuring device which is analogous to an electrical voltmeter and can 

 be calibrated by the thermophone ^ or other means to measure the 

 excess pressure in a medium resulting from a sound wave. Other 

 acoustic measuring devices such as the Rayleigh disc, thermal devices 



1 "The Thermophone," E. C. Wente, Physical Review, Vol. XIX, No. 4, April, 

 1922. 



1 



