140 BELL SYSTEM TECHNICAL JOURNAL 



the "response" of the loud speaker as measured at a specified position 

 or positions and is expressed in transmission units (TU) relative to a 

 fixed arbitrary reference condition of 1 volt, 1 ohm, and 1 bar. In 

 other words the acoustic power density at a specified position or the 

 average acoustic power density at specified positions in the medium 

 produced by the loud speaker under test per unit of available electrical 

 supply power, is expressed relative to the acoustic power density 

 produced by a fictitious standard loud speaker which when placed in 

 the location of the loud speaker under test will produce a mean square 

 pressure of one unit at the specified position or positions in the room 

 when the ratio 



e^ 

 -= 1. 



r 



The response in TU is thus expressed by the relation : 



t _ 



TU= 101ogio-^= lOlogio^' 



= 20 1ogioy, 



where p is the r.m.s. pressure at a specified position or the square root 

 of the mean square r.m.s. pressures at specified positions in the medium; 

 and e and r are as defined above. 



Measuring Considerations in a Reflectionless Medium.— -In a medium 

 where there are no sound reflections from the bounding surfaces, two 

 factors are most likely to cause the measured response of a loud 

 speaker to vary with frequency. These factors are independent and 

 their effects of about equal importance. The first and most apparent 

 is the inherent dynamical characteristics of the loud speaker which 

 involves its ability to transfer maximum power from the electrical 

 supply to the acoustic medium. Any variation with frequency in the 

 acoustic power output of a loud speaker when supplied by constant, 

 available electrical power will, of course, cause corresponding variations 

 in the response provided the square of the pressure at the measuring 

 position is indicative of the power transmitted through this position. 

 In order that this latter condition be strictly true, the measuring 

 position in general should be, at a distance from the loud speaker, 



