254 MEASUREMENTS 



A. Response Frequency Characteristic. 

 1. Response ^'*. — The response of a loud speaker is a measure of the 

 sound produced at a designated position in the medium with the electrical 

 input, frequency and acoustic conditions specified. 



Absolute response is the ratio of the sound pressure (at a specified point 

 in space) to the square root of the apparent electrical power input. It is 

 given by the equation 



Absolute response = = 11.8 



e e 



■Vze 



where p = measured sound pressure, in dynes per square centimeter, 

 e = effective voltage applied to the voice coil, in volts, and 

 Ze = absolute value of the impedance of the voice coil, in ohms 

 {ze is a function of frequency). 



The absolute response characteristic is obtained by measuring the 

 sound pressure ^ as a function of frequency with constant voltage e on 

 the voice coil, and measuring impedance Ze as a function of the frequency, 

 and correcting the measured sound pressure for the measured impedance 

 in accordance with the equation. The resulting characteristic represents 

 the sound pressure as a function of the frequency which would be obtained 

 from the speaker if fed from the generator which would automatically 

 dehver constant apparent power c'^/ze to the voice coil over the frequency 

 range. 



The response may be expressed by a value equal to the above ratio or 

 may be expressed in decibels relative to an arbitrary value of response 

 corresponding to one volt, one ohm and one dyne per square centimeter. 



Ze 



/■\/ze 



Absolute response = 20 logio — z = 20 logio ^ ,— 11.9 



i/VI 



The outdoor or free space response frequency characteristic is the 

 simplest method for making loud speaker measurements which may be 

 duplicated and compared among laboratories. Outdoor measurements 

 have the disadvantage of being dependent upon the weather and noise con- 

 ditions and, for this reason, nearly all development and routine work on 



" Standards on Electroacoustics, Institute of Radio Engineers, 1938. 



