TESTING OF LOUD SPEAKERS 



259 



obtain the factor ^/^ in the formula for absolute response (equation 11.8) 

 is shown in Fig. 11.10. This arrangement has the feature that it does not 

 require an absolute calibration of the measuring system. 



GENERATOR 



LOUD SPEAKER MICROPHONE 



L-IA'VWV 



—WW^j—^o- 



MEASURING 

 SYSTEM 



Fig. 11.10. Schematic arrangement for obtaining the inctor p/e in the formula for absolute 



response. 



Referring to Fig. 11.10, the absolute response is given by 



Absolute response, in decibels, = 20 logio - V z^ 



= [A - B - C -T)] + \Q logio Ze 11.10 



where A = output of measuring system, in decibels, with the microphone 

 picking up sound from the loud speaker with iS" open, 

 B = output of measuring system, in decibels, with S closed and the 



microphone shielded from sound, 

 C = open circuit voltage output of the microphone, in decibels 

 above one volt for one dyne per square centimeter, in a 

 free progressive wave, 

 D = 20 log ^i/^2 = 20 log (r^iH- rE2)/rE2, and 

 Ze = impedance of loud speaker, in ohms. 

 rE2 should be sufficiently small compared to the impedance of the micro- 

 phone or, in other words, the output of the microphone should not change 

 when rE2 is short circuited, rsi should be so selected as to obtain a value 

 of B in the range of the values obtained for A. 



4. Outdoor or Free Space Response ^^. — In all outdoor tests the loud 

 speaker and microphone should be removed a sufficient distance from 

 reflecting surfaces so that the only sound striking the microphone is 

 direct sound (fifteen to twenty feet from the ground or other reflecting 

 surfaces). Any microphone calibrated in terms of plane progressive wave 

 pressure may be used. 



For the purpose of these tests, loud speakers are divided into two 

 classes. The first class includes direct radiator loud speaker units which 

 are designed for operation in some additional structure such as a baffle or 



22 Olson and Massa, " Applied Acoustics," 2nd Ed., p. 325, P. Blakiston's Son 

 and Co., Philadelphia. 



