TESTING OF LOUD SPEAKERS 



255 



loud speakers is generally carried on in rooms. Each laboratory can cor- 

 relate its own indoor with outdoor measurements. The testing of loud 

 speakers under actual conditions is also useful, such as the operation of a 

 radio receiver in a living room. As the art in sound measurement pro- 

 gresses, this type of measurement will become more important. 



2. Apparatus Jor Measuring the Sound Pressure Frequency Relationship 

 of a Sound Source. — An arrangement for obtaining the sound pressure 

 characteristic by the semiautomatic method ^^ is shown in Fig. 11.6. 



n=OCl 



OSCILLATOR o 



* 



MICROPHONE 



.qLOGARITHMIC o 

 DETCCTOR 



Fig. 11.6. Schematic arrangement of the apparatus for manually recording the sound 

 pressure frequency characteristic of a sound source. (After Wolff and Ringel.) 



This method yields a response frequency curve on semilogarithmic paper 

 in about three minutes. Rotation of a condenser governing the beat 

 frequency and coupled to a drum on which the paper record is made gives 

 the abscissas for the curves, values which are proportional to the logarithm 

 of the frequency due to the manner in which the condenser plates are cut. 

 The drive may be manual or by motor. 



A linear or logarithmic detector ^^' ^'^ may be employed. In the latter, 

 the deflection of the meter is a logarithmic function of the sound pressure. 

 The resulting curve is recorded directly in decibels. A variation of this 

 method is sometimes used in which the recording pen is coupled to a gain 

 control'in the amplifier, the operator manipulating the control in such a 

 manner that the output indicated by the meter remains constant. Either 

 a linear or a logarithmic co-ordinate scale may be obtained by suitable 

 design of the gain control. 



The acoustic level recorder ^^ is an automatic device which records the 

 gain settings required to keep the amplifier output constant as the fre- 



1^ Wolff and Ringel, Proc. I. R. E., Vol. IS, No. 5, p. 363, 1927. 



" Ballantine, S., Jour. Acous. Soc. Amer., Vol. 5, No. 1, p. 10, 1933. 



17 Hackley, R. A., Broadcast News, No. 28, p. 20, July, 1938. 



1* Wente, Bedell and Swartzel, Jour. Acous. Soc. Amer., Vol. 6, No. 3, p. 121, 1935. 



