TESTING OF LOUD SPEAKERS 261 



A modification of the above test procedure is to use a stationary micro- 

 phone and warble the frequency (20 cycles + 10 per cent of the mean 

 audio frequency as a maximum total band width) to average out reflection 

 errors. This method tends to average out very short or abrupt variations 

 in the loud speaker response. The check response frequency measure- 

 ment taken close to the loud speaker with no warble should be made to 

 determine if there are any abrupt variations in its response. 



5. Living Roo7n Measurements ^°. — The performance of a radio receiver 

 in a living room has been discussed in Sec. 12.2/. The characteristics 

 shown in Fig. 12.12 were obtained with the cathode ray response mea- 

 suring system described in Sec. \\3A2. However, any of the systems 

 described in Sec. 1\.3A2 may be used. It is customary to obtain a large 

 number of characteristics for each position of the receiver in the room. 



6. Theatre Measurements. — The performance of a loud speaker in a 

 theatre has been discussed in Sec. 12.25. The characteristics for the 

 various parts of the theatre may be obtained with any equipment described 

 in Sec. 11.3yf2. However, the high speed response measuring equipments 

 are preferable for this type of work. 



7. Automobile Measurements. — The conditions under which an auto- 

 mobile radio receiver operates differ widely from those of a loud speaker 

 in a room. For this reason it is very important to test the performance 

 under actual operating conditions. The response frequency characteristic 

 should be obtained by placing the microphone at the ear position in each 

 of the normal listening positions in the automobile. In the case of back 

 seat measurements persons should be seated in the front seat to sim- 

 ulate actual conditions. Measurements should be made with the windows 

 open and closed. In general, the response frequency characteristics will 

 differ widely for the front and back seats. It is customary to favor the 

 front seats in determining the optimum response frequency characteristic. 

 At high speeds wind, road rumble and engine noises are quite high and 

 mask the reproduced sound. The power output should be sufficient to 

 override these noises and give intelligible speech. In view of the fact 

 that the sound level delivered by the loud speaker is quite high under 

 these conditions, it is important that the response frequency character- 

 istic be smooth, otherwise the reproduced sound will be disagreeable. 



The response frequency characteristics may be obtained with any equip- 

 ment described in Sec. \1.3A2. 



B. Directional Characteristic. — The directional characteristic of a 

 loud speaker is the response as a function of the angle with respect to some 



25 Wheeler and Whitman, Proc. I. R. £., Vol. 23, No. 6, p. 610, 1935. 



