A VOICE AND EAR 



315 



The tube used in the measurements is associated with a condenser 

 transmitter.*^ One end of this tube, having a relatively large opening 

 is acoustically coupled to the transmitter diaphragm. The calibration 

 of the transmitter is made witn the aid of a Rayleigh Disc, in a highly 

 damped sound-proof box, as shown in Fig. 18. The sound source is 

 a loud speaking receiver of large power capacity,^ terminated with an 



SEARCH 

 TRANSMITTER 



BAFFLE 



RECEIVER 



Fig. 17 — Diagrammatic arrangement employed in receiver calibrations on 



human ears. 



adjustable tube. With the mouth of the tube as origin, an approxi- 

 mately spherical progressive sound wave is produced. F'rom the disc 

 deflection, the particle velocity and hence the alternating sound 

 pressure is determined in the space occupied by the disc. The search 

 transmitter (the bulk of it wrapped in felt) is placed with its mouth 

 as near the disc as possible, and its voltage output is measured. 



The search transmitter calibration is made under conditions con- 

 forming to the following essential requirements: 



^ Loc. cit. 

 * Loc. cit. 



