ABSOLUTE CALIBRATION OF CONDENSER TRANSMITTERS 103 



the calibrating apparatus, while relatively small in either case, is not 

 the same for both methods. At higher frequencies other factors 

 contribute. At the highest frequencies, say above 10,000 c.p.s., the 

 pressure on the diaphragm probably is more uniform in the present 

 method than in Method 1. 



Constant Field Calibration 



For constant field calibration methods it is difficult to provide a 

 plane progressive wave over a sufficiently large wavefront. Instead a 

 small source in a chamber lined with highly absorbing material is used. 

 The resultant progressive spherical wave, at sufficient distance from 





igSIN 2ujt 



TO AMPLIFIER- 

 RECTIFIER 



Fig. 8 — Electrostatic method — auxiliary third electrode driving diaphragm. 



the source, gives approximately the desired sound field. The measur- 

 ing device must give the absolute value of the undistorted field in- 

 tensity. We shall not consider the thermal, optical and sound radi- 

 ation pressure methods possible, on account of the experimental 

 difficulty which they present. One other absolute method is more 

 readily available: 



The Rayleigh Disc, which on certain assumptions gives the absolute 

 value of the particle velocity in the sound wave. In the sound field 

 presupposed for the field calibrations, the corresponding sound pressure 

 is easily computed.^ 



Another procedure is to measure the sound pressure with the aid of a 

 "search transmitter." This is a transmitter whose dimensions are so 



