106 BELL SYSTEM TECHNICAL JOURNAL 



the air particles in the recess aperture all move in phase and normally 

 to the diaphragm. 



At still higher frequencies the doubled pressure effect largely persists 

 and superposed on it are a number of rather complicated diffraction 

 effects. These involve radial wave propagation across the diaphragm 

 recess while the above two effects are due to normal plane waves. 

 The marked dip at 11,200 c.p.s. corresponds to a sound wave-length 

 such that 



-^li^PQ)' + (PAY -PA=\ 



"2^ 

 (see Fig. lA). 



So far normal incidence of the sound wave has been assumed. For 

 other directions of arrival, substantially different field calibrations are 

 obtained. Since the transmitter is symmetrical about any diaphragm 

 diameter, the effect of direction may be given in terms of the azimuth 

 angle of incidence. A set of azimuth curves for various frequencies 

 are given in Fig. 11, all expressed relative to the normal field cali- 

 bration. In general, the higher the frequency the greater the effect of 

 azimuth. For a large range of angles that effect is as great as or greater 

 than the difference between the pressure and the normal field cali- 

 brations. It is interesting to note that the anomalous azimuth curve 

 at 11,200 c.p.s. corresponds to a pronounced dip at that frequency in 

 the normal field curve. 



Relation of Field Calibration to Actual 

 Transmitter Performance 



We now consider the bearing of field calibrations upon the response 

 of the No. 394-Type Transmitter under one or two conditions of 

 actual use. 



First, consider the case of a person speaking directly toward the dia- 

 phragm. The normal field calibration approximately applies, provided 

 the distance is not great enough for reflected waves to be comparable 

 with the direct wave and the distance is not so small that the transmitter 

 reacts back on the source (the voice), or that pronounced standing 

 waves are set up between the transmitter and the head. Outdoors and 

 in a well damped room distances ranging say from 6 inches to 3 feet are 

 likely to be within the above limits for the important voice frequencies. 



On the other hand, for much of indoor work the distances from the 

 microphone to the source and to the several reflecting surfaces are such 

 that waves reaching the microphone by reflections are comparable 

 with and often predominate over the direct sound. Besides, the 

 microphone often is so placed that the direct sound strikes it more 



