408 



BELL SYSTEM TECHNICAL JOURNAL 



-95 



500 

 FREQUENCY IN 



1000 

 CYCLES PER SECOND 



5000 10,000 20,000 



Fig. 3- 



-Field response of a laboratory model of the new 630-A non-directional micro- 

 phone for several angles of sound incidence. 



Diffraction of Sound Around the Microphone 



The effect of the shape of the microphone on the directional response 

 has been brought out by study of the diffraction effect of different 

 geometrical objects of equal diameter. The diffraction of a sphere was 

 first treated by Rayleigh ^ and evaluated for a point on the sphere for 

 normal incidence by S. Ballantine,^ and for other angles of incidence 

 by H. C. Harrison and P. B. Flanders.* The effect for a circular plate 

 has been given by L. J. Sivian and H. T. O'Neil.^ Figure 4 shows the 

 calculated diffraction effect of the cylinder, cube, and sphere * as a 

 function of frequency and angle of incidence in terms of the ratio of the 

 disturbed to the undisturbed sound pressure at a point located centrally 

 in the surface of the object. The abscissae are given as the ratio of the 

 diameter of the object to the wave-length of sound, but the table at the 

 bottom indicates the corresponding frequencies for diameters of 1 inch, 

 2 inch and 4 inch. If a microphone were built having any one of these 

 shapes, and its diaphragm were made very small and located at the 

 point for which the curves were computed, its response would be 

 increased or decreased approximately in correspondence with these 

 curves as the angle of incidence is changed from + 90° to — 90°. It 

 will be noticed that both cylinder and cube show a marked directional 



* The calculated values for the diffract ion of the circular plate and cube have been 

 taken from an unpublished work of G. G. Muller of the Hell Telephone Laboratories. 



