400 BELL SYSTEM TECHNICAL JOURNAL 



Magnetostriction also has found little application in microphones 

 for air-borne waves at audio frequencies, although this principle has 

 been applied with notable success in both the generation and detection 

 of ultra-audio waves by G. W. Pierce and in the generation of audio 

 frequency waves of high intensity in liquids.^^ 



Piezoelectric Microphones 



The application of piezoelectric action in the construction of acoustic 

 microphones was first made by A. M, Nicolson,^^ who used Rochelle 

 salt as the active material. Rochelle salt is unique in that its piezo- 

 electric constant is about a thousand times as great as that of any 

 other crystal. It has, however, several characteristics which would 

 appear to render it unsuitable for use as a measuring microphone. It is 

 mechanically fragile and its piezoelectric activity, under normal condi- 

 tions, varies greatly with temperature, falling to a very low value for 

 temperatures above 23° C. R. D. Schulwas-Sorokin ^^ found, how- 

 ever, that by the application of a static stress the temperature coeffi- 

 cient could under certain conditions be greatly reduced and the ac- 

 tivity extended to higher temperatures. C. B. Sawyer ^* found that 

 if two thin slabs are cut and cemented together in such a way that one 

 of the slabs will expand and the other contract when a potential is 

 applied between the interface and the two outer surfaces, variations 

 of activity with temperature are reduced to a low value. Presumably 

 stresses are set up in the slabs by temperature variations which reduce 

 the temperature coefficient of activity in accordance with the experi- 

 ments of R. D. Schulwas-Sorokin. Sawyer has utilized these so-called 

 bimorphic slabs in the construction of microphones. Single elements 

 can be constructed of sufficiently small dimensions to avoid diffraction 

 of the sound. In order to obtain microphones of greater practical effi- 

 ciency a number of elements may be used in combination. If these 

 elements are mounted symmetrically the translating efficiency will 

 be the same in all directions about the axis of symmetry, as is the case 

 for any microphone having an axis of symmetry. The amount of 

 variation in respect to other directions depends upon the relation 

 between the dimensions and the wave-length. According to the 

 published data the sensitivity of a multiple element microphone of 

 this type is about 25 db below that of a moving coil instrument."''^ 



" Gaines, Physics 3, 209 (1932). 



32 Trans. A. I. E. E. 38, 1315 (1919). 



^Zs.f. Physik 73, 9-10, 700 (1932). 



^Proc. I. R. E. 19, 2020 (1931). 



=»A. L. Williams, Jour. S. M. P. E. 23, 196 (1934). 



