ACOUSTICAL INSTRUMENTS 393 



Measurement of Periodic Changes in Density 

 As the optical index of refraction of an elastic medium depends upon 

 the density, it is possible to measure sound by letting one of the paths 

 of the light beams of an interferometer pass through the sound field 

 while the other is shielded therefrom. The interference fringes of the 

 interferometer will be displaced periodically in synchronism with the 

 periodic variations in density of the wave. This method was first 

 used by Boltzmann and Toepler ^^ who in this manner observed the 

 rather large variations in density within a sounding organ pipe. This 

 method has the advantage that the measurements are independent of 

 frequency but it is not very sensitive and at best is rather cumbersome. 

 An interesting modification ^^ of this method has recently been applied 

 in measurements of high-frequency sound waves in liquids. At these 

 high frequencies the wave-lengths are so small that the spatially 

 periodic variations of the density of the medium can act as a diffrac- 

 tion grating for light waves. This phenomenon has provided a neat 

 means of picturing the propagation of high-frequency sound waves in 

 liquids.''^ 



Instruments Employing Diaphragms and Optical Magnification 

 In the phonautograph of Scott (1857) a circular diaphragm is ac- 

 tuated by sound waves and the motion is recorded on a moving strip 

 of smoked paper by a stylus attached to the center of the diaphragm. 

 The recorded amplitudes are no greater than the actual amplitudes of 

 motion of the diaphragm which, except at the resonance frequency 

 or for very intense sounds, are so small that they cannot be accurately 

 determined from the record. Small motions can be observed and re- 

 corded if the stylus is replaced by an optical lever. This arrangement 

 in various forms has been used in the past by a number of investigators. 

 It reached its highest state of development in the well-known phono- 

 deik of D. C. Miller. ^^ In this instrument a horn is used for increasing 

 the sound pressure acting on the diaphragm, the motion of which is 

 magnified in some forms of the instrument by as much as 40,000 times. 

 By refinements in mechanical design and construction a remarkably 

 uniform sensitivity was achieved. 



Microphones 



The instruments discussed so far operate without the benefit of 



electric-current amplifiers. The important role that these amplifiers 



^^ Pogg.Ann. 141, 321 (1870). 



i» Debye and Sears, Proc. Nat. Acad. Sci. 18, 409 (1932). Lucas and Biquard, 

 Jour, de Physique et le Radium 3, 464 (1932). 



13 R. Baer and E. Meyer, Phys. Zeits. 34, 393 (1935). 



2" Science of Musical Sounds, The Macmillan Company (1922). 



