194 



MICROPHONES 



to the particle velocity in a sound wave. The response of a pressure gra- 

 dient microphone as a function of the distance from a point source and the 

 frequency is shown in Fig. 9.26^. 



B. Velocity Microphone '^^'^^''^'^. — During the past few years free rib- 

 bon microphones have been used for all types of sound collection. Essen- 

 tially these microphones consist of a loosely stretched ribbon suspended 

 in the air gap between two pole pieces (Fig. 9.13). In addition to sup- 

 plying the flux to the air gap the pole 

 pieces serve as a bafile for acous- 

 tically separating the two sides of 

 the ribbon. The configuration and 

 dimensions of the baffle determine 

 the effective sound path between the 

 two sides of the ribbon. Under the 

 influence of a sound wave the ribbon 

 is driven from its equilibrium position 

 by the difference in pressure between 

 the two sides. The motion of the 

 ribbon in the magnetic field induces a 

 voltage between the two ends of the 

 ribbon. The electrical output of this 

 system under certain conditions cor- 

 stDE VIEW responds to the particle velocity in a 

 Fig. 9.13. The essential elements of a ^ound wave. Accordingly, the term 

 velocity microphone. velocity microphone has been applied 



to the free ribbon microphone. In 

 past analysis it has been customary to treat the system as an acoustic 

 doublet. This method is essentially accurate when the effective dimen- 

 sions of the baffle are small compared to the wavelength. When the 

 effective dimensions are comparable to the wavelength, there Is consider- 

 able discrepancy between the simple doublet theory and the actual per- 

 formance. It is the purpose of this section to develop the theory of the 

 conventional baffle type velocity microphone. 



Approximate solutions for the diffraction of sound by a circular and 

 square plate have been obtained.^^ These analyses may be applied to the 

 problem of the baffle type ribbon microphone. 



FRONT VIEW 



20 Olson, H. F., Jour. Soc. Mot. Pic. Eng., Vol. 16, No. 6, p. 695, 1931. 

 •^1 Olson, H. F., Jour. Acous. Soc. Amer., Vol. 3, No. 1, p. 56, 1931. 



22 Olson, H. F., Proc. Inst. Rad. Eng., Vol. 21, No. 5, p. 655, 1933. 



23 Sivian and O'Neil, Jour. Acous. Soc. Amer., Vol. 3, No. 4, p. 483, 1932. 



