PRESSURE MICROPHONES 



183 



the high impedance of the crystal. A cross-sectional view of a diaphragm 

 actuated crystal microphone is shown in Fig. 9.5. 



D. Moving Conductor Microphones. — A moving coil microphone is a 

 microphone in which the output results from the motion of a conductor in 

 a magnetic field. The conductor may be in the form of a circular coil 

 which is termed a moving coil microphone or in the form of a straight 

 conductor which is termed an inductor microphone. These microphones 

 are also termed dynamic microphones. 



DIAPHRAGM^ SUSPENSION 



CROSS -SECTIONAL VIEW 

 m, '^i- 



III, - 



fM 





EQUIVALENT CIRCUIT 



1000 

 FREQUENCY 



m-.ru 



^TUBE 

 CROSS - SECTIONAL VIEW 



fM 



EQUIVALENT CIRCUIT 



2 

 O .1 



.001 



1000 

 FREQUENCY 



Fig.- 9.6. Cross-sectional view and equivalent circuit of a diaphragm, coil and suspension is 

 shown above. The velocity frequency characteristic for a unit force and a unit resistance 

 is indicated as Curve 1 on the graph. The same for a resistance of 60 units is indicated as 

 Curve 2. Cross-sectional view and equivalent circuit of a dynamic microphone is shown 

 below. Curve 3 on the lower graph is the same as Curve 2 on the upper graph. Curve 4 

 is the response with the tube mzvz added. Curve 5 is the response with the compliance 

 Cm2 added. 



1. Moving Coil Microphone {Dynamic Microphone) ^^. — A cross- 

 sectional view of a moving coil microphone is shown in Fig. 9.6. The 

 motion of the diaphragm is transferred to a coil located in a magnetic field. 



^° Wente and Thuras, Jour. Acous. Soc. Amer.^ Vol. 3, No. 1, p. 44, 1931. 



