ACOUSTICAL INSTRUMENTS 



397 



measuring instrument. Not even with an optical interferometer 

 could we hope to evaluate displacements so small. 



Moving Coil Microphone 

 The condenser microphone has inherently a high electrical im- 

 pedance, so high in fact that any attempt to connect the microphone to 

 an amplifier by leads of appreciable length results in a loss of voltage. 

 To avoid this loss an amplifier of at least one stage has generally been 

 placed in close connection with the microphone. However, since the 

 input impedance of a vacuum tube is also high, the microphone can be 

 connected to it without the use of an impedance transformer, a distinct 

 advantage at the time when transformers of good frequency charac- 

 teristic were not available. During the last few years, through the 

 development of new magnetic materials and advances in design, it 

 has been possible to build transformers having a substantially uniform 

 response over the whole acoustic frequency range. This development 

 has made it possible to design microphones operating on electro- 

 magnetic principles, which have a good response-frequency character- 

 istic and a greater sensitivity than the condenser microphone. They 

 have an important advantage over the condenser microphone in that, 

 because of their relatively low and constant impedance, they may be 

 connected to the amplifier by a relatively long cable without appreci- 

 able loss. One such instrument ^* is shown diagrammatically in Fig. 1. 



Fig. 1 — Moving coil microphone. 



The diaphragm has attached to it a coil which lies within a radial 

 magnetic field. As the voltage generated by an axial motion of the 

 coil is proportional to the velocity, if the same voltage is to be gener- 

 ated at all frequencies under a given sound pressure, the impedance of 

 the moving element must be independent of frequency. This type of 

 impedance characteristic over a wide frequency range is obtained by 

 properly proportioned air chambers and resistances in back of the 

 ^* Jour. Acous. Soc. Anier. 3, 44 (1931). 



