CHAPTER IX 

 MICROPHONES 



9.1. Introduction. — A microphone is an electroacoustic transducer 

 actuated by energy in an acoustic system and delivering energy to an 

 electrical system, the wave form in the electrical system being substantially 

 equivalent to that in the acoustic system. A pressure microphone is a 

 microphone in which the electrical response is caused by variations in pres- 

 sure in the actuating sound wave. A velocity microphone is a microphone 

 in which the electrical response corresponds to the particle velocity result- 

 ing from the propagation of a sound wave through an acoustic medium. 

 All microphones in use to-day may be classified as follows: pressure, ve- 

 locity or a combination pressure and velocity. For the conversion of the 

 acoustic variations into the corresponding electrical variations the follow- 

 ing transducers may be used: carbon, magnetic, dynamic, condenser, 

 crystal, magnetostrictive and hot wire. 



Microphones may also be classified as directional or nondirectional. 

 The particular configuration of the acoustic elements which constitute the 

 vibrating system determines the directional properties of the microphone. 

 It is the purpose of this chapter to consider the microphones in most com- 

 mon, use to-day from the standpoint of the above classifications. 



9.2. Pressure Microphones. — A. Carbon Microphones. — Acarbon micro- 

 phone is a microphone which depends for its operation on the variation in re- 

 sistance of carbon contacts. The high sensitivity of this microphone is due 

 to the relay action of the carbon contacts. The carbon microphone is almost 

 universally employed in telephonic communications where the prime 

 requisite is sensitivity rather than uniform response over a wide frequency 

 range. For high quality reproduction the distortion may be reduced by 

 employing two buttons in a push-pull arrangement. It is the purpose of 

 this section to consider single and double button carbon microphones. 



1. Single Button Carbon Microphone. — A typical carbon microphone 

 is shown in Fig. 9.1. The carbon button consists of a cylindrical cavity 

 filled with carbon granules. The carbon granules are usually made from 

 anthracite coal. The carbon granules make contact with the diaphragm 

 and the cylindrical cup. Suitable washers are used to prevent leakage of 



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