212 MICROPHONES 



9.5. Miscellaneous Microphones. — A. Lapel Microphone. — Lapel 

 microphone ^^^' "^^^ is a term applied to a small microphone which can be 

 hooked into the buttonhole of the coat of a speaker. The principal purpose 

 of a lapel microphone, as contrasted to a stationary microphone placed 

 somewhere in front of the speaker, is to allow the speaker freedom to 

 move about the stage or lecture platform or to turn away from the 

 audience without any loss in intensity of the amplifier due to variation in 

 distance from the microphone. The response frequency characteristic 

 of a lapel microphone is usually adjusted so that the output is the 

 same as that of an ordinary microphone located directly in front of the 

 speaker. 



The carbon, crystal, and ribbon velocity microphones are the principal 

 types in use for lapel microphones. These microphones are usually made 

 small in size and light in weight. Save for these characteristics, the lapel 

 microphones are essentially the same as those described in the preceding 

 sections. 



B. Throat Microphone. — Throat microphone is a term applied to a 

 microphone which is held in place against the throat near the larynx by a 

 strap around the neck. The vibrations produced by the vocal cords are 

 transmitted to the microphone by flesh conduction. The sibilant sounds 

 are transmitted by conduction through the throat and by air conduction. 

 By suitable compensation with respect to the frequency, tolerable intel- 

 ligibility may be obtained. 



The throat microphone allows the wearer more freedom of action than in 

 the case of the conventional microphone. It is particularly useful for air- 

 plane pilots because it does not interfere with vision, the use of oxygen 

 apparatus, etc. 



In order to obtain high outputs and thereby reduce the size of the 

 amplifier, carbon microphones are generally used in aircraft throat micro- 

 phones. 



C. Hot Wire Microphone. — The hot wire microphone depends for its 

 operation upon the cooling effect of a sound wave, with the resultant change 

 in resistance, on an electrically heated fine wire. The cooling effect is 

 primarily due to the particle velocity in the sound wave. Two changes in 

 resistance occur, namely: a steady change and an alternating change of 

 twice the frequency of the sound wave. The steady change in resistance 

 may be used to measure or indicate the intensity of a sound wave by plac- 



30^ Olson and Carlisle, Jour. Inst. Rad. Eng., Vol. 22, No. 12, 1934, p. 1354. 

 3°^ Olson and Massa, " Applied Acoustics," P. Blakiston's Son and Co., Philadel- 

 phia. 



