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BELL SYSTEM TECHNICAL JOURNAL 



ton wire. Recently A. E. Johnson ^^ has been able to make thermo- 

 couples with exceedingly small heat capacities with which measure- 

 ments have been made up to 5,000 cycles and it is stated that they are 

 usable up to several hundred thousand cycles. They are so small that 

 they do not alter the sound field by diffraction and are free from reso- 

 nance effects inherent in most instruments depending upon mechanical 

 movement. As compared with other types, thermocouple micro- 

 phones have a low sensitivity, at least 100 db below that of the moving 

 coil microphone, according to the data given by Johnson. 



Velocity Microphones 

 All the preceding types of microphones depend ultimately for their 

 operation upon pressure variations in the sound wave. As the two 

 primary characteristics of sound are pressure variations and alternat- 

 ing flow of the air particles, it is possible also to design microphones 

 which generate voltages in accordance with the velocity of the air 

 particles. 



Hot Wire Microphone 



One form of microphone of this character depends upon the change 

 in resistance of a heated fine wire resulting from changes in tempera- 

 ture produced by the transverse flow of air. A microphone operating 

 on this principle was first devised by Tucker ^"^ and used extensively 

 during the war for locating enemy artillery. In order to increase the 

 sensitivity and reduce distortion a steady stream of gas should be 

 passed across the wire. An application of this principle to the con- 

 struction of a microphone is shown in Fig. 4. Maximum response is 



\/////y-7-//^///////// /^^^/ ;/// n 



OPEN 

 END 



'^ 



V^////,l^////////7^ry 



FINE WIRE 

 RESISTANCE 



MEMBRANE TRANSMITTING 

 SOUND BUT IMPERVIOUS 

 TO DIRECT AIR FLOW 



Fig. 4 — Hot wire velocity inicrophone. 



^Thys. Rev. 45, 645 (1934). 

 *Thil. Trails. 221, 389 (1921). 



