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BELL SYSTEM TECHNICAL JOURNAL 



If we have a simple source of constant strent;;th A cos kct, the pressure 

 at a distance r is given by 



-^ sin k{ct 



r), 



where p is the density, c the velocity of sound, and k is equal to o/c. 

 The particle velocity is given by 



where ^ is a function of X and r. It will be seen that the pressure 

 varies inversely with the distance for all frequencies, while the relation- 

 ship between velocity and distance involves the wave-length, or fre- 

 quency. In Fig. 6 are given some response-frequency characteristics 



50 100 500 1000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 6 — Response of velocity microphone as a function 

 of distance from source. 



for several distances from the source of a velocity microphone having 

 a uniform characteristic for plane waves. The change in character- 

 istic as the instrument is brought close to the source is very marked. 

 Some care is therefore required in interpreting the results of measure- 

 ments made with this type of instrument. On the other hand, a pres- 

 sure microphone, except in so far as diffraction may modify the sound 

 field, will exhibit the same form of response-frequency characteristic 

 at all distances from the source, so the wave form of the voltage 

 generated by a pressure microphone will be the same for all positions 

 in the free sound field of a simple source. This dilTerence in character- 

 istics of the two types of instruments is easily observed by comparing 

 reproduced speech when the microphones are first placed near and 

 then at some distance from the speaker's mouth. 



Ribbon Microphone 

 A form of microphone, which has been extensively used in recent 

 years, is the ribbon microphone. Essentially it consists of a very thin 



