ACOUSTICAL INSTRUMENTS 403 



obtained when the direction of the sound wave coincides with the 

 direction of the steady stream. At a given frequency the resistance 

 variation is nearly proportional to the product of the steady stream 

 velocity, the particle velocity of the sound wave and the cosine of the 

 included angle. Since velocity in contrast with pressure is a vector 

 quantity, a velocity microphone will respond selectively to sound 

 coming from certain directions even at low frequencies. This charac- 

 teristic is of considerable advantage in certain types of measurements, 

 for it is often possible to so place and orient the instrument that its 

 response is a minimum for an interfering or disturbing sound and a 

 maximum for the sound to be measured. An illustration of such an 

 application in sound measurement or pick-up is shown in Fig. 5 



SOURCE 



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Fig. 5 — Method of reducing effect of interfering'waves 

 with velocity^^microphone. 



where the instrument is so placed that it will receive the sound directly 

 from the source, but is insensitive to the sound reflected from the 

 floor or neighboring wall. Other examples of acoustic measurements, 

 where benefit is derived from the directional characteristics of the 

 velocity microphone, are discussed in a recent paper by Wolff and 

 Massa."! 



There is one other important difference in the performance of 

 velocity and pressure microphones. In a plane progressive wave 

 particle velocity and pressure are strictly proportional at all fre- 

 quencies. For a spherical sound wave, the radius of curvature of 

 which is small compared with a wave-length, this is no longer true. 



" Jour. Acous. Soc. Amer. IV, 217 (1933). 



