VELOCITY MICROPHONES 



189 



due to the obstacle effect, see Sec. 1.11, the pressure on the ribbon increases 

 at the higher frequencies and the output is practically independent of the 

 frequency. 



B°40 

 r KLMULN^-T FREQUENCY 



Fig. 9.9. ^. The impedance characteristics ofthe elements of the pressure microphone, xar 

 ribbon acoustic reactance, xaa air load acoustic reactance. rAA air load acoustic re- 

 sistance. XAP pipe acoustic reactance (negative), tap pipe acoustic resistance. B. The 

 volume current Uy of the pressure ribbon for a sound pressure of one dyne per square centi- 

 meter. phase angle between the ribbon velocity and the driving pressure. 0i leading, 

 02 lagging. 



9.3. Velocity Microphones. ■ — A pressure gradient microphone is a 

 microphone in which the electrical response corresponds to the difference 

 in pressure between two points in space. In general, when the distance 

 between these two points is small compared to the wavelength, the pres- 

 sure gradient corresponds to the particle velocity. A velocity microphone 

 is a microphone in which the electrical response corresponds to the particle 

 velocity resulting from the propagation of a sound wave through an acous- 

 tic medium. The acoustical and electrical elements which form the coup- 

 ling means, between the atmosphere and the electrical system, for trans- 

 forming the sound vibrations into the corresponding electrical variations 

 may be arranged in innumerable ways to obtain pressure gradient or ve- 

 locity microphones. It is the purpose of this section to consider pressure 

 gradient and velocity microphones. 



A. Pressure Gradient Microphone ^^' ^^' ^^' ^^' ^^^. — The response of a pres- 

 sure gradient microphone, as the name implies, is a function of the difference 

 in sound pressure between two points. Obviously, a pressure gradient 

 microphone may be built in a number of ways. One type of pressure 



IS Olson, H. F., Jour. Soc. Mot. Pic. Eng., Vol. 16, No. 6, p. 695, 1931. 

 1^ Olson, H. F., Jour. Acous. Soc. Amer., Vol. 3, No. 1, p. 56, 1931. 



18 Olson, H. F., Proc. Inst. Rad. Eng., Vol. 21, No. 5, p. 655, 1933. 



19 Massa, F., Jour. Acous. Soc. Amer., Vol. 10, No. 3, p. 173, 1939. 



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

 phia. 



