NONLINEAR DISTORTION IN MICROPHONES 



223 



effect of the second by streamlining the microphone and the third is mini- 

 mized by reductions in the first and second sources. 



The customary wind screen ^^^ consists of a frame covered with silk en- 

 closing the microphone (Fig. 9.425). Very sheer silk reduces the response 

 to wind without appreciable attenuation of the sound. A spherical shape 

 has been found to offer the best shielding properties. The shielding proper- 

 ties increase with the volume of the shield. 



In general, the response to wind is much higher for directions normal to 

 the diaphragm by applying the principles of hydrodynamics. A wind 

 screen has been developed which reduces the wind response of the micro- 



MICROPHONE 



Fig. 9.42. Wind screens for microphones. A. Bernoulli wind screen applied to a dynamic 

 microphone. B. Wind screen consisting of a wire frame covered with sheer silk. 



phone. The Bernoulli " wind screen is shown in Fig. 9.42^^. The wind 

 pulses travel through the screen and exert a pressure on the diaphragm. 

 These same pulses cause a reduction in pressure at the periphery 1. These 

 two effects tend to balance each other and, therefore, the response to wind 

 is reduced. This type of screen reduces the wind response about 12 db. 

 9.8. Nonlinear Distortion in Microphones. — The sources of distortion 

 in microphones are, in general, the same as in the case of loud speakers. 

 The two principal causes are due to nonlinear mechanical or acoustical 

 elements and nonuniform magnetic field in dynamic types. The latter 

 type of distortion can be made negligible in well-designed units. For 

 example, in a velocity microphone the amplitude of the ribbon for a plane 



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

 phia. 



37 Phelps, W. D., RCA Review, Vol. 3, No. 2, p. 203, 1938. 



