224 MICROPHONES 



wave of 100 dynes per square centimeter at 30 cycles Is less than a milli- 

 meter. The distortion due to a variation in the field over this distance is 

 less than jV of a per cent. In the case of the velocity microphone the 

 system is mass-controlled and there are no nonlinear elements. The mea- 

 sured distortion (see Sec. 11. 2C) in a velocity microphone for sound pres- 

 sures up to 1000 dynes per square centimeter is less than | of a per cent at 

 80 cycles. The most common source of nonhnear distortion in dynamic 

 microphones originates in the suspension system. In some cases at the 

 lower frequencies the harmonic distortion for a sound pressure of 100 dynes 

 per square centimeter may be several per cent. This very high distortion 

 is usually caused by instability of certain portions of the suspension due to 

 dissymmetry of the corrugation and inhomogeneity of the material. As 

 already pointed out, the distortion in carbon microphones is very high due 

 to the nonlinear characteristics of granular contacts. Considerable im- 

 provement has been made in carbon materials in recent years and the dis- 

 tortion, although still high, has been materially reduced. 



9.9. Transient Response of Microphones. — The subject of transient 

 response of vibrating systems, together with applications to loud speakers 

 has been considered in Sec. 7.16. The measurement of transient response 

 of loud speakers will be considered in Sec. 11. 3G. The transient response 

 of a microphone may be predicted from the equivalent circuit by the use 

 of the Operational Calculus. 



In the case of the mass-controlled system of the velocity microphone the 

 response to transients is very good. In the more complicated microphones, 

 having vibrating systems of several degrees of freedom, the transient re- 

 sponse may be very poor. This is particularly true of multiresonant sys- 

 tems with relatively low resistance. 



9.10. High Sensitivity Microphones (Motional Impedance). — The 

 considerations in this chapter have assumed the mechanical or acoustical 

 impedance, due to the electrical system, to be small compared to the other 

 mechanical or acoustical impedances of the vibrating system and may, 

 therefore, be neglected. In all the high quality microphones discussed in 

 this chapter these assumptions are satisfied. In the case of the batteryless 

 telephones or other highly sensitive microphones the effect of the electrical 

 system upon the vibrating system must be considered in order to predict 

 the performance of the system. The discussions of Sees. 6.2 and 7.2 and 

 Fig. 7.3 are of course applicable to the moving conductor, dynamic and 

 ribbon microphones. The mechanical or acoustical impedance due to the 

 electrical circuit is in series with the actuating force or pressure. In the 

 equivalent circuit of the mechanical system of Fig. 7.3 the actuating force in 



