202 



MICROPHONES 



shown in Fig. 9.22. It will be seen that this force leads the particle velocity 

 by 90° for small values of R/\. The phase angle between the voltage out- 

 put of the ribbon and the particle velocity is also shown in Fig. 9.22. For 

 small values of R/\ the voltage output of a mass controlled dynamic ribbon 

 microphone with a baffle corresponds to the particle velocity in the sound 

 wave. 



The above analysis has been concerned with a ribbon located in a cir- 

 cular baffle. Irregular baffles instead of circular baffles are used in com- 



FiG. 9.22. The phase angle, in degrees, between the actuating force and the particle velocity 

 for a mass controlled ribbon with a circular baffle as a function of R/X. The phase angle 

 between the voltage output of a mass controlled electrodynamic ribbon located in a mag- 

 netic field as a function of R/X. 



mercial microphones for two reasons: first, a suitable magnetic field results 

 in an irregular baffle and, second, the sound path lengths between the two 

 sides of an irregular baffle differ and, as a consequence, it is possible to 

 obtain uniform directional response characteristics over a wider frequency 

 range. An analytical solution of the irregular plate is difficult. However, 

 the graphical method may be used and is very effective. 



In well-designed velocity microphones which have been built in the past 

 the effective sound path introduced by the baffle has been made less than 

 one half wavelength for all frequencies within the useful range. There 

 are two reasons for this selection of sound path: first, the response up to 

 this frequency is quite uniform, while above this frequency the response 

 falls off rapidly with increase of the frequency; second, in the case of an 

 irregular baffle the directional characteristics are of the cosine type to 

 within a few per cent of this frequency limit. A commercial microphone 



