UNIDIRECTIONAL MICROPHONES 



203 



is shown in Fig. 9.13. It will be seen that the effective baffle is irregular 

 in shape. The directional characteristics of the microphone of Fig. 9.13 

 are shown in Fig. 9.23. Further, the deviation from a cosine characteristic 

 is very small. 



The above considerations have been concerned with a plane wave. As 



HORIZONTAL 

 PLANE 



.2 !r. 



Fig. 9.23. The directional characteristic of the velocity microphone shown in Fig. 9.13. The 

 polar graph depicts the output, in volts, as a function of the angle, in degrees. The maxi- 

 mum response is arbitrarily chosen as unity. 



in the case of the pressure gradient microphone, it can be shown that the 

 output of a baffle type velocity microphone corresponds to the particle 

 velocity in a spherical wave. The response of a baffle type velocity micro- 

 phone as a function of the distance from a point source and the frequency 

 is shown in Fig. 9.'16A. 



9.4. Unidirectional Microphones ^4. 2^- ^^ — The unidirectional micro- 

 phone consists of the combination of a bidirectional microphone and a 

 nondirectional microphone. The performance of this system is a function 

 of the distance from the source, the spacing of the units, the sensitivity of 

 the units and the phase angle between the units. These fundamental 

 characteristics will now be considered. 



A unidirectional microphone consisting of a ribbon velocity element and 

 a ribbon pressure element (see Sec. 9.2D3 and Fig. 9.8) is shown in Fig. 9.24. 

 The damped pipe terminating the back of the pressure ribbon is folded in 



24 Olson, H. F., Jour. Acous. Soc. Amer., Vol. 3, No. 3, p. 315, 1932. 

 -^ Weinberger, Olson and Massa, 'Jour. Acous. Soc. Amer., Vol. 5, No. 2, p. 139, 

 1933. 



26 Olson, H. F., Jour. Soc. Mot. Pic. Eng., Vol. 27, No. 3, p. 284, 1936. 



