204 



MICROPHONES 



the form of a labyrinth and enclosed 

 in a case. The velocity and pressure 

 sections are formed from a single con- 

 tinuous ribbon. A common magnetic 

 structure is used for both the velocity 

 and pressure sections. Due to a 

 finite length of pipe for the pressure 

 section the velocity of the pressure 

 ribbon leads the pressure in the sound 

 wave at the low frequencies. See 

 Sec. 9.2D3 and Fig. 9.9. The resis- 

 tance (silk cloth) introduces a corre- 

 sponding phase shift in the velocity 

 section. At the high frequencies the 

 phase shifts in the two elements are 

 made the same by suitable geometrical 

 configurations of the field structure. 



A unidirectional microphone consist- 

 ing of a ribbon velocity element and 



CXJ 



CkJ 



Fig. 9.24. Unidirectional microphone 

 with the screen removed. Ribbon type 

 pressure and velocity elements. 



a dynamic pressure element is shown 

 in Fig. 9.25. Equalizers are used to 

 correct the amplitude and phase of 

 the dynamic element to conform 

 with the velocity element. 



A. The Response of the Unidirec- 

 tional Microphone as a Function of 

 the Distance and the Frequency ". 

 — The low frequency response of 

 the velocity microphone is accentu- 

 ated when the distance between 

 the source and the microphone is 

 less than a wavelength. The same 

 effect occurs to a smaller extent in 

 the unidirectional microphone. It 



VELOCITY 

 ELEMENT 



PRESSURE 

 ELEMENT 



EQUALIZER 



FRONT VIEW 



CROSS SECTION A-A 



Fig. 9.25. Unidirectional microphone consist- 

 ing of a ribbon type velocity element and a 

 dynamic type pressure element. 



"" Olson. H. F., Broadcast News, No. 30, p. 3, May, 1939. 



