188 MICROPHONES 



pipe shows that for the mid and high frequency range the impedance is an 

 acoustic resistance. 



The acoustic resistance of the pipe referred to the ribbon is, 



42 

 Tap = — 9.24 



Ap 



where Ap = area of the pipe, in square centimeters. 



The acoustic impedance due to the electrical circuit may influence the 

 motion of the ribbon. The acoustical impedance due to the electrical 

 circuit is 



^.. = ^' 



Ab^Zt 



where Zt = total electrical impedance in the ribbon circuit, in abohms, and 

 Ar = area of the ribbon, in square centimeters. 



The acoustic impedance Zae, due to the electrical circuit, and the acous- 

 tic impedance Zas, due to the aperture between the ribbon and pole pieces, 

 are in general small compared to the other impedances in the system save 

 at the very low frequencies. 



The acoustic impedance characteristics of the elements of a pressure rib- 

 bon microphone are shown in Fig. 9.9. 



The volume current of the ribbon, in cubic centimeters per second, is 

 given by 



U = — ^ : ^ 9.26 



rAP + Taa + JXar + JXaa — JXap 



The volume current of the ribbon and the phase angle between the vol- 

 ume current and pressure computed from equation 9.26 is shown in Fig. 9.9. 

 The velocity of the ribbon, in centimeters per second, is 



Ak 



9.21 



The voltage, in abvolts, generated in the ribbon is given 



e = Blx 9.28 



where B = flux density, in gausses, and 



/ = length of the ribbon, in centimeters. 

 The shape of the voltage curve will be the same as that of Uy in Fig. 9.9. 

 This assumes that the pressure is the same for all frequencies. However, 



