186 



MICROPHONES 



section and the action is the same. A transformer housed in the magnet 

 structure is used to step up the low impedance of the conductor to that 

 suitable for transmission over a line of several hundred feet. 



3. Ribbon Type. — The pressure ribbon microphone i3, u, ua consists of a 

 light metallic ribbon suspended in a magnetic field and freely accessible to 

 the atmosphere on one side and terminated in an acoustic resistance on the 



EQUIVALENT CIRCUIT 



Fig. 9.8. The elements of the pressure microphone and the equivalent electrical circuit of the 

 acoustical system. Mar inertance of the ribbon. Car acoustic capacitance of the ribbon. 

 ^AA acoustic resistance of the air load on the ribbon. Maa inertance of the air load upon 

 the ribbon. %ap acoustic impedance of the pipe terminating the ribbon, p driving sound 

 pressure. 



Other side. The essential elements are shown schematically in Fig. 9.8. 

 These elements may take various forms as, for example, the pipe is usually 

 coiled in the form of a labyrinth. See Fig. 9.24. 



The equivalent circuit ^^ of the pressure ribbon microphone is shown in 

 Fig. 9.8. 



The inertance and acoustic capacitance of the ribbon is given by Mar 

 and Car- 



The resistance and mass of the air load upon the ribbon are designated by 

 Vaa and Maa- The expression for the air load upon the ribbon will now 

 be derived. The pressure, in dynes per square centimeter, at a distance 



13 Olson, H. P., U. S. Patent 2,102,736. 



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

 '^^■^ Olson and Massa, " Applied Acoustics," P. Blakiston's Son and Co., Philadel- 

 phia. 



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



