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VELOCITY-OPERATED MICROPHONES
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electron tube. Polarizing voltage for operation of the condenser microphone is taken from the plate supply.
Velocity-Operated Microphones
Velocity-operated microphones such as the RCA 44BX (Fig. 46) are used where weight and size are less important than smoothness and fidelity. The diaphragm resonance of diaphragm-type microphones occurs in the middle of the speech range at about 800 cps for moving-coil microphones, or further up in the speech range between 3000 and 8000 cps for condenser microphones. The ribbon resonance, on the other hand, occurs below the speech range at 10 to 15 cps. The ribbon used is light, thin, and relatively short ; the ribbon in the RCA 44BX microphone
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Fig. 46A. Direction-response characteristic of RCA MI-3027-E microphone.
Fig. 46B. Response-frequency characteristic of typical RCA MI-3027-E microphone.
is a duralumin corrugated strip about 2 in. long, 3/16 in. wide, and about 0.0002 in. thick. The ribbon is suspended between poles of an Alnico permanent magnet, with the length of the ribbon perpendicular to — and its width in the plane of — the magnetic flux passing between the two magnet poles. The ends of the ribbon, across which the voltage appears from the movement of the ribbon in the magnetic field, are connected to a transformer whose output is connected to the output terminals provided for the microphone. Such a microphone is bidirectional; it picks up equally well from either of two opposite sides in a direction perpendicular to the plane of the ribbon. The microphone is directional for both low and high frequencies; for practical purposes the response may be con