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PEESSUEE-OPEEATED MICEOPHONES
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instinctively jump from script to microphone during a recording. Because of the image spread that occurs on film — an effect which is quite serious for variable-area recording if films of other than best resolving power are used in other than the best developers — pressure-type microphones are likely to cause excessive or distorted recorded sibilants because of their comparatively "jagged" response-frequency characteristics even with relatively small deviations from the best in films and in processing. In the more obvious cases, either the "s" sounds are very annoying because the effect is like that of a very bad set of false teeth, or they may be distorted to the point where the "s" sounds are reproduced as "f's" or "th's" and even worse. While the use of a 5300 cycle lowpass filter (which is a part of the band-pass filter of range F that limits the frequency range to 110 to 5300 cps) may limit the distortion to an acceptable value on occasion, the substitution of one of the more expensive
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Fig. 45A. Typical free-field response curve of Western Electric 640AA. dbm
equals db relative to a power level of 1 milliwatt in a resistive load corresponding to the nominal amplifier output impedance.
types of microphone such as a cardioid type will often eliminate this difficulty and, at the same time, result in recordings of somewhat lower recorded noise level and better smoothness.
For a number of years prior to World War II condenser microphones had been almost completely displaced by the pressure-operated moving coil microphones (such as those just described) despite the popularity of the condenser microphone some 15 years ago. Now it seems that the condenser microphone* can stage a comeback as the recent redesign in the Western Electric 640 A A (Fig. 45) overcomes many of the disadvantages of the earlier condenser microphones with regard to size, frequency characteristic, and certain maintenance difficulties. As the directional properties of a pressure-operated microphone depend upon the ratio of the diaphragm diameter to the wavelength, the reduction of the diameter
* Another recent design is the Altec-Lansing ; this is even smaller.