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CARDIOID MICROPHONES 227
phone is suspended on the end of a microphone boom; this is a long "fish pole" used to place the microphone in the best possible position in the sound field. Heavy microphones (such as the RCA 44BX ribbon type) require a heavy, complicated, clumsy, expensive boom that is often noisy. Controls, usually in the form of cranks, are provided at the boom tripod to permit twisting and tilting of the microphone while recording is in progress in order to "point" the microphone at the sound source. It is usually difficult to locate and to manipulate a large microphone so that its shadow does not appear in the picture and so that the noise from the cables, cranks, wheels, etc. of the boom are not picked up by the microphone. Not of least importance is that a "fancy" boom with a large number of controls needs several highly skilled personnel to run it, because of the close coordination required among the members of the sound-recording crew.
One of the earlier designs of cardioid microphones was the RCA 77A. Although its directional properties were something of an improvement over the bidirectional ribbon microphone, it was as large and about as heavy and thus did not overcome the size disadvantage and the weight disadvantage of the otherwise excellent ribbon microphone. The RCA 77A and the later designs, such as the RCA 77B, 77C, and 77D and the Western Electric 639A and 639B are in reality two mechanisms combined within a single housing — a pressure-actuated element and a velocity-actuated element. The unidirectional characteristic is obtained by electrically combining the nondirectional characteristic of the pressure-actuated element with the bidirectional characteristic of a separate velocityactuated element.
The RCA 77B is one of the lightest and smallest of the unidirectional microphones of this kind; its weight is about 2 pounds and its length about 10 inches. Its front-to-back discrimination is about 20-to-l ; its directional characteristic is quite uniform in front for low and high frequencies. At the rear, however, it is sharply directional in its pickup at high frequencies (at 8000 cps) where its response along the rear axis is 10 db less than its response along the front axis. The low-frequency pickup, however, is quite small at the rear.
The RCA 77C and the Western Electric 639A each have a switch in the microphone housing that permits selection of a ribbon, a dynamic, or a cardioid type of response merely by moving the switch setting. This is accomplished by utilizing only the appropriate parts of the microphone mechanism for energizing the microphone output terminals. Incidentally