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228 IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT
there are important design differences between the Western Electric and the RCA products; the RCA microphone utilizes a thin ribbon for the bidirectional element and a second ribbon which is pressure-loaded for the pressure element ; the Western Electric products use a relatively thick and stiff ribbon for the bidirectional element and a small diaphragm-actuated moving coil for the pressure-actuated element. The RCA 77 C weighs 3 pounds and is 8 1/2 inches long; the Western Electric 639 A weighs 3 1/4 pounds and is 7 1/2 inches long.
Fig. 47. Western Electric 639B cardioid microphone. Specifications Response-frequency characteristic : Essentially uniform from 40 to 10,000 cps. Sensitivity: Open circuit voltage 64 db below 1 volt per 10 dynes per square centimeter; equivalent to 84 db below 1 volt per 1 dyne per square centimeter. Signal-to-noise Ratio: 78 db above thermal agitation noise generated within the microphone for a signal of 10 dynes per square centimeter; 58 db for 1 dyne per square centimeter. Directivity : Six patterns — R, D, C, 1, 2, and 3 selectable through a six-position screwdriver-operated switch. At the angle of minimum response, the average discrimination with respect to the 0° response is 20 db over the range from 40 to 10,000 cps. Impedance: Varies somewhat throughout the frequency range; average value is 40 ohms. It is intended for use with equipment haveing a rated source impedance from 25 to 50 ohms. Power output level : 56 dbm for 10 dynes per square centimeter, 76 dbm for 1 dyne per square centimeter, when the microphone is terminated with a resistance equal to its internal impedance. Approximately 10 dynes per square centimeter sound pressure is produced at conversational speech level three feet from a microphone. Dimensions and weight: Height 7 1/2" including the plug terminal; Length 4 7/16"; Width 3 7/16"; Weight 3 1/4 lbs.