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IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT
The Western Electric 639B (Fig. 47) and RCA 77D (Fig. 48) not only provide a choice of the three directional characteristics provided for the Western Electric 639 A and the RCA 77 C, but also provide a choice of three additional directional characteristics that alter in relative amounts the contributions of the velocity-actuated and pressure-actuated elements. The size and weight of the 639B is about the same as the 639A; that of the 77C is about the same as the 77D.
Specifications
Directional characteristic (adjustable) : Bi-directional, uni-directional and non-directional. Output impedances (tapped transformer) : 50/250/600 ohms. Effective output level: -57 dbm*. Hum pickup level: 118 dbm**. Frequency response: See Fig. 48B. Finish: Satin chrome and umber gray. Mounting: \" pipe thread. Dimensions, overall: Height 11$"; Width 3f"; Depth 2£"; Weight (unpacked inluding mountings) : 3 lbs. Cable (MI-43 3 conductor shielded): 30' less plug. Stock identification: MI4045-A.
Accessories Microphone plug: MI-4630-B. Protective cloth bag: MI-4087.
Fig. 48. RCA 77D cardioid microphone (polydirectional — MI-4045-A).
One manufacturer, Shure Brothers of Chicago, has been marketing a unidirectional microphone, #556 (Figs. 49 and 49A), with but a single pressure-actuated diaphragm-type element. In this design, an acoustical network is used with the single moving coil element to provide the unidirectional characteristic. This design has merit because of its simplicity. The concept of the single element with appropriate electrical and acoustical networks has been applied to velocity-actuated elements in the RCA MI-10,001 microphone shown in Figures 49B, 49C, and 49D. With sim* Keferred to one milliwatt and a sound pressure of 10 dynes/cm2.
** Level referred to a hum field of 1 x 10~3 gauss.