The RCA Type BK-1A semi-directional pressure microphone (1952)
RCA Type BK-1A, MI-11007, on Desk Stand KS-11, MI-11008
Description The Type BK-1A pressure Microphone is a high-fidelity instrument of the pressure-actuated type, especially designed for announcing and remote pickup. Its smooth response and frequency range (60 to 10,000 cycles) make it suitable for reproducing both music and speech. It is effectively non-directional when mounted vertically and is semi-directional when mounted horizontally.
The moving element consists of a lightweight molded diaphragm attached to an annular coil assembly which is placed within a magnetic field. Coupled to the diaphragm is an acoustic circuit so proportioned that the diaphragm velocity remains essentially constant for a constant sound pressure from 60 to 10,000 cycles. The coil is connected to a transformer which provides output impedances of 30, 150, and 250 ohms.
When mounted vertically the BK-1A Microphone is non-directional with the higher frequencies uniformly attenuated. When mounted horizontally, the microphone is essentially non-directional for frequencies below 2,000 cycles, and the higher frequencies are attenuated more as the angle with the perpendicular line to the diaphragm increases. Figure 3 shows the relative directional output or pickup with the microphone horizontally positioned.
The BK-1A Microphone is particularly recommended for broadcast announcing and remote pickup. It is a small, lightweight microphone which may be carried in the hand for interview and mobile use or used with a stand. The relatively high output level, which provides a good signal-to-noise ratio, is advantageous for remote work. When it is used in the open air, its construction makes the effect of air currents practically negligible. The microphone is also excellent for many studio applications which require a non-directional or semi-directional microphone.
Caution: Airtight joints above the foot of the BK-1A Microphone are essential to its proper operation. Use a good grade petroleum jelly to seal the joints when re-assembling after servicing.