The RCA Type BK-11A
bi-directional ribbon microphone
The RCA Type BK-11A Velocity microphone, as shown above, is a dependable, bi-directional microphone, specially designed for AM, FM, and TV studio use where a microphone of highest quality reproduction is desired. The BK-11A is constructed to withstand mechanical shocks and to retain its sensitivity and frequency response regardless of changes in temperature and humidity. The microphone is exceptionally well shielded and can perform satisfactorily in high hum fields. Acoustically designed, sturdy stainless steel screens protect the microphone from mechanical injury.
The moving element is a thin, corrugated metallic ribbon supported at the ends and placed between the pole pieces of a magnetic circuit. Because of its light weight, the motion of the ribbon corresponds very closely to the velocity of the air particles and the voltage generated in it is therefore a reproduction of the sound waves which traverse it. The microphone is free of the effects of cavity resonance, diaphragm resonance and pressure doubling.
The uniform frequency response, 30 to 15,000 cycles, is suitable for high fidelity pickup of voice and music. The three-position (M, V1, V2) screwdriver switch may be easily adjusted to the position suitable for the application depending on the distance between the sound source and the microphone.
The directional patterns are shown in Figure 3. The bi-directional pattern of the velocity microphone permits a pickup distance 1.73 times that of a non-directional microphone for an equal signal to random acoustical noise. This is the same increase in distance that is obtainable with a uni-directional microphone having a cardioid pattern. The bi-directional pattern may be used to greater advantage when the unwanted sound or reflections are directed in to null plane. The usual vertical position permits cancellation of floor and ceiling reflections, and by careful orientation, other positions may be used to reduce reverberation and unwanted sound.
The BK-11A response to sounds originating in the null plane is shown by the 90° curves in Figure 4. Unlike other types of directional microphones, the null plane provides excellent rejection of sound over the entire audio frequency range. This is particularly valuable for instance, when the microphone is positioned to eliminate feedback from sound-reinforcing loudspeakers.
The frequency response to a sound source more than three feet from the microphone (plane wave) is shown in Figure 4, which includes curves for various angles of pickup. The response to sound sources located over the useful angle of pickup, in the front or rear of the microphone, is more uniform than has been previously obtained in a ribbon velocity microphone; therefore, there is little loss in quality with off-axis pickup.
When the sound source is closer than three feet, this type of microphone has a rising response characteristic at low frequencies, To compensate for this effect, the BK-11A incorporates a shunting reactor and a three-position, screwdriver-operated switch. The switch positions and approximate source distance for uniform response are as follows:
Text and illustrations are from RCA Type BK-11A Velocity Microphone Instruction Book No. IB-24895
Figure 3—Directional Characteristics of Type BK-11A
A replica of a 1960 RCA advertisement.
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Seven photos above are courtesy of Dennis Schrank.
Note the older style embossed logo.