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The RCA Type BK-11A
bi-directional ribbon microphone

RCA BK-11A

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RCA BK-11A

RCA BK-11A

RCA BK-11A

RCA BK-11A

RCA BK-11A

RCA BK-11A

RCA BK-11A

RCA BK-11A

Description
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.

Directional Pattern
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.

Frequency Response
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:

Interior
Photo courtesy of Mitch Manger


Text and illustrations are from RCA Type BK-11A Velocity Micro­phone Instruction Book No. IB-24895

BK-11A parts

BK-11A frequency response

BK-11A polar patterns
Figure 3—Directional Characteristics of Type BK-11A

Ad replica
A replica of a 1960 RCA advertisement.
There are no microphones for sale on this web site.

RCA BK-11A

RCA BK-11A

RCA BK-11A

RCA BK-11A

RCA BK-11A

RCA BK-11A

RCA BK-11A
Seven photos above are courtesy of Dennis Schrank.
Note the older style embossed logo.

1967 RCA catalog page
 

 

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