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The RCA Types BK-5A and BK-5B
Uniaxial ribbon microphones


Three photos above are courtesy of blues musician Chas. DelPlato.

The RCA Type BK-5A Uniaxial Microphone is a depen­dable, high-quality ribbon instrument possessing an im­proved unidirectional characteristic, and is designed for broadcast use in AM, FM, and TV stations. The micro­phone has a frequency response that is essentially uniform from 50 to 15,000 cycles. Its excellent response and frequency range, combined with its unexcelled cardioidal directional characteristic, makes it ideal for reproducing both speech and music.

Perspective view of the BK-5A showing the high frequency and low frequency directional characteristics about the major axes. At low fre­quen­cies, the pickup pattern is a true cardioid. At 5000 CPS and above, the pattern becomes fan shaped. This can be utilized to reduce pickup of scraping noises from the floor of a television studio set.

The microphone has been engineered with the television studio in mind. Since maximum sensitivity lies on the major mechanical axis, it is a one-axis, or uniaxial type mic. This directional characteristic simplifies micro­phone and camera placement problems. Incorporated in the unit is a blast filter which reduces damage to the micro­phone from gun blasts and other violent noises. In addition, the small size, light weight, unobtrusive yet attractive gray finish and appearance render it especially suitable for television, but it is also admirably suited to general broadcasting and high-fidelity sound systems.


The Type BK-5A Microphone is a unidirectional micro­phone in which the moving element is a thin corrugated metallic ribbon clamped under light tension to cause it to vibrate at its own resonant frequency. The ribbon is placed between the pole pieces of a magnetic circuit. One side of the ribbon is open to the atmosphere and the other opens onto an acoustical labyrinth which has phase-shift openings giving the instrument its improved uni­directional characteristics. The labyrinth of the mic houses an impedance matching transformer and switch for selecting response characteristics for voice or music.

Wind Screen

A unique feature of the BK-5A is a blast filter consisting of two separate cloth layers supported by perforated metal screens. The filters reduce damage to the mic from gun blasts and other loud noises encountered in broad­cast programming. In addition, the transformer is ex­ceptionally well shielded against stray magnetic fields and can perform satisfactorily in high hum fields. As further protection for the sensitive vibrating ribbon, a wind screen is available for use with the instrument. Its use is recommended if the mic is to be used outdoors.

Blast screen effect Wind Screen

Gun Blast Resistance
There have been many occasions when ribbon mics have been dam­aged during television shows by guns being fired close to the mic. Tests at RCA’s David Sarnoff Research Center showed that the ribbon dam­age was not done by the steep-fronted blast wave, but rather by the concussion wave that followed as the air moved into the vacuum created by the explosion. The concussion wave contains many low-fre­quen­cy com­po­nents. If it were possible to isolate the ribbon from these com­po­nents, a con­sid­er­able measure of protection would be effected. To ac­com­plish this, two layers of fine mesh cloth are supported in front of the ribbon by the horn-like structure shown in Fig. 8. These form an acoustical filter which reduces the concussion wave of gun blasts.

Blast baffles

Figure 8

During laboratory tests it was possible to fire .38-caliber blanks as close as three feet from the micro­phone and .32 caliber blanks as close as one and one-half feet without any measurable effect on the performance. The tests were made with the gun fired at right angles to the micro­phone and directly in front of it.

The ribbon chamber

The integration of the blast filter, acoustic phase-shift network and specially designed connector to couple the ribbon to the labyrinth is responsible for the unique uni­axial characteristic of the BK-5A, and uniform frequency response over the aural spectrum. The micro­phone is sup­ported on a cushion mounting which has a half-inch internal pipe thread to fit RCA desk or floor stands. An improved shock mount based on panel meter mounts designed for military use is incorporated in the optional Boom Unit. This new mount isolates the mic from vibration and shock transmitted by the boom. There are no rubber band mountings to wear out. A thirty-foot flexible cable supplied with the micro­phone makes use of tinned cadmium bronze wire to provide longer life.

Shock Mount

The small size and axial directivity aid in placing the BK-5A in inconspicuous fixed locations. There are no shiny external parts to reflect light and draw attention to the instrument. The axial directivity combined with the Boom Mount (MI-11012) make the micro­phone easy to handle to keep the sound source in focus. The addition of the wind screen to this combination does not cause a loss of the sense of the pickup axis.

Horizontal Longitudinal Vertical

Polar Response

Music-Voice selector Frequency Response

Frequency Response

RCA BK-5B data

Copied from the BK-5B RCA Instructions book IB-24887-4


Copied from another RCA source (note differences, such as stand fitting).

RCA BK-5A Impedance settings Ellis Jeff BK-5B BK-5A B Sheet BK-5A B Sheet 1967 RCA catalog page 1967 RCA catalog page

See a brand new (old stock) in the box BK-5B.

See another new-in-box BK-5B.

New logo

The new RCA logo on a Type BK-5B.

Photo credits: Library of American Broadcasting, University of Maryland.
Broadcast News, Vol. 83, May, 1955, pp. 44-47, Radio Corporation of America.

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