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

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

Description
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.

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.

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.

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 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.

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.