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Type B   RCA BK-6B
Miniature dynamic microphone

Type-B is a miniature dynamic microphone, developed and manufactured around 1958 by RCA in Camden 1 (New Jersey, US) for the US Central Intelligence Agency (CIA). The device is based on the professional TV broadcast microphone BK-6B, but has a sound port 2 that can be extended with a hollow pipe, or probe, allowing it to be burried inside a wall behind plaster or wallpaper.

In 1958, RCA introduced the BK-6B microphone as the successor to its omni-directional BK-6A. It was about half the size of its predecessor and was intended for the professional TV broadcast market. Nicknamed PEEWEE it could be carried around the neck with a lanyard in interviews [A].

According to the brochure [A], it could easily be concealed in a man's hand. For use as part of a covert listening device (bug), APD 3 provided a new enclosure that allowed the sound port to be extended with a probe. The APD-modified BK-6B became known within the CIA as the Type-B [C].
Type-B (BK-6B) with probe

Extending the sound port of a microphone with a non-metallic hollow tube, allows the device to be hidden or embedded in a concrete or wooden wall, and reduces the chance of detection by means of a metal detector, as the actual microphone is located deep inside the wall. The hollow pipe, or probe, extends to the surface of the wall, and ends just behind the plaster or wallpaper, with a tiny pinhole aperture allowing the sound in the room to reach the microphone's diafragm.

According to a CIA memorandum of 6 August 1958 [2], the probe enclosures of the Type-B were obtained by APD from ASD 4 as samples of a program done in Frankfurt (Germany). In March 1959, a Type-B microphone was supplied to the Dutch Radar Laboratory (NRP) for use with an Easy Chair Mark III passive covert listening device (bug), as an alternative to the Shure MC-30 [3]. Because of the higher fidelity of the Type-B it was found to be more suitable in certain situations.

  1. RCA = Radio Corporation of America. It was once one of the most important developers of domestic and professional audio and video appliences in the US. Today, RCA is no more than a licensable brand name.
  2. The enclosure was developed and made by an unknown party in Frankfurt (Germany) [1].
  3. APD = Applied Physics Division if the. CIA.
  4. ASD = Audio Support Division of the CIA.

Type-B (BK-6B) with probe Side view of microphone with hollow pipe (probe) Type-B microphone (RCA BK-6B in special APD probe enclosure) BK-6B / Type-B Sound port Bare Type-B microphone Looking into the sound port Type-B microphone with 3'' probe
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Type-B (BK-6B) with probe
2 / 8
Side view of microphone with hollow pipe (probe)
3 / 8
Type-B microphone (RCA BK-6B in special APD probe enclosure)
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BK-6B / Type-B
5 / 8
Sound port
6 / 8
Bare Type-B microphone
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Looking into the sound port
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Type-B microphone with 3'' probe

Frequency response
At a CIA meeting of 24 July 1958 it was decided that frequency response curves of the BK-6B (Type-B) with and without the new probe enclosure would be made [2]. In March 1959, these curves were made available to the Dutch Radar Laboratory (NRP) [C][D]. Comparisons were made between pinhole apertures directly on the microphone, and when using 3", 6", 12" and 18" probes. The curve for the 3" hollow pipe shown in the photographs above, is shown here:

Orignal frequency response curve for the Type-B with a 3

 See the full report

  1. Miniature Dynamic Microphone Type BK-6B leaflet
    CM302604/A. RCA, Catalog B.1019. 5511B. Date unknown but probably 1958. 2 pages.

  2. BK-6B Miniature Dynamic Microphone - Instructions
    CM302604/B. RCA, IB-24776-I. Date unknown but probably 1958. 8 pages.

  3. Notes on the Type B Microphone
    CM302604/C. Internal CIA note. Retrieved 9 March 1959.

  4. Type-B frequency response curves with and without probe
    CM302604/F. CIA. Date unknown but probably August 1958.
  1. CIA, Collection of documents related to the Type-B and MC-30 microphones
    Crypto Museum Archive, CM302604 (see above).

  2. CIA, Memorandum for the record, Meeting of 24 July 1958
    6 August 1958. 1

  3. Technical Manual of EC. Mk. III equipment
    CM302533/F, August 1959 (est.). Page 10.
  1. Partially released by the CIA on 3 May 2011 under the FOIA.

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Crypto Museum. Created: Sunday 02 April 2017. Last changed: Thursday, 27 September 2018 - 13:45 CET.
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