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Subminiature covert microphones

Knowles is a manufactuer of miniature microphones, based in Itasca (Illinois, USA). The company was founded in 1946 by Hugo Knowles and was among the first to manufacture subminiature microphones and speakers for hearing aids. In 1969, NASA even took their microphones to the moon. Knowles microphones are also found at the heart of many covert listening devices (bugs).

Knowles company logo

Knowles microphones come in many flavours and sizes, ranging from the first minature dynamic microphones that measured approx. 10 x 10 x 5 mm, to state-of-the-art MEMS-based variants that measure no more than a couple of millimetres. During the Cold War, much of the research and development at Knowles was financed by the US Central Intelligence Agency CIA, who was one of their main customers. The research was fed back to the CIA in a series of reports, e.g. [E].

Ironically, Knowles microphones were also used by the CIA's counterparts in East-Europe, often under the pretence that they were needed for the manufacture of hearing aids. A good example is the 31216 radio bug of the repressive and feared East-German (DRR) security service, the Stasi.

Knowles microphones on this website
1500 MHz covert transmitter with noise-based speech masking
ATET S-35 professional radio bug
290 MHz bug with TP audio masking
350 MHz bug with RP audio masking
DDR telephone line carrier bug
Stopfen RF bug 31217-111 (Stasi) with microphone in wooden plug (960 MHz)
Holzwurm (woodworm) RF bug 31217-1 in vertical stick (950 MHz)

Some examples of Knowles microphones. At the front is a 2016 Knowles MEMS microphone.

The Knowles 1501, also known as BA-1501, 1 was a very popular dynamic element for military headsets as well as for the early generations of covert listening devices (bugs) used by the CIA. It is also known as NSN 5965-00-015-7408.

In the 1960s and 1970s, the 1501 was used in a variety of bugs, such as the SRT-107. Measuring just 10 x 10 x 5 mm, the 1501 was one of the smallest microphones at the time.
Knowles BA-1501

The BA-1502 is basically identical to the BA-1501, except that it has its sound port at one of its sides, rather than on the top surface.

The image on the right shows the two variants side by side. Depending on the application, the most suitable shape was used. The two micro­phones shows here have been used in real life bugging operations, in which they were covered in polyether foam, that has clearly left its marks.
Knowles BA-1501 (left) and BA-1502 (right)

Knowles also produced sub-miniature electret condenser micro­phones, such as the BT-1750, BT-1751, etc. These micro­phones are generally known as the BT-Series, and were also used during the Cold War by the MfS (Stasi) of the former DDR (East-Germany) for use in their listening devices (bugs).

 Download datasheet
Click to see more

SWM-44 B
This microphone was developed especially for the CIA. It was intended for use in combination with covert equipment such as covert agent radios and covert listening devices (bugs). It consists of a subminiature Knowles electret microphone element that is cast in silicone.

The silicone body reduces any low-frequency vibrations caused by conductance (e.g. through the walls of a building) and makes it easier to handle the device.

 More information

Gel microphone compared to the size of a hand

  1. The letters 'BA' stand for Balanced Armature.

Knowles BA-1501
Knowles BA-1501
Knowles BA-1501
Collection of Knowles microphones
Collection of Knowles microphones
Collection of Knowles microphones
Small, smaller, smallest
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Knowles BA-1501
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Knowles BA-1501
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Knowles BA-1501
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Collection of Knowles microphones
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Collection of Knowles microphones
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Collection of Knowles microphones
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Small, smaller, smallest

Other Knowles devices
Below is a non-exhaustive list of other types of Knowles devices that have been reported [3]:

  • 1568
  • 1582
    Receiver (center-tapped)
  • 1588
  • 1634
    Receiver (center-tapped)
  • 1681
    Electret microphone
  • 1687
    Microphone with active pre-amplifier
  • 1714
    Dynamic microphone
  • 1722
  • BT-1750
  • BT-1751
    Electret microphone with sound port at the front
  • BT-1959
    Same but with large sound port at the top surface
  • 1784
  • 19x43
  • 1806
    (dual port)
  • BC-1520
  • TI 11214G
    Dynamic microphone
  1. Knowles BT-Series datasheet
    Issue 02, March 2001. 1

  2. Vibration Sensitivity of Knowles Microphones
    Knowles Technical Buletin TB19. S-321-0989. 1

  3. Knowles EA-21842 microphone datasheet
    April 2006. 1

  4. Effects on variation of supply voltage on the sensitivity of the BL microphone
    Application note S-083-566. Date unknown.

  5. Evaluation of the Knowles Ceramic Type Microphone (final report)
    Knowles/CIA. June 1970.
    Document kindly provided by Detlev Vreisleben [2].

  1. Knowles website, Knowles history
    Retrieved January 2017.

  2. Detlev Vreisleben, Knowles BT datasheet
    Personal correspondence, March 2017.

  3. Pete McCollum, Personal correspondence
    March 2017.
Further information
Some examples of Knowles microphones. At the front is a 2016 Knowles MEMS microphone.

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Crypto Museum. Created: Sunday 29 January 2017. Last changed: Wednesday, 04 November 2020 - 07:49 CET.
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