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Sennheiser MM-23
Buttonhole microphone

MM-23 is a miniature microphone, also known as buttonhole microphone or lapel micro­phone, developed and manufactured from the mid-1960s onwards by Sennheiser Electronic Laboratories in Wennebostel (Germany). During the Cold War it was one of the favorite microphones of both Western and Eastern intelligence and law enforcement agencies, because of its high audio quality.

The body of the microphone measures 19 x 12.6 x 6.7 mm. It has a coaxial socket for connection of the audio cable at the bottom, and a button-type sound port at the front, allowing it to be worn in clear view through the buttonhole of a jacket. The complete microphone weights just 4 grams and has an excellent frequency response, that covers the 400 - 4000 Hz voice spectrum.

Like the Sennsheiser MM-61 Concealed Fountain Pen microphone, the MM-23 was initially meant for use in combination with dictation machines and the first generations of (large) hearing aids.
  
Sennheiser MM-23 buttonhole microphone

However, it was soon adopted as a concealed microphone for covertly making recordings and transmissions. As it was worn overtly (i.e. not concealed) it delivers a far better audio quality than a concealed one, which is often hidden under the operative's clothing. The MM-23 was first sold in 1964, and was available for no more than DM 43 1 . It was in production until the mid-1970s.

  1. List price in 1964: DM 43 (approx. EUR 22).

Sennheiser MM-23 buttonhole microphone Cable socket Seen from the top Bottom view Side view MM-23 compared to the size of a hand
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Sennheiser MM-23 buttonhole microphone
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Cable socket
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Seen from the top
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Bottom view
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Side view
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MM-23 compared to the size of a hand

Variants
The MM-23 was available in a number variants, each haused in a similar enclosure, but with a different sound port. The image on the right shows the different versions we've seen so far.   

Specifications
  • Frequency
    400 - 4000 Hz
  • Impedance
    2000Ω
  • Type
    Dynamic
  • Connection
    Special coaxial socket
  • Size
    12,6 x 19 x 15.5 mm
  • Weight
    4 grams
  • Material
    Metal
  • Shape
    Rectangular with button-type sound port
  • First sold
    1964
  • Last sold
    1975
References
  1. Radiomuseum, Mikrofon MM 23
    Retrieved April 2017.

  2. Museum in der Runden Ecke, Magnetisches Knopfloch-Mikrofon MM 23
    (German) Retrieved April 2017.
Further information
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Crypto Museum. Created: Wednesday 05 April 2017. Last changed: Tuesday, 13 June 2017 - 10:59 CET.
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