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Sennheiser MM-21
Miniature microphone - under construction

MM-21 is a miniature dynamic microphone, developed and manufactured from the mid-1960s onwards by Sennheiser Electronic Laboratories in Wennebostel (Germany), for use is small devices like hearing aids. During the Cold War it was one of the favorite covert 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 two solder terminals at the long sides and a small circular sound port at the front surface. The complete microphone weights just 4 grams and has a good frequency response that covers the 400 - 4000 Hz voice spectrum. It is the basic model of a series with different types of sound port, like the MM-23 and MM-26.

Like the Sennsheiser MM-61 Concealed Fountain Pen microphone, the MM-21 was initially meant for use in combination with dictation machines and the first generations of (large) hearing aids.
Sennheiser MM-21 microphone

However, it was soon adopted as a concealed microphone for covert listening devices (bugs), both in the west and behind the Iron Curtain. The MM-21 was first sold in 1964, and was in production until the mid-1970s. In 1971 it was available for a price of $ 9.90. 1

  • Frequency
    400 - 4000 Hz ±3dB
  • Impedance
  • Sensitivity
    0.14 mV/µbar @ 1000 Hz with 2kΩ load
  • Type
  • Connection
    Standard electronic flash light socket
  • Size
    18.9 x 15.5 x 12.6 mm
  • Weight
    5.4 grams
  • Material
  • Shape
    Rectangular with circular sound port
  • First sold
  • Last sold
  1. Sennheiser microphones price list 1971
    Sennheiser Electronics Corporation. 1 June 1971. Retrieved June 2018.

  2. Sennheiser Micro-Revue 70-71
    1970. MM microphones pp. 55.
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Crypto Museum. Created: Tuesday 18 September 2018. Last changed: Tuesday, 18 September 2018 - 10:59 CET.
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