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Sennheiser MM-301
Sub-miniature microphone

MM-301 is a sub-miniature dynamic microphone capsule, or module, introduced around 1970 by Sennheiser Electronic Laboratories in Wennebostel (Germany). It is the base design for many other Sennheiser capsules of the era and was also sold to other manufactuers as an OEM part.

During the Cold War, the MM-301 was one of the favorite microphones of both Western and Eastern intelligence and law enforcement agencies, mainly because of its high audio quality and small size.

The body of the microphone measures just 9.5 x 7.3 x 4.4 mm, which means that it takes about 1/7 of the space of other Sennheiser capsules of the era, such as the MM-21 and the MM-23. It has two solder terminals for connection of the audio cable at one of the short sides, and a pin-hole sound port at the centre of its largest surface.
Sennheiser MM-301 dynamic microphone
The complete microphone weights just 0.8 grams and has an excellent frequency response, that covers the 500 - 6000 Hz voice spectrum. Although it was officially designed for hearing aids and dictation machines, it was soon adopted by the intelligence community for use in combination with covert listening devices (bugs) and covert recorders, such as listed elsewhere on this site.

They were used for example by the US Central Intelligence Agency (CIA) with radio bugs of the 1970s — like the Easy Chair devices developed by the Dutch Radar Lab — as an alternative to Knowles microphones. Being of the dynamic type, they do not require a separate DC power source and do not consume any battery current.

But the microphones were equally popular at the other side of the Iron Curtain, such as in the former DDR (East Germany) where they were used by the much-feared Stasi (MfS) with wired audio bugs like the Bremen 20 and the 31550-6.
Sennheiser MM-23 and MM-301 (front)

The image above shows the MM-301 in front of an MM-23, another popular Sennheiser capsule of the era. As it is much smaller than the other members of the Sennheiser MM-series, it is even more suitable for covert applications. The impedance of the microphone is 4500Ω at 1000 Hz (more than twice that of the MM-23) making it ideal for transformerless transistor applications.

Sennheiser MM-301 microphone Sennheiser MM-301 dynamic microphone Sennheiser MM-301 dynamic microphone Sennheiser MM-23 and MM-301 (front) Sennheiser MM-22 and MM-301 (front) Sennheiser MM-301 microphone seen from the top Sennheiser MM-301 microphone with wiring Sennheiser MM-301 microphone close-up
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Sennheiser MM-301 microphone
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Sennheiser MM-301 dynamic microphone
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Sennheiser MM-301 dynamic microphone
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Sennheiser MM-23 and MM-301 (front)
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Sennheiser MM-22 and MM-301 (front)
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Sennheiser MM-301 microphone seen from the top
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Sennheiser MM-301 microphone with wiring
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Sennheiser MM-301 microphone close-up

Equipment that uses the MM-301
East German wired audio bug 'Bremen 20' used by the Stasi Stasi wired audio bug (NF) RF bug 31217-132 (Botond) concealed in piece of wood
  • Frequency
    500 - 6000 Hz
  • Tolerance
    500-1000 Hz ±3dB, 100-5000 Hz ±4dB
  • Impedance
    4500Ω @ 1000 Hz
  • Sensitivity
    0.12 mV/µbar @ 1000 Hz with 5kΩ load
  • Type
  • Connection
    Solder terminals
  • Size
    9.5 x 7.3 x 4.4 mm
  • Weight
    0.8 grams
  • Material
  • Shape
    Rectangular with pin-hole sound port at the centre
  1. Sennheiser microphones price list 1971
    Sennheiser Electronics Corporation. 1 June 1971. Retrieved June 2018. p. 6.

  2. Sennheiser Micro-Revue 70-71
    1970. MM microphones p. 55.
Further information
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Crypto Museum. Created: Monday 18 June 2018. Last changed: Friday, 21 September 2018 - 12:32 CET.
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