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Sennheiser
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.
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The complete microphone weighs
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.
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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.
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Equipment that uses the MM-301
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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 Dynamic Connection Solder terminals Size 9.5 x 7.3 x 4.4 mm Weight 0.8 grams Material Metal Shape Rectangular with pin-hole sound port at the centre
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© Crypto Museum. Created: Monday 18 June 2018. Last changed: Wednesday, 05 November 2025 - 11:43 CET.
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