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Sennheiser
Covert microphone capsules

Sennheiser is a German privately held manufacturer of professional audio equipment, specialising in the development and production of a wide range of high-end products, such as (wireless) microphones, headphones, telephony accessories, avionics headsets, conference equipment and other professional and business applications. It has its headquarters in Wedemark (Germany).


Over the years, Sennheiser produced a wide range of (bare) microphone elements, that were used in products from other manufactuers (OEM). During the Cold War, such microphones were used in covert listening devices (bugs) and covert recording equipment, by both sides of the Iron Curtain.

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Sennheiser microphones on this website
Wired Sennheiser microphone disguised as a fountain pen Sennheiser lapel microphone Sennheiser sub-miniature dynamic capsule
Variants
The MM-23 was available in a number variants, each haused in a similar enclosure, but with a different sound port. Below are the Sennheiser capsules that we have identified so far.   
Sennheiser MM-23, MM-22, MM-26 and MM-301 (front)

MM-21
This is the basic design of the MM-20 range. It consists of a rectangular metal enclosure with curved edges, and is available in different versions, each with a different sound port and an different frequency response curve, adapted for a variety of applications.

The MM-21 has a small pin-hole sound port at the top surface. At present, no image of an MM-21 is available, but it is very similar to the MM-26, albeit with a smaller sound port.

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Sennheiser MM-21 microphone

MM-22
This is basically the same microphone, but with a slotted sound port at one of the short sides.

The MM-22 shown here as seen quite some action and shows sign of corrosion. It was used for many years by the US Central Intelligence Agency (CIA) with an SRT-56 bug.
  
Sennheiser MM-22 microphone with slotted sound port at the front

MM-23
This is a special variant of the MM-20 range and was known as button or lapel microphone. It has a rather special sound port at the top surface that can be fitted through a buttonhole.

Unlike the other MM-20 variants, it does not have solder terminals. Instead is has a socket to which a standard electronic flash light plug can be fitted. Suitable plugs and cables were widely available at the time.

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Sennheiser MM-23 buttonhole microphone

MM-26
This is the same capsule as the MM-21, but with a larger sound port at the top surface. As a result, it has a better frequency response curve.

The one shown here is glued onto a metal bracket that was probably used to attach it to the operator's clothing. It was found with a Tesla E-120 Mini Corder and shows that during the Cold War, the Western Sennheiser microphones were also popular behind the Iron Curtain.
  
Sennheiser MM-26 with metal bracket

MM-61
MM-61 is a dynamic microphone disguised as a fountain pen. Although officially announced for use in combination with dictation machines, it is clear that it was much more suitable for covert recording of conversations.

Despite its 'modern' look, the design is rather old. The MM-61 was manufactured from 1962 to 1975 and was used by both West and East.

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Microphone concealed as a fountain pen made by Sennheiser

MM-301
This microphone capsule was introduced around 1970 and occupies about 1/7 of the space of the other MM capsules, making it even more suitable for use with covert listening devices (bugs).

Although it was originally intended for with with hearing aids, it soon became the preferred micro­phone for bugs, not just in the West, but also in Eastern Block countries like East Germany (DDR).

 More information
  
Sennheiser MM-301 dynamic microphone

Documentation
  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. 54—55.
Further information
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Crypto Museum. Created: Tuesday 19 June 2018. Last changed: Tuesday, 18 September 2018 - 11:02 CET.
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