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Contact microphone

Styk 1 is a normal-mode contact microphone (German: Körperschall-Mikrofon), developed in the mid-1980s — most likely in Czechslovakia, for the secret state police StB — and used by the repressive state security service of the former DDR (East-Germany), the Stasi. By design, a contact microphone picks up audio vibrations in a room through contact with solid objects. Its main advantage is that it does not require a (pin) hole in the wall or floor, which improves security.

Unlike a regular microphone, a contact micro­phone is almost completely insensitive to air vibrations. Most versions exploit the piezo-electric effect of certain ceramic materials; the same effect that is used in low-cost buzzers [4].

The image on the right shows a typical contact microphone that was used by the intelligence service Stasi of the former DDR, for overhearing conversions in an adjacent room, commonly as part of a wired or wireless covert listening device (bug). In Stasi-terminology, equipment used for audio bugging was referred to as B-Technik.
East-German Körperschall-Mikrofon (contact microphone)

The contact microphone shown here has a cylindrical body with a white rubber sleeve. The body is approx. 47 mm long (without the tip), has a diameter of 21 mm and weights 24 grams. At the front (i.e. the acoustic sensitive point) is a strong treaded metal spike. Before putting it to use, the spike is removed, so that it can be hammered into, say, a wall or a floor. Once that is done, the body is screwed onto the spike again, after which it is connected to the listening equipment.

The microphone was typically used by the Stasi in combination with wired audio frequency bugs, wired carrier frequency bugs and with wireless (radio) bugs, but the pre-amplifier of the bug had to be suitable (or adapted) for the different frequency response of the contact microphone.

Like most contact microphones, the one shown here uses the piezo sensor in longitudinal mode. Although this produces resonable results, the sound quality if far less than with a piezo sensor that is used in shear mode, like the SWM-25 that was used by the US Central Intelligence Agency.
Spike removed from the body

That being said, the SWM-25 is much heavier, needs power for an internal pre-amplifier and can not be removed easily, as it has to be glued to, say, a contrete wall [5]. The main advantage of a shear-mode device however, is that it is only sensitive to transversal waves that enter the device perpendicular — along its longitudinal axis — and far less to waves coming in from the side [5].

  1. In original Stasi documents, the device is referred to as Styk [3]. In Czech and Polish, Styk means Contact.

East-German Körperschall-Mikrofon (contact microphone) Spike removed from the body Contact microphone compared to the size of a hand Contact microphone compared to the size of a hand Contact microphone with (two) connection wires
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East-German Körperschall-Mikrofon (contact microphone)
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Spike removed from the body
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Contact microphone compared to the size of a hand
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Contact microphone compared to the size of a hand
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Contact microphone with (two) connection wires

The diagrams below show the difference between a piezo crystal in normal or longitudinal mode, and a piezo crystal in shear mode. A regular piezo sensor consists of a disc of piezo-electric ceramic that is glued against a metal disc (generally brass). In this case, the piezo disc is in between the two contacts. When the crystal is compressed, it delivers an electric voltage.

Piezo crystal in longitudinal or normal mode

A piezo sensor that uses shear mode, generally consists of two concentric cylindrical bodies that are firmly tied together. The cylinder at the center is the piezo crystal that is attached to the pick­up axis. The outer cylinder is a heavy mass that acts as the second contact. When exited by sound waves, the tension at the boundary between the crystal and the mass, causes an electric voltage.

Piezo crystal in shear mode

  1. Detlev Vreisleben, 31217-1, technical description and operating instructions
    Personal correspondence, May - August 2018.

  2. Bundesbeauftragte für die Stasi-Unterlagen (BStU) 1
    Federal Commissioner for the Stasi-Records.

  3. Stasi, Inventurliste, Aufnahmebereich: Aufgabe B
    Inventory list B-technology (German).
    GVS MfS o035-2043/87, 10 September 1987. Page 24. 2

  4. Wikipedia, Contact microphone
    Retrieved September 2018.

  5. Crypto Museum, SWM-25 shear-mode contact microphone
    September 2018.
  1. Full name: Bundesbeauftragte für die Unterlagen des Staatssicherheitsdienstes der ehemaligen Deutschen Demokratischen Republik (DDR) — Federal Commissioner for the Records of the State Security Service of the former German Democratic Republic (GDR) — officially abbreviated to BStU.
  2. Obtained from BStU [2] via Detlev Vreisleben[1], September 2018.

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© Crypto Museum. Created: Friday 21 September 2018. Last changed: Monday, 24 September 2018 - 15:35 CET.
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