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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.
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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.
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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 weighs 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.
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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.
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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].
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In original Stasi documents, the device is referred to as Styk
[3].
In Czech and Polish, Styk means Contact.
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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.
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Piezo crystal in longitudinal or normal mode
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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.
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Piezo crystal in shear mode
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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.
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Obtained from BStU [2]
via Detlev Vreisleben[1], September 2018.
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© Crypto Museum. Created: Friday 21 September 2018. Last changed: Wednesday, 05 November 2025 - 11:42 CET.
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