SWM-25 is a Piezo Ceramic Shear Tube Contact Transducer, also known as a
or non-air microphone,
developed in 1969/1970 by an unknown manufacturer
for the US Central intelligence Agency (CIA).
It was intended for listening through thick concrete walls in covert
The main advantage of a contact transducer over a traditional pinhole
microphone, is the absense of a pinhole, which greatly improves the
security of a clandestine operation [A].
The device works by passing vibrations from the wall to a piezo-ceramic
element operating in shear mode.
The image on the right shows the SWM-25 microphone, which has a diameter of
1 inch (2.54 cm) and is 2 inches long (about 5 cm).
In order to avoid noise as a result of the very faint signals, a small
pre-amplifier is built inside the microphone's enclosure. Initially this
was a FET-amplifier, but this was later changed into a
because of the lower voltage required.
This allows the SWM-25 to be powered by a single military-grade
1.35V mercury cell.
The image on the right shows two early variants of the SWM-25: a prototype
and an engineering sample. The cable fitting was later improved.
In October 1970, the CIA provided several SWM-25 microphones
to the Dutch Radar Laboratory (NRP),
with the request to adapt an existing
SRT-56 covert listening device (bug)
for its use. 1 The NRP first developed a new amplifier with built-in compressor
for the SWM-25, as the amplifier supplied by the manufacturer
was found to be inadequate.
Next, an existing SRT-56 video encoder 2 was modified for non-air use [E].
This was necessary, as the frequency response curve of the SWM-25
is different from the curve of a standard dynamic (air) microphone.
As visible in the above images, three wires are needed for the connection
of the SWM-25. In the final version, this was
replaced by a single 4-wire cable.
The image on the right clearly shows the internal construction of the SWM-25.
Both ends of the brass cylinder are sealed with O-rings, in order to
prevent the oil from leaking out. The ends are held in place by
miniature screws. The top end contains the actual microphone
(here visible at the far right) and is sealed with another O-ring.
The other end of the cylinder contains a small
two-transistor pre-amplifier, 3
which is housed in a milled-out section of the bottom end plate.
The two unconnected (black and green) wires are normally soldered
to the microphone element.
In operation, the microphone was usually attached (glued) with its
flat top-end (here visible at the right) to a wooden or concreate
separation wall, in such a way that vibrations in the wall enter the
microphone perpendicular, along its longitudinal axis. The
is slightly different from the ones shown here, in that
it has a single 4-wire cable with a cone-shaped strain-relief [D].
A disadvantage of a contact microphone, is that it is also sensitive to
vibration from sources other than speech, which adds to the
low-frequency noise, and reduces the intelligibility.
Examples of such noise sources
are air conditioning machines, people walking through the building,
water running through pipes, slamming doors, cars driving by,
wind blowing against the building, etc.
In practice, this was solved by filtering off the lower frequencies
and using a pre-emphasis.
This was done under the Easy Chair
In this context, the expression video encoder is used for the
audio masking unit of the SRT-56.
In the first protype, a single FET pre-amplfier was used, but this
required a higher supply voltage.
The diagram below shows a cross-section of the SWM-25, in which the position
of the various parts is clearly visible. In this drawing, the bottom end
(through which the cables are fed-in) is at the left. The sensitive
side that contains the piezo-ceramic sensor, is at the right.
For an optimum transfer of acoustic vibrations, it is glued to
the wall, which was commonly made of concrete.
In order to reduce noise and hum, the weak signal from the piezo-ceramic
sensor is first amplified in a two-stage amplifier that is housed inside
the same enclosure. The amplifier was usually powered by a single 1.35V
mercury cell, separate from the batteries of the
The diagram above shows an exploded view of the cross section, in which
the individual parts are visible. The actual element is
an integral part of the top of the cylinder. The ceramic material
is rigidly mounted onto the central (brass) axis and is surrounded
by a heavy metal seismic mass, which is made of Mallory metal, that is
twice as heavy as steel . The shear-mode piezo effect takes place
at the outer surface of the ceramic element where it meets the Mallory
Current40 µA at 1.5V supply
Output500 µV ± 6 dB at 2 kHz, with 100 µg acceleration drive
FrequencyFlat ± 3 dB from 600 Hz to 3000 Hz
Resonance5 kHz (min.)
Input drive5 mg acceleration (max.) for undistorted output
Temperature-17 °C to +65 °C
Dimensions1" x 2" (~ 2.5 cm x 5 cm)
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© Crypto Museum. Created: Sunday 02 April 2017. Last changed: Tuesday, 13 June 2017 - 11:01 CET.