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Contact microphone

SWM-25 is a Piezo Ceramic Shear Tube Contact Transducer, also known as a contact micro­phone, 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 operations. 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 transistor circuit because of the lower voltage required. This allows the SWM-25 to be powered by a single military-grade 1.35V mercury cell.
SWM-25 contact microphone

To avoid corossion when embedded inside a concrete wall, and to provide internal damping, the device is hermetically sealed with several O-rings, and the voids inside the enclosure are filled with oil. Detailed technical and operational documentation is available for download below, including a test report by the CIA and an improved amplifier circuit developed at the NRP [E].
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. 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 1 was modified for non-air use [E].
Two slightly different SWM-25 models

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, 2 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.
Inside the SWM-25

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 final version 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.
  1. In this context, the expression video encoder is used for the audio masking unit of the SRT-56.
  2. In the first protype, a single FET pre-amplfier was used, but this required a higher supply voltage.

Two slightly different SWM-25 models SWM-25 contact microphone Shear-mode piezo-ceramic transducer Pre-amplifier Cylindrical enclosure Two SWM-25 models Two SWM-25 models

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.

SWM-25 - cross section

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 SRT-56.

SWM-25 - cross section - exploded view

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 [2]. The shear-mode piezo effect takes place at the outer surface of the ceramic element where it meets the Mallory seismic mass.
  • Power supply
  • Current
    40 µA at 1.5V supply
  • Output
    500 µV ± 6 dB at 2 kHz, with 100 µg acceleration drive
  • Frequency
    Flat ± 3 dB from 600 Hz to 3000 Hz
  • Resonance
    5 kHz (min.)
  • Input drive
    5 mg acceleration (max.) for undistorted output
  • Temperature
    -17 °C to +65 C
  • Dimensions
    1" x 2" (~ 2.5 cm x 5 cm)
  • Weight
    185 grams
  1. Contact Transducer System - Final Report
    CM302605/A. (CIA). Date unknown.

  2. Test report on the Piezo Ceramic Sheer Tube Contact Transducer SWM-25
    CM302605/B. CIA, Equipment Suitability Groups (ESG), Project 69-156.

  3. SWM-25 Pre-amplifier notes
    CM302605/C. (CIA) Author and date unknown.

  4. Ceramic Shear Tube Contact Transducer Microphone - Technical Manual
    CM302605/D. (CIA). August 1970.

  5. Audio amplifier compressor for non-air path microphones
    CM302605/E. NRP, October 1970.

  1. CIA, Collection of documents related to the SWM-25 contact microphone
    Crypto Museum Archive, CM302605 (see above).

  2. Wikipedia, Mallory metal
    Retrieved April 2017.

Further information

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Crypto Museum. Created: Sunday 02 April 2017. Last changed: Saturday, 22 April 2017 - 14:25 CET.
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