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Wall probe microphone
Audiotel wall probe kit

When eavesdropping on a conversation it is not always necessary to use an RF transmitter. If you have access to the building or an adjacent room, it might also be possible to 'listen through the wall'. This can be done with a contact microphone, or with a so-called wall probe microphone.
 
The image on the right shows an example of such a wall probe, that was manufactured by Audiotel in Corby (UK) in the late 1980s. The kit consists of a highly sensitive amplifier with a wide dynamic range, an extremely sensitive electret microphone in a metal enclosure and a pair of earphones to prevent acoustic feedback.

The kit comes with a set of ultra thin metal tubes that can be attached to the front of the microphone. If the tube is entered into a small hole in the wall, the eavesdropper can listen to any sound or conversation in the adjacent room.
  
Audiotel wall probe microphone with amplifier

Over the years, Audiotel developed a range of highly sensitive amplifiers for use with their wall probe microphones. The one featured on this page is the VA-095, which was manufactured in 1989. It is so sensitive that you can easily pick up whispering from the other side of the room.
 
In order to cope with the enormous dynamic range that is required for this kind of bugging, the VA-095 amplifier has 6 sensitivity ranges plus a separate volume setting (also ON/OFF switch). A built-in automatic compressor/limiter is used to suppress excessive sounds.

The standard microphone supplied with the kit is shown in the image on the right. It consists of a small metal cylinder of which the front can be unscrewed. Inside the cylinder is a sensitive electret microphone with a three-wire cable to the VA-095 amplifier's 3.5 mm jack socket.
  
Microphone with tube

The front part of the microphone enclosure has a tiny hole through which one of the supplied metal tubes can be inserted. Once the tube is inserted, the tip can be screwed back onto the microphone casing. The wall probe microphone, with the selected tube, is now ready for use.
 
Audiotel wall probe microphone with amplifier The sensitive probe microphone Long metal tube Metal tube inserted in the tip of the microphone Microphone with tube VA-095 amplifier with wall probe microphone Adjusting the volume Connections for earphones and a tape recorder

 
Controls
All controls are at the front panel of the VA-095 amplifier. The ON/OFF switch is combined with the volume adjustment at the centre. The microphone should be connected to the 3.5 mm jack socket at the top left, whilst a suitable pair of headphones is connected to the EAR socket at the bottom right. The GAIN control, to the left of the volume knob, provides six sensitivity levels.

Front panel of the VA-095 amplifier. Click to enlarge.

At the bottom right is a socket pair that can be used for the connection of an external tape recorder. Such recorders were commonly used in the 1980s, and most of them used a combination of a 3.5 mm jack (for the audio signal) and a 2.5 mm jack (for the remote control). The slide switch at the top right controls the recorder. When set to RUN, recording is activated. Setting the switch to VOX starts recording only when an actual conversation is taking place.
 
Eavesdropping
Once a suitable tube is mounted to the front of the microphone, it can be inserted into a hole in, say, the wall or the ceiling. Whenever possible, the investigator will try to use existing holes, such as a keyhole, an existing borehole of a cable, or a pipe of the central heating system.
 
The image on the right shows how the probe microphone, fitted with a short tube, is inserted into the keyhole of a door. Once the microphone is in place, it should be fixated, for example with a piece of duck tape. The operator should also ensure that there is no acoustic leakage to his own room, as this may lead to feedback noise (even when a pair of headphones is used).

The microphone is then connected to the VA-095 amplifier, the headphones are connected and the amplifier is switched on. If necessary, an extension cable can be used for the microphone.
  
Using a keyhole to eavesdrop on a conversation in the next room

If it is not possible to use an existing hole, the investigator can use a special drilling kit that enables him to drill a tiny hole through the wall without leaving a mark at the other side. In order to cope with varying thickness in walls, ceilings, etc. four different length tubes are supplied.
 
Four different tubes VA-095 front panel VA-095 amplifier with wall probe microphone Kit with various tubes VA-095 amplifier with wall probe microphone in use Using a keyhole to eavesdrop on a conversation in the next room Using a central heating pipe to get through the ceiling from the room above Using an existing hole in the wall to reach the next room

 
Interior
The metal housing of the VA-095 amplifier consists of two half case shells, that are held together with a large bolt at the center of the top half. After removing the large bolt with a screwdriver or a coin, the top half can be removed, giving access to the interior and to the standard 9V battery.
 
The image on the right shows the interior of the VA-095. The double-sided PCB has all its components on the top side and has a cut-out for the 9V block battery in the rear corner.

The circuit is very well designed and built. Immediately behind the microphone input is a ZN459 ultra low noise pre-amplifer, followed by several LM358 operational amplifiers that form the automatic fast-attack slow-release limiter.

Finally, the audio signal is amplified to earphone level using a standard LM386 audio amplifier.
  
VA-095 interior

In the 1980s, tape recorders or cassette recorders were commonly used for recording of conversations, intercepts and taps. As the length of the tapes was limited to, say, 90 minutes, automatic voice operated circuits (VOX) were used to record only the actual conversation. In the VA-095, the VOX circuit consists of another LM358 which finally drives a small Siemens relay. The latter was needed because it directly controlled the motor of the connected tape recorder.
 
Opened microphone VA-095 interior Top view of the interior Close-up of the wiring

 
Further information

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Crypto Museum. Last changed: Friday, 18 September 2015 - 21:51 CET.
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