The eavesdropper has at his disposal microphones that are smaller than the head of a pin which will pick up a whisper at up to twenty feet. There are several different types of common microphones, such as the carbon, condenser, magnetic and electret microphones. The professional eavesdropper also has a virtually unlimited selection of others to choose from, including tube microphones, piezo film, hydrophones and even electrical switches, which are sometimes microphonic.
Of all the microphones utilized by the eavesdropper, the contact and spike microphones are perhaps the only ones specifically designed for this purpose. These microphones contain a special crystal which, when slightly compressed, will produce a very small electrical signal. If placed against a vibrating wall or attached to a rigid probe which is touching one of the vibrating surfaces, the crystal will produce small electrical signals which correspond to the vibrations. If these vibrations are caused by room conversations, the electrical signal will correspond to those conversations.
The pneumatic cavity microphone is an electronic version of a glass tumbler against the wall, historically recognized as one method of monitoring adjacent room conversations. This electronic version is substantially superior, as it is highly responsive to surface vibrations at audio frequencies found in human speech. Several manufacturers offer these microphones as electronic stethoscopes.
One of the most frequently used microphones is a simple speaker usually found in a radio. T.V. or intercom. Additionally, numerous ceiling speakers, typically used for background music, are commonly found throughout most office areas.
Most speakers are structurally similar to a magnetic microphone with a coil of wire positioned in a magnetic field. When used as a simple speaker, electrical current is passed through the coil which vibrates the speaker to provide sound. Most speakers show varying degrees of reciprocal performance and can therefore be used as a microphone. When acoustical energy impinges on an unused speaker cone and vibrates the coil of wire in the permanent magnetic field, small amounts of electrical energy are produced which can be transmitted by a radio frequency transmitter or over wires to a listening post.
Hardwired Audio Transmission
Regardless of which type of microphone that is used by the eavesdropper, intelligence must be transmitted out of the target premise. With the exception of a radio transmitter with a built-in microphone, the eavesdropper will normally connect the microphone to an amplifier, transmitter or tape recorder by conductive paint or extremely small wires. Wires thinner than a human hair, and as flexible, can be purchased at most local electronic shops. These wires can be sewn in the carpet or hidden in a variety of different ways. Once outside the target area, the conductive paint or tiny wires are usually connected to unused wires in a ceiling or wall, and then carry the signal to the listening post or to a radio transmitter.