In the modern era of bugging, no conversation in office or home is safe from eavesdroppers. The bugging is usually done by smuggling a small but sensitive microphone into the room.
One type of bug uses a low-powered radio transmitter attached to the microphone to send its signals to a receiver a few yards away. Another type has the microphone connected to a receiver by a cable.
Although a wired-in system requires careful concealment of the cable, it has the advantage of not broadcasting a radio signal, which is easy to detect. And most rooms or offices have so much cable already installed that the extra wire goes unnoticed. The microphone can be designed to resemble a household appliance, like a light bulb, television or telephone.
Provided a listening post can be found within a reasonable radius of up to a kilometre and a half, every word spoken in the target room will be relayed over the wire. An existing cable, such as a spare telephone line, is usually used to carry the voices.
One of the reasons why diplomatic buildings are so closely guarded when under construction is the fear that a foreign intelligence agency might try to incorporated cable conduits into the fabric of the building. This occurred in Moscow in 1987, where all the steel beams delivered to the site of the Us Embassy were discovered to be hollow.
If an eavesdropper cannot gain access to a room, he can put a bug called a stethoscope to one of the walls from the outside. It is a simple microphone pressed to the wall, and rigged to an amplifier. Alternatively, a hole can be drilled through the wall from the outside, ending as a tiny pinhole on the inside of the room. A microphone in a tube is placed in the hole and connected to a tape recorder outside. A variation is the ‘spike’ which penetrates only part of the wall, but nevertheless picks up all the sounds in a room.
Inserting a device into a target room can be difficult if the room’s occupants know they may be spied on. So the laser has been exploited to help the eavesdroppers. A laser beam is focused on the window of the room. When a conversation takes place inside the room the glass in the window vibrates to the voice waves, and the microscopic movements of the glass can be detected by measuring the tiny variations in the length of the fixed laser beam. This information can then be converted back electronically into an intelligible form.
The cavity microphone is probably the ideal cordless device, requiring neither batteries nor maintenance. It consists of a capsule, about 1in (25mm) wide, containing a sensitive aerial and a diaphragm (a thin disc). When a radio beam is directed at the room from outside, the capsule is transformed into a radio transmitter that picks up sounds in the room, allowing any conversation to be overhead.
One of these bugs was found in a model of the American Great Seal that had been presented to the US Ambassador by the Mayor of Moscow in 1952 and had hung over his desk for years.
Picture Credit : Google