Bugs (Microphones) and Bug Detectors

Bugs (Microphones) and Bug Detectors


A key part of intelligence gathering and surveillance is the installation of listening devices. The classic Cold War image of Soviet espionage agents secretly planting "bugs" in an office of the United States embassy is an accurate historical picture of the use of these listening devices. Police forces and private investigators also use bugging devices (with legal approval).

The use of listening devices is often a race to acquire information before the devices are discovered and removed. For example, rooms where top-secret intelligence activity occurs are frequently examined, or "swept", for bugs.

A typical electronic bug consists of a microphone and a radio transmitter. The microphone receives sound waves and either vibrates a thin membrane called a diaphragm (a dynamic microphone) or a thin metal ribbon suspended in a magnetic field (a ribbon microphone). Vibration of the diaphragm produces an electrical signal. Vibration of the metal ribbon produces a voltage change, which can be converted to an electrical signal.

The electric signals are then beamed out of the transmitter portion of the bug to a receiver. The conversation

Sinn Fein President Gerry Adams dsiplays an electronic tracking and listening device, found in a car used by Sinn Fein leaders, during a press conference in Belfast, Northern Ireland in 1999. AP/WIDE WORLD PHOTOS.
Sinn Fein President Gerry Adams dsiplays an electronic tracking and listening device, found in a car used by Sinn Fein leaders, during a press conference in Belfast, Northern Ireland in 1999.

transmitted by the bug to the receiver can be recorded or listened to directly. Other types of bugs exist. For example, radio frequencies passing through the electrical wiring of a building can be intercepted. Bugs can also intercept the electrical transmissions from portable phones, wireless computers linked to a network, and even from a computer monitor.

The designation of secret listening devices as bugs is entirely suitable, given their small size. Modern bugs can be concealed in pens, calculators, and even buttons (although the latter need to be replaced frequently, as their power supply is so small).

The miniaturization of electronics has made it possible to pack more devices into the small package. For example, video equipment can be contained in a bug, enabling sight as well as sound surveillance.

Up to the 1980s, bugs operated using very high frequency, or VHF, radio waves. However, the development of mobile communications technology, particularly digital telephones, paved the way for the development of bugs that operate using ultrahigh frequency wavelength or microwaves. This has made the detection of bugs more difficult than simply detecting the output of radio waves. Some modern bugging devices can also disguise the output signal or vary the frequency of the signal, which can thwart detection.

Some bugs contain voice-activated recorders that are capable of storing up to 12 hours of conversation. The information can then be rapidly sent to a receiver in a "burst" transmission. Because detection of the bug is geared toward the frequencies emitted during transmission, the detection of these bugs is difficult. Counter systems are designed to try and activate the bug and then detect it. The transmission range of bugs has improved from mere yards to miles. Some bugs can even transmit to satellites, making monitoring from thousands of miles away feasible.

Another surveillance option is the use of a microphone. Conventional microphones operate electronically; the electrical signals representing the converted sound waves are passed through a wire to a receiving device located elsewhere. Microphones that operate using magnetic fields also exist.

Shotgun microphones equipped with a parabolic reflector can record conversation outside at a distance. Electronic filters screen out extraneous background noise in order to enhance the sensitivity of the microphone.

Laser microphones bounce a laser beam off of an object that is near the conversation. The object must be something that resonates, or is able to move as pressure waves created by noise in the room encounter it. As the object vibrates back and forth due to the sound waves from the conversation in the room, the distance traveled by the laser beam will become slightly shorter and longer. These length differences can be measured over time, and the pattern of the vibrations translated into the text of the conversation.

Microphones are extremely hard to detect, especially when used in a room where other electrical appliances (i.e., computers, telephones) are operating.

Bugs are detected by virtue of the frequencies they emit. Essentially a bug detector is a receiver. When brought near an operating bug, the detector will collect and amplify the bug's transmission. Bug detectors are now portable enough to be carried in a "sweep" of a room.

Bugs and microphones have moved from the arena of political espionage to the boardrooms of corporate offices and police surveillance operations. Recognizing the prevalence of electronic eavesdropping devices and their threat to privacy, the United States Congress passed the Electronic Communication Privacy Act in 1986, which made bugging illegal. Nonetheless, the use of eavesdropping devices and detectors is widespread in the intelligence and business communities. One estimate places the annual sales of such devices in the United States alone at $888 million.



Shannon, Michel L. Bug Book: Everything You Ever Wanted To Know About Electronic Eavesdropping…But Were Afraid To Ask. Boulder, CO: Paladin Press, 2000.

Shannon, Michel L. Don't Bug Me: The Latest High-Tech Spy Methods. Boulder, CO: Paladin Press, 2002.


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