This was originally posted on http://www.wisegeek.com/what-is-audio-surveillance.htm, credit should go them as this is a interesting article.
Audio surveillance is the act of listening to third-party conversations and recording them. This technique is frequently used by law enforcement,Â private detectivesÂ and government spy agencies. Most audio surveillance consists of either bugging a room, wearing a wire, tapping a phone or distance listening. Each provides distinct advantages and disadvantages, depending on the situation.
WiretappingÂ is one of the most common and simple form of audio surveillance. This is preferred because it is highly inconspicuous and allows for two sides of a conversation to be clearly recorded. Small audio devices, commonly called bugs, are attached to the internal circuitry of a telephone to pick up a conversation. A signal is wirelessly transmitted to another device that records the conversation. The drawback of this method is getting access to a subject’s telephone to properly wiretap it.
A room microphone is another audio surveillance technique that often is utilized. This involves planting a wireless microphone in a room to pick up conversations. Disguised room microphones are available to look like pens, clocks, stuffed animals and a variety of other covert forms. This microphone sends a signal to a receiver, just like a wiretap does, and the signal can be directly recorded. The disadvantage here is access to some rooms and getting only one side of a phone conversation if it takes place in that room.
Concealable transmitters known as body wires are well-known devices that have been featured in many television shows and movies. A small microphone and transmitting device are worn under the clothes of a person in order to send a signal back to a receiver and record a conversation. This allows the person wearing the wire to ask questions and get specific details that simply listening to other people’s conversations could not provide. The disadvantage of this method is getting access to the person needed to be recorded and also concealing the microphone in a way that hides it but allows for clear recording.
Long-distance microphones are another covert means of audio surveillance. A parabolic microphone, often called a shotgun microphone because of its long shape, has a powerful ability to pick up conversations up to 300 feet (91.4 m) away. Its main disadvantage is its high sensitivity. It can pick up other noises and cannot function if obstructions, such as trees and automobiles, are between the microphone and the conversation.