Traditionally bugs have two main components: a microphone to capture the audio and a radio transmitter to relay the conversation to the listener. The microphone is activated when sound reaches a threshold level, or when a radio signal of the correct frequency is sent to the device.
Newer bugging techniques take advantage of modern mobile phone and laser technology. GSM (Global System for Mobile Communications) audio bugs contain a SIM card. When called, the bug will automatically – and silently – answer, and pass on audio to the listener’s phone. Laser listening devices, on the other hand, do not require a microphone at all, but detect the changes in vibration of objects as sound waves hit them. A laser aimed at a reflective surface, like a window, can detect minute changes in the vibration caused by shifts in air pressure as people talk. Laser bugs require the listener to be in line of sight of the target, but unlike microphone-based devices, no one needs to break into a property to plant the bug.