Audio Bugging

Microphone Surveillance technologies

Contact Microphones - these are generally based on a non piezoelectric transducer, and can pick up speech through 15cm of concrete. However their effectiveness is dramaticallky impeeded by foam insulation or even something as simple as thicker than average wallpaper.

Directional Microphones - Good directional mikes can pickup normal conversation from a distance of up to 20 m.yet remain small enough to be covertly held in the hand and pointed at conversations in a line of sight.

Microphones on a wire– If an intruder has access to ventilation ducts ceiling voids, hollow walls etc, then they can pass a small high sensitivity omni directional microphone through these voids and listen in at the other end.

GSM Bugs

However all of these ways of recording covertly have mostly been replaced with GSM bugs that use the mobile phone network to record info and send it anywhere in the world that a listener happens to be.