Listening from a distance without radio transmission

• Ultra-directional microphones, or parabolic microphones. These are like the microphones seen on camcorders, or carried by sound technicians. They are constructed to receive signals only from one direction. The most high-tech directional microphones can eavesdrop on conversations from a hundred metres away or more. Microphone arrays can be used as well.

• Laser microphones. These are very expensive and highly technical to operate. A laser beam is bounced off a window, or off any object near to the conversation monitored. Any object which can resonate/vibrate (for example, a picture on a wall) will do so in response to the pressure waves created by noises present in a room. The electronics detect the minute difference in the distance travelled by the light to pick up this resonance and reproduce the sound causing it.

• Some equipment may exhibit microphonics and can therefore, unsuspected by the party listened to, act as a microphone.

• The adversary can use a trojan horse to acquire access to microphones connected to a computer.

• Telephone lines can be used as the transmission medium for devices called "infinity transmitters" or "harmonica bugs". These are covert listening devices connected either inside a target's telephone or somewhere along the telephone line and activated by calling the number. The circuitry silences the ringer long enough for the eavesdropper to send a control tone that activates the microphone. This allows surveillance to be conducted from anywhere in the world, hence the name "infinity". With the advent of remotely programmable mobile telephone technology (smartphones, etc.), this technique can be used without having to plant anything. Called "roving bugs", it involves the upload of surveillance software to the target phone.