Listen Through Wall Device

A Listen Through Wall Device is a type of electronic stethoscope microphone and amplifier combination, used to pick-up the sound vibrations that travel through different materials such as concrete, wood, metal and glass; vibrations are usually picked up by a contact microphone, and then fed into an amplifier unit where the vibrations are turned into audible sound which can be heard through headphones, or routed to speakers. Some devices also allow the recording of amplified signals to a secondary device.
Contact Microphone
Such devices use a microphone, commonly referred to as a contact or concrete microphone, that picks up vibrations using a protruding pin on a spring. When the pin is placed against a solid surface, minute vibrations traveling through the contact surface are transferred to the pin. These vibrations are turned into electrical signals inside the microphone unit via a transducer, and sent back to the main amplifier unit for processing.
A variation on the standard contact pin microphone is a needle microphone, consisting of a protruding hollow needle attached to a pickup at the base. This is intended to be inserted into cracks and holes in the contact surface.
Amplifier
Once the vibrations have been picked up and turned into an electrical signal, they are processed by the amplifier unit before being output to the user. Amplifiers vary greatly in power and definition, and alongside microphone sensitivity dictate how much or little the user can hear.
Basic devices have smaller amplifier circuits which may only hear the top of a loud conversation happening through a 30cm wall. More advanced units might be able to resolve not only the conversation in whole, but also very fine sounds of breathing and movement through a much thicker wall.
Amplifier units may also feature a filter control which allows the user to select a specific audio band to focus on, such as increased treble or bass. Others feature an automatic volume reduction when the unit detects a significant increase in decibel level.
Uses
Such devices are usually advertised for use by rescue workers and law enforcement, for hearing trapped people in collapsed buildings, ticking mechanisms in bombs, and gathering evidence against criminals where direct access to a property is not possible.
In reality, however, such devices can be utilized for activities which may break local laws and violate human rights, such as invasion of privacy and harassment.
Variations
As well as the classic wired listen through wall device, it is possible to acquire variations to fit specific needs.
Wireless devices use a contact microphone with its own independent power source and antenna which broadcasts its electrical signal to the long-range amplifier unit via radio signals. Such a device allows the user to be quite a distance from the target when monitoring.
Multiple-Transducer devices use several microphones in one unit to deliver greater fidelity of sound.
All-In-One devices combine the microphone and amplifier in one unit.

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