A Laser Microphone bounces a beam of laser light off a rigid object, and detects the sounds made near that object by analyzing the changes in the reflected beam.
The harder the object, the more readily it conducts sound, and the closer the conversation, the better the sound quality. A glass windowpane or metal hatch are perfect, concrete or brick walls less so, and insulated walls are very poor. The target object must be within direct line of sight.
The most common versions of this device use an invisible infrared laser to perform the recording, though this limits the range of the device to about 500 meters.
The device is the size of a large pistol. It has a connector for a computer to gather and record the data. It also has a small tripod for stable mounting of the device.