Spy Review

Have you always wanted to listen through walls to hear conversations? Sure X-ray vision would be more fun, but at least listening through walls is possible! This is a Wall Listening Device, which is essentially a very sensitive (and I mean very sensitive) microphone, with a little black box for amplification and pair of earphones. You can listen through solid walls and windows up to several inches thick, providing there’s no air-gap between you and the target you are trying to listen to. Typically priced around £225 or $500, they’re not cheap, but they are a great deal of fun!

The microphone of the listen-through-walls-device has a magnetic base, with a small ball-like element in the middle of the magnetic base. The protruding element is physically connected to the microphone diaphram, passing on any vibrations from the wall to which its attached. For the device to work through walls, the wall needs to be solid.

Here’s the physics bit! Sounds are basically pressure waves in air. When these pressure waves hit a solid surface, they pass on some of their energy to the solid surface, causing vibrations. A high density solid will transmit sound waves better than any low density solid, due to the proximity of particles in the solid at a moleculer level. The closer the particles (and hence denser the solid), the more energy that is transmitted between neighbouring particles. If there is an air gap between two surfaces (i.e. one where sound waves are hitting it, and one where you are listening), the air will lose some of the vibrational energy due to its low density compared with a solid. This means you will hear less through a wall with an air gap, compared to a solid surface, such as a pane of glass or a wooden door.

Having understood that, you’ll now know how to get the best sound quality from the wall listening device! It also has a tape recorder output, so you can record what you’re hearing too. An expensive bit of kit, but it is a professional listening device.