The ability to listen through walls is invaluable for many high-level police and government investigations to proceed, in cases where evidence can otherwise be challenging to compile. But that's not the limit to this incredibly advanced spying technology. Contact wall microphones are sophisticated gadgets using the latest wireless technology. As well as being used to gather evidence about terrorist activities, or noisy neighbours (a popular application).

This can be essential for a wide range of purposes, such as:

  • search and rescue - medical and military teams commonly use these devices to find people trapped under rubble, following earthquakes and explosions
  • bomb detection - they are regularly used to listen into packages sent through the post, or through airports, as they can often detect timers hidden inside
  • mechanical diagnostics - similarly, but less sinisterly, problems in engines and other mechanical apparatus can be detected
  • pest control - locating infestation from rodents and insects in buildings
  • plumbing - discovering where leaks originate.

As these varied uses suggest, microphones used to listen through walls are precise, sturdy and have the ability to cope with thick walls and other noise barriers. Generally you can expect to be able to easily pick up on audio conversations through walls that are up to twelve inches thick - which is a lot of wall. There are some devices than can record audio through walls that are twice as thick as this! The great advantage is that you don't have to access the building being spied on (as with hidden cameras) - often the key issue for anyone who needs to listen through a wall. Simply place the flat microphone on the wall and listen in and record the information you need. There have been great advances in microphone technology over the last generation.

Audio quality has improved so much, especially now we can further refine recordings using audio software computer programmes to filter out background noise. Most high quality microphones come with built-in sound filters and limiters, much needed as otherwise you could damage your eardrums listening into the loud background fuzz, or sudden foreground bangs. Modern day listen through wall devices are also incredibly slight and discreet, while still robust, thanks to their chrome protective casings. Some listen through wall devices are equipped with contact pins - as the name suggests, these are tiny pins which attach the microphone to the surface you need to listen through. If there is no contact pin included, then you can use magic putty. This is also well-named, it leaves no marks behind after you remove the device - just like magic, all traces disappear. The most sophisticated and sensitive listen through walls devices use remote infra-red technology. Amazingly they can easily pick up locations of people 800 metres away and enable users to listen through walls which are up to 23 inches lick - even solid concrete walls present no problems. Even better, this is absolutely legal to use across the European Union. Using Infra-red with UHF transmitting doesn't contravene any of the region's various privacy laws.