As the name implies, through wall microphones eliminate barriers and allows for a stealthy insertion of audio monitoring. Obviously, this ability makes a thru-wall microphone perfect for covert audio surveillance of almost any room or building, and especially when gaining access is problematic. Beyond static surveillance, thru-wall microphones find themselves naturally at home with tactical teams desiring to gain intel prior to no-knocks and stand-off situations. In either of the tactical scenarios, a thru-wall microphone with a stereo capability can also be used to judge the approximate location of individuals inside the room or structure.
Thru-wall microphones are also sometimes called contact microphones and the name contact microphones help to explain how the magic happens. Essentially when the microphone makes contact with the material being penetrated, the material then becomes part of the microphone, or in other words, the material becomes a giant microphone capturing minute amounts of vibration. The denser the material is, the better thru wall microphones work…i.e. concrete, drywall (gypsum board) and brick are excellent conductors of sound and work very well with this type of microphone.
Material that poses the greatest challenge for thru wall microphones is insulation. In a recent test at our facility in Texas…we found it difficult penetrating an unusually thick wall that had eighteen inches worth of insulation. However, still in the same commercial type structure, we tested through a firewall that had four sheets of ¾” inch sheetrock (gypsum board) and six inches of insulation, with excellent results…we could discern speech from more than thirty feet on the opposite side of the wall. In contrast, residential structures typically have walls substantially thinner than commercial walls, making home walls little or no challenge whatsoever.
When purchasing a through wall microphone system, here are some of the key features that I recommend:
- MEMS Sensors
MEMS stands for Microelectromechanical Systems and the more advanced of through-wall microphones will employ this technology over the older, cheaper type microphones known as electrec (ECM). MEMS-based through-wall microphone systems allow for the microphones to smaller, are better tuned to the human voice spectrum and are dramatically better at eliminating unwanted noise. I should point out that due to the sophistication of MEMS type contact microphones they are typically 4 to 6 times the cost of electrec type systems. However, the results are very different between the two and are significant enough to where the old adage “you get what you pay for” applies. So no matter what or whom you buy from, ensure you’re getting a MEMS type system. Please see the MANTIS as an example system.
Through-wall microphone systems are available in a mono-type format. However, by not having stereo you’ll be missing out on a lot of information such as location. With stereotype systems, general direction and to the trained ear….distance can be ascertained. If you find a system with stereo, also check to make sure it has a balance adjustment which also can aid in determining target locations.
- Dual Audio Out
Two is better than one! With an extra audio output, you can greatly expand the benefits of the contact microphone system. For instance, in a SWAT situation, you can allow for two team members to monitor and compare observations. Alternatively, the second output can be used for live broadcast to a SWAT commander. In a surveillance role, the dual outputs can serve the needs for both live broadcast via RF, internet or cellular, and recorded locally.
Yes, spikes! When using the thru-wall microphone for surveillance activity, its implied that the microphone is going to be in one spot for a long time. With that in mind, a well-designed system should include a wall punch and spikes that allow the microphones to be mounted during the surveillance mission. Along with providing a way to fasten the microphones to a wall, spikes also provide a deeper penetration, rendering even better audio performance.
Thanks for reading my article. Should you have any comments, questions or suggestions for future articles…please give me a shout.