Sensing intruders through closed doors

Cocoon Labs


We have been working with our cocoon development systems for a while. These powerful little devices are packed with a variety of sensors. One of the useful things we can do with them is to detect infrasound.

Infrasound is another term for low-frequency sound waves. Basically a note so low you can’t hear it (perhaps in the 0-20Hz range). They are useful to us because they are generated by any significantly-sized body moving in air (e.g. a human walking or a door opening) and also because of they way they move around a house.

When any wave fits through a gap it will spread out a bit, just like waves of the sea around rocks:


How much it spreads out depends on the wavelength of the wave compared to the size of the gap. The bigger the wavelength, the more it will spread out. Very low frequency waves, such as infrasound, will spread out a lot. In effect this allows them to be transmitted around a building much more easily than the higher frequency sound we can hear.

For Cocoon, this means that we can detect things such as a door opening and human movement from another room in the same house – the waves generated travel around the house and through even the tiniest gaps beneath closed doors to our sensors.  We can process the infrasound signature in real time and use that as part of the input in our decision making process – deciding if there is something going on at home that you would like to know about.

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