Really long time ago at an boot sale I bought essentially complete security system made in early 90's by some smallish company that was probably spun-off from Tesla (ie. the elecronics manufacturer "Tesla n.p.") as part of privatization and then probably failed outright. The thing consisted of obviously repurposed parts of other systems and looked like one giant kludge.
But what was the interesting feature of this was that most sensors were based on infrasound, notionally they were meant to detect shattering glass, but somehow the sensitivity was tweaked in such way that they even detected opening of doors/windows (this was mentioned in manual as feature) and even could detect person moving around the room.
But what was the interesting feature of this was that most sensors were based on infrasound, notionally they were meant to detect shattering glass, but somehow the sensitivity was tweaked in such way that they even detected opening of doors/windows (this was mentioned in manual as feature) and even could detect person moving around the room.