Ultrasonic security systems

Have you felt or heard something funny at the local pharmacy or department store? Like a high pitched whine or pressure in your head? Not sure what it was? It could be their ultrasonic intrusion sensors. Intrusion sensors are normally active during off-hours to detect unwanted, er, intruders. One would think that these would be shut off during business hours, because, well, people go shopping. Doh!

I encountered an ultrasonic detection system unexpectedly at a local Sears store this week. As I walked into the tools section, I experienced a very uncomfortable buzzing sensation between my ears that felt, well it felt like my brain was being sawed in two. It was disturbing. However, having recently encountered unexpected discomfort near a sound source (see my post on Hammered by a wind turbine), I backed off quickly and fired up the iPhone SignalScope application. I walked toward the previous location with SignalScope running and captured a 21 kilohertz signal that peaked in intensity right at the spot where I had experienced the strongest and most uncomfortable sensations. Bingo! It was an active ultrasonic volumetric motion or “occupant” detector system- running during daytime hours.

Did other people hear or feel it? My sweetheart said she felt a pressure on her head at that location. The sales associate was cordial but heard nothing.

It’s worth noting that a standard sound level meter, even a Type 1 Precision sound level meter, wouldn’t register this acoustic tone at all. Depending on the model, sound level meters cover from 20 Hertz to 20,000 Hertz. This security system tone falls outside sound meter range!

So- if you’re in a store and suddenly feel a really strange sensation in your head, it’s probably not your imagination or something you ate. It’s probably the store’s ultrasonic security system!

An ultrasonic sensor is volumetric, meaning it floods an area within its coverage pattern. This allows it to detect persons behind partitions and other obstructions. Any moving object within its coverage area will disturb the sound wave pattern, creating a Doppler shift and altering the signal returning to the sensor. False activation may also occur from vibration, air movement, high sound levels, and audible sounds having ultrasonic components. Ultrasonic sensors have sensitivity adjustments to minimize environmental effects. Reducing sensitivity will also reduce the coverage area. If ultrasonic sensors are mounted close to each other, they must operate at different frequencies to avoid interfering with each other.