Would you consider including ultrasonic alarm sensors in security applications? We’re looking for something different in a challenging area on a very large site.

A: Ultrasonics can be used in glazed office space (the glass keeps signal from spilling) and in big volume sites comprising irregular internal structures. You’d need to use the multi-head or multi-frequency unit for both reliability and economy in big sites.

What’s good about ultrasonics is that they can be used in extremes of temperature, but these extremes do need to be stable. There are a few things to bear in mind with ultrasonics. They can be set off by machinery noise, fans or air ducts, vibrations with frequencies in the 50-100Hz range and telephones.

In addition, ultrasonic technology is based on measuring Doppler shift effects in sound with frequencies between 16,000Hz and 40,000Hz. Lower frequencies will penetrate walls, while higher ranges will be less susceptible to this problem. All ultrasonic emissions can cause headaches in humans with long exposure and these sensors shouldn’t be active where people are likely to work for very long periods.

Finding an ultrasonic sensor from a reputable security manufacturer is not likely to be easy – we don’t know of any designed for security applications, though it would be possible to make something on the workbench using components, or you could select a compact waterproof NO/NC ultrasonic sensor. If you customise, be sure to do plenty of beta testing before applying to a customer’s premises.