Science: Ultrasonic Alarm

A new kind of burglar alarm, just patented last week, has already put a crimp in the burglar business. Developed by Samuel Bagno and manufactured by the Alertronic Corp. of Long Island City, the Alertronic alarm has one or more “loudspeakers” that generate sound waves with a frequency of 19,000 cycles a sec. This is too high-pitched for normal human ears, whose upper limit is about 18,000 cycles a sec., so the office or bank protected by Alertronic seems silent to a burglar, although every cubic inch of its air is in rapid vibration.

The machine can hear its own racket, of course, through listening devices. When nothing is moving in the room, the reflected waves that enter its microphones are all of the same frequency—19,000 cycles a sec.—and the Alertronic holds its peace. But when a burglar creeps towards the safe, the waves reflected from his body have a slightly different frequency. The machine detects the altered waves and rings a police station alarm.

A problem for the designers of Alertronic were intruders smaller than burglars, i.e., pets and mice. To keep it from giving alarm whenever a mouse scampers into its sound field, the machine has to be set below its maximum sensitivity. It lets mice frisk undetected, but their delicate ears can hear its high-pitched sound, and the uproar frightens them so much that they die of a heart attack.