| Date | Text | |
|---|---|---|
13 Oct 1953
Ultrasonic alarm |
Ultrasonic alarm In 1953, the first U.S. patent for a burglar alarm operated by ultrasonic sound was issued to Samuel Bagno of New York City (No. 2,655,645) on a system for movement detection in a confined space by intruders or fire. It was manufactured as the Alertronic and first sold in Jun 1950. A sound source produced waves of 19,000 hertz, a frequency too high for normal human hearing. An intruder could be detected by the alarm by a difference in frequency of the reflected waves from the moving body (the Doppler effect). The device would then sound an alarm. |
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