Editing Security alarm (section)

===Ultrasonic detectors===
These active detectors transmit ultrasonic sound waves that are inaudible to humans using frequencies between 15&nbsp;kHz and 75&nbsp;kHz. [[Doppler effect|The Doppler shift principle]] is the underlying method of operation which detects a change in frequency due to object motion. This detection occurs when the object must cause a change in the ultrasonic frequency to the receiver relative to the transmitting frequency.

The ultrasonic detector operates by the transmitter emitting an ultrasonic signal into the area to be protected. Solid objects (such as the surrounding floor, walls, and ceiling) reflect sound waves, which the receiver will detect. Because ultrasonic waves are transmitted through air, hard-surfaced objects tend to reflect most of the ultrasonic energy, while soft surfaces tend to absorb the most energy.

When the surfaces are stationary, the frequency of the waves detected by the receiver will be equal to the transmitted frequency.  However, a change in frequency will occur as a result of the Doppler principle when a person or object is moving towards or away from the detector. Such an event initiates an alarm signal. This technology is not active in many properties as many consider this obsolete.