A motion detector that operates at a frequency of 20 to 40 KHz is known as what type of detector?

A motion detector that operates at a frequency of 20 to 40 KHz is classified as an ultrasonic detector. This type of detector uses high-frequency sound waves, typically above the audible range for humans, to detect motion. When an object moves within the vicinity of the detector, it alters the sound waves emitted by the device, leading to detection.

Ultrasonic detectors are commonly used in security systems because they can cover larger areas and are not easily obstructed by objects. This frequency range allows them to effectively sense movement from a variety of angles and distances.

In contrast, microwave detectors operate at much higher frequencies, typically in the gigahertz range, and rely on the reflection of microwave energy. Passive infrared (PIR) detectors function differently by detecting changes in infrared radiation caused by the movement of people or animals and are not frequency-based like ultrasonic detectors. Photoelectric detectors work on the principle of light interruption and are used in different applications, not related to motion detection through sound waves. Thus, the distinguishing characteristic of the ultrasonic detector is its operation in the 20 to 40 KHz frequency range.