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

The correct answer is that a motion detector operating at a frequency of 20 to 40 KHz is known as an ultrasonic detector. Ultrasonic motion detectors function by emitting high-frequency sound waves, which are typically above the range of human hearing (20 KHz and higher). When these sound waves encounter an object in their path, they bounce back to the detector. The device measures any changes in the frequency or phase of the returning sound waves to detect movement.

This technology is particularly effective in sensing motion, as it can detect even slight movements in its coverage area, making it common in various security applications.