Ultrasonic Proximity Detectors
Ultrasonic waves are acoustical vibrations in the range above human hearing.
The difference between a radio wave and ultrasonic waves is that the radio wave is electromagnetic, whereas the ultrasonic wave is acoustical. That is, the ultrasonic wave consists of variations of the air pressure.
A 40 KHz electromagnetic wave is a radio wave, whereas a 40 KHz acoustical wave is an ultrasonic wave.
For the purposes of proximity detection, acoustical frequencies between 20 KHz - 60 KHz are used.