How do diffuse, retroreflective, and through-beam ultrasonic sensors differ?
Ultrasonic frequencies are above the human audio range – higher than 20 kHz. Operating in the ultrasonic frequency range, ultrasonic sensors are commonly used for measuring proximity (distance), determining the level in storage tanks, detecting objects, and more. By measuring the time it takes for sound to travel to and reflect off the surface (time
Source: designworldonline.com