Ultrasonic

Ultrasonic sensors measure distance by using ultrasonic waves and measuring the time taken between sending a signal and receiving an echo back. Parking sensors on cars use ultrasonic sensors.

The distance can be calculated with the following formula:

Distance L = 1/2 x T x C, where L = the distance, T = the time taken between the sending and receiving, and C is the sonic speed. (the value is multipled by 1/2 as the T is the time for the send and receive distance).