ULTRASONIC-SENSORS HC-SR04.
 
An ultrasonic sensor is an electronic device that measures the distance of a target object by emitting ultrasonic sound waves, and converts the reflected sound into an electrical signal. Ultrasonic waves travel faster than the speed of audible sound (i.e. the sound that humans can hear). Ultrasonic sensors have two main components: the transmitter (which emits the sound using piezoelectric crystals) and the receiver (which encounters the sound after it has travelled to and from the target).
 
In order to calculate the distance between the sensor and the object, the sensor measures the time it takes between the emission of the sound by the transmitter to its contact with the receiver. 
The formula for this calculation is D = ½ T x C (where D is the distance, T is the time, and C is the speed of sound ~ 343 meters/second). For example, if a scientist set up an ultrasonic sensor aimed at a box and it took 0.025 seconds for the sound to bounce back, the distance between the ultrasonic sensor and the box would be:
D = 0.5 x 0.025 x 343
 
Resources:
1. Complete Guide for Ultrasonic Sensor HC-SR04 with Arduino
2. How HC-SR04 Ultrasonic Sensor Works & Interface It With Arduino
3. Ultrasonic Sensor HC-SR04 and Arduino – Complete Guide