Ultrasonic sensor makes use of the principle of sound in air and more importantly the echo theory thought in elementary physics to determine the distance between the sensor and the next obstacle.

Sonic simply speed of sound in air which is within the range of 336m/s to 346m/s. This is a very interesting lesson in my fluid mechanics class.There is a dimensionless number called mach number I'm sure you much have heard ma 1 , 2 etc. Mach number is ratio of the speed of air object to the speed of sound in air. If this ratio is less than 1 we will say is a SUBSONIC , that is the object is running at a speed less than the speed of sound and this simply means that you will hear the sound of such object before the object gets to you. We have supersonic if mach is greater than 1 and this means the object must have passed before you will passive it sound. Quite interesting ? Fear anything supersonic please. Ultrasonic very high speed. I will write a post on complete working principle of ultrasonic sensor later this is getting too much for just a comment