How does ultrasound work?

Humans hear sound between 20 and 20,000 Hz. Ultrasound uses frequencies of sound which is 2-15 MHZ, far above range in normal daily life. Ultrasound started with sonar, sending beam in the water and hits the submarine and it bounces back to originating source and multiplied how long it took it go out and come back. Ultrasound is based on algebraic equation distance=rate x time. Speed of sound in human body is fixed 1540 meters/sec in every single ultrasound machine. If you know the rate, you must calculate time to find the distance. The distance in ultrasound is from skin to a structure in the body.