The sounds that are generated are measured by the echoes received. In today's society ultrasonic imaging has become an accepted and integral part of exploration into the body. Technology has allowed a medical view into the human body. Ultrasound is the only source of looking into the body without surgery that uses sound waves rather than electromagnetic waves. The pictures produced from ultrasound is based on echolocation which finds a target by sending out a signal and waiting for an echo to return.
Animals use echolocation such as the bat and porpoise to find food and avoid obstacles.
Echolocation is a phenomenon of nature, but was not known about when sonar was developed. The distance is determined by the time it takes for the sound to travel round trip. Obtaining ultrasound pictures from objects within the human body required modifications of the sonar technique. In sonar, the targets are much larger, obviously, than in medical imaging. In order to get an echo from a small object, the wavelength needs to change size.