Ultrasonic waves are sound waves with a frequency above the human audible range of 20,000 Hz. They are used for applications like SONAR to calculate depth of water by sending and receiving echoes, medical imaging of internal organs by scanning and receiving echoes, cleaning small parts like watch parts, and defect detection in metals by scanning for cracks that allow ultrasonic waves to pass through. Some animals like bats and dolphins use echolocation to navigate and find objects by emitting ultrasonic sounds and perceiving the returning echoes.