Vibrational or stress waves in elastic media that have a frequency above 20 kilohertz, the highest frequency of sound waves that can be detected by the human ear. They can be generated or detected by piezoelectric transducers (seepiezoelectricity). High-power ultrasonics produce distortion in a medium; applications include ultrasonic welding, drilling, irradiation of fluid suspensions (as in wine clarification), cleaning of surfaces (such as jewelry), and disruption of biological structures. Low-power ultrasonic waves do not cause distortions; uses include sonar, structure testing, and medical imaging and diagnosis. Some animals, including bats, employ ultrasonic echolocation for navigation.