Ultrasound is sound waves with frequencies
higher than the upper audible limit of human hearing.
Ultrasound is not different from "normal"
(audible) sound in its physical properties, except that humans cannot
hear it. This limit varies from person to person and is approximately 20
kilohertz (20,000 hertz) in healthy young adults.
Ultrasound devices operate with frequencies
from 20 kHz up to several gigahertz.