Ultrasound is an oscillating sound pressure wave with a frequency greater than the upper limit of the human hearing range. Ultrasound is thus not separated from 'normal' (audible) sound based on differences in physical properties, only the fact that humans cannot hear it.
The use of ultrasound can induce hyperemia, which increases blood flow as tissue is heated. This thermal effects speeds up the healing process while reducing inflammation. Ultrasound is also used to vibrate tissue, stimulating cell membranes and heightening their repairing effects. Chiropractors often use ultrasound therapy to help their patients reduce swelling, increase blood flow, and decrease pain, spasms, and stiffness.