Ultrasound refers to mechanical waves with frequencies higher than the upper audible limit of human hearing (more than 20 kilohertz). Ultrasound always requires a medium, such as air, liquid, solid or human tissue, to propagate, using pressure variations to travel. Ultrasound propagates well in liquid substances, but when it comes to the liquid–air interface, it is reflected back almost completely. The decay of ultrasound depends on the frequency and the properties of the medium, and may therefore not occur predictably and evenly.
Airborne ultrasound primarily affects the external organs of the body, such as ears and eyes. The effects of ultrasound in direct skin contact on the body are either based on the tissue warming up (thermal effect) or the cavitation phenomenon. The thermal effect normally becomes dominant when the frequency is increased from kilohertz to megahertz levels. Cavitation, on the other hand, can occur at a low frequency and at a low power. When being propagated through liquid, ultrasound may produce bubbles and cause them to grow, vibrate or collapse.