It's too high pitched for human hearing, but many animals, such as dogs, cats and bats can hear ultrasound. Therefore, ultrasound is not different from the normal audible sound in its physical properties except in that man cannot hear it. Ultrasound wave moves as a pressure wave through a medium causing mechanical disturbance within that medium and when the medium is a patient, the wavelike disturbance is the basis for use of ultrasound as a diagnostic tool. Appreciation of the characteristics of ultrasound waves and their behavior in various media is essential to understanding the use of diagnostic ultrasound in clinical medicine. Sound waves are longitudinal waves consisting of areas of both compression and rarefaction. Particles of a material or tissues when exposed to a sound wave will oscillate about a fixedpoint rather than move with the sound wave itself. As the energy within the sound wave is transferred to the material, itwill cause oscillation of the particles of that material.