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. Ultrasonic devices are used to detect objects and measure distances. Ultrasonic imaging (sonography) is used in both veterinary medicine and human medicine. We use this principle to work out how fast blood cells move. Ultrasound reflects off the blood cells and causes a Doppler shift. Ultrasound probes eliminate ultrasound waves. Ultrasound is a kind of energy which is absorbed by tissue, causing heating. We can get images of the body by recording echoes of ultrasound. Ultrasound can produce good images of the soft tissue.