Ultrasounds are 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, only in 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.Ultrasound is used in many different fields.