All ultrasonic waves share the common property of being mechanical waves with a frequency higher than the upper limit of the human hearing range. The only difference in physical properties between ultrasonic and sub-ultrasonic waves is that ultrasonic waves cannot be heard by humans while sub-ultrasonic waves can.
The limit varies from person to person, with upper cut-off frequency in the average adult being around 20 kilohertz. Ultrasound devices operate at frequencies from the kilohertz range to the gigahertz range.
Applications of ultrasound include distance measurement, object detection, imaging, non-destructive testing, cleaning, mixing and chemical process acceleration.
Distance measurement and object detection using ultrasound is employed in ultrasonic range finders. An ultrasonic pulse is generated in a specific direction. Objects in the path of the wave reflect the pulse back to the transmitter as an echo. A mounted receiver measures the properties of the reflected sound wave and the time it took for the wave to bounce back. These reflected waves properties give information on the distance, topography, size and composition of the object.
Medical imaging using ultrasound is called medical sonography. The higher the frequency of the ultrasound wave, the clearer the produced image becomes. Frequencies above 2 megahertz are commonly used in imaging applications. Although the frequency of the sound waves is high, the power density is generally lower than 1 watt per centimeter to prevent tissue damage.