A very high-frequency sound is called ultrasound. It is characterized as a sound with a frequency higher than 20,000 Hz, which is outside the upper limit for human hearing.

Ultrasound has several applications in healthcare. It is used to treat kidney stones. The vibrations caused by the ultrasound shake apart kidney stones, breaking them up. It is also used to scan the inside of the body, creating a picture of something that cannot be seen directly, such as an unborn baby in the womb. Ultrasound is what bats use to echolocate.

Ultrasound occurs in almost every situation. There is noise around us all the time, but we may not be hearing all of it. Any sort of movement can create a vibration, which in turn creates sound waves. These can be in the ultrasonic range as well as being audible. A lot of the electrical equipment we use on a daily basis...