Ultrasound waves have a frequency higher than 20,000 Hz, which is outside the upper limit of human hearing. The cochlea in the human ear is only stimulated by a limited range of frequencies. This means that humans can only hear certain frequencies. The range of normal human hearing is 20 to 20,000 Hz, but some animals like dogs can hear the ultrasound.

Ultrasound has several applications in healthcare. It is used to break kidney stones. The vibrations caused by the ultrasound shake apart kidney stones, breaking them up. The principle is the same as the opera singer's trick, where a glass may shatter if the singer makes a high-pitched sound near the glass. 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 occurs in almost every situation. There is noise around...