Ultrasound
The frequency of sound waves is measured in hertz, which has the symbol Hz. The bigger the number, the greater the frequency and the higher the pitch of the sound. Human beings can generally hear sounds as low as 20 Hz and as high as 20,000 Hz (20 kHz).
High pitched
Sound with a frequency of more than 20,000 Hz is called ultrasound. It is too high pitched for humans to hear, but other animals (such as dogs, cats and bats) can hear ultrasound.
Ultrasound has many applications in medicine, including ultrasound scans to check on the health of unborn babies.
Sound and ultrasound are pressure waves that transfer energy from place to place. Ultrasound can be used to clean jewellery. The jewellery is placed in an ultrasonic bath, where the rapid vibrations shake the dirt loose. Ultrasound can also be used for physiotherapy. Its energy is absorbed by soft tissue in the body, bringing relief from sprains and arthritis (painful joints).