Sound waves with frequencies above 20 kHz are called ultrasound, which exceeds the audible range for humans.
Application
In medical : Ultrasound is used to observe the internal parts of the body, especially in the development of a foetus. Ultrasound pulses are emitted into the body by a transmitter. The echoes reflected by any surfaces within the body are received. Through times interval the depth of the reflecting surfaces is calculated.
In cleaning: Transmission of high energy ultrasound may cause cavitation bubbles at sites of rarefaction. These bubbles displace contaminant from surfaces. This effect also permits fresh chemicals to come into contact with the contaminant remaining on the surface to be removed.
Pitch, Loudness and Quality
A pitch is relative i.e. it is determined in relation to other sounds. It is related to the physical quantity of frequency. The higher the frequency the higher the pitch. Doubling the frequency increases the pitch by one octave.