Ultrasound is a very high pitch sound. Within sound there are different frequencies. The human ear can detect sound with frequencies between 20 Hz and 20,000 Hz. Ultrasound is like the ordinary sound we hear except that is has a higher frequency then that to which the human hearing system responds. Dogs for example can hear a higher pitch of sound then we can.
We read frequency of sound in Hertz (Hz) or Megahertz (MHz). Hertz relates to one cycle of sound per second. Megahertz relates to one million, therefore, one Megahertz is 1, 000, 000 Hz.
For example within music middle C has a frequency of about 500 Hz.
Some animals like bats and dolphins can send out sound as well as receive sound at frequencies above 20 kHz. Within Ultrasound we produce sound through mechanical vibrations at above 20,000 Hz, any sound at this level or above this is defined Ultrasound.
The Ultrasound level we use in medical diagnostic imaging are between 3 to 15 MHz.
We use a tool called a transducer or probe to produce and receive the ultrasound. They have hundreds of tiny special crystals in them. The Transducers come in different shapes and sizes, depending on which type of scan we need to do.