Sound is carried by a medium such as air or water. The sound source creates vibrations in the surrounding medium. As the source continues to vibrate the medium, the vibrations propagate from the source at the speed of sound, forming the sound wave. The physical reception of sound for humans is limited to a range of frequencies. Humans normally hear sound frequencies between approximately 16 Hz and 18,000 Hz (no higher than 18 KHz). This range might differ slightly for everyone, and the upper limit decreases with age.
An ultra sound is a sound wave which has a frequency of 18 kHz to 10 MHz and thus is inaudible to humans.