Sound is a mechanical vibration carried through a liquid, gas or solid medium through which energy moves away from the source in waves. The number of waves per second is called the frequency and is measured in hertz, or Hz. Human ears can detect frequencies between 20 Hz and 20,000 Hz. Pressure variations with frequencies that are lower than 20 Hz are called infrasound, and frequencies higher than 20,000 Hz, written as 20 kHz, are called ultrasound.
