Before considering the auditory system, it is useful to review some basic properties of sound waves.
Sound waves are sinusoidal in shape.
Sinusoidal waves can be easily generated by using a tuning fork. When struck, the tines of the fork move in two directions. These changes in density of the air molecules leads to changes in local air pressure. Hz to 20 kHz. Human infants can hear frequencies slightly higher than 20 kHz.