We name the distance between these waves the frequency of the sound. The shorter the distance between them, the higher the frequency.
The most commonly used unit for frequency is hertz (hz), which is just another way of saying events per second, or in the case of simple wave forms: one wave per second.
An undamaged ear can hear sounds from 20 to 20000 hz. As we get older we tend to lose the ability to hear higher frequencies.