The way we measure sound waves, audio waves, and other types of waves is by their length and their height (amplitude). The length of a wave (or wavelength) refers to the distance between the peak of one wave and the peak of the next wave. When the wave reaches its highest point, we use that as the first measure. Then the wave usually goes down to a low point before it begins to go back up, all the way to a high point again. This second point is the next measure. We then simply use the distance between these points to identify the length of the wave.