Here's what happens:
Notice how the same thing occurs with passing cars when you are standing still? Maybe I am about to describe the doppler effect, who knows but as I understand the situation, here's what happens:
Wavelengths are measured in meters and centermeters (or w.h.y.) and take a period of time to complete a cycle. If the object is moving towards you, the movement during the cycle will compact it and increase its frequency. Vice versa when the object is moving away from you.