On this page, there's an animated diagram (aptly done in shockwave) about
normal shockwave formations:-

http://selair.selkirk.bc.ca/aerodyna...ed/Page2c.html

Something confuses me though. As diagrammed, the red pressure wave at the
rear has a low frequency at the front and a high frequency at the back. Why
would this happen (if indeed it does)? Why is the frequency HIGHER in the
direction that the airflow is moving?

Thanks in advance.