That shockwave effect can occur at sub-sonic speeds. If you ever go to a Blues show, the commentator does a good job of explaining it.
It's a showpiece move and you'll see it in a lot of Blues shows. If the conditions are right, they'll do it.
It's definitely not supersonic. They manage to pull it off at SF most years, although not that low usually! Just google it.
Note the 2 kayakers at the start of the video... haircut, sir?
This is an example of when it doesn't happen:
http://my.break.com/content/view.aspx?ContentId=351527
Taken at SeaFair 07 (great show, by the way).