As my news server doesn't let me post to a.b.p.aviation, this is the
next best place I can think of for this reply. In answer to a question
about pictures of the SR-71 showing a shock wave, several people have
responded saying that the SR-71 is not supersonic at low level. Looking
for confirmation, I found this URL

http://www.wvi.com/~lelandh/exec12.jpg

- a page from "The SR-71 Reconnaissance System Executive Handbook",
written by Bill Majors (Lockheed ADP), which gives the performance
envelope of the SR-71. And yes, it is subsonic to about 20,000 feet.