Here ya go ... facts come fast on FR.
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/soubar.html
The Prandtl-Glauert Singularity
enjoy ...
Very nice site, Blueflag!
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/soubar.html
Thank you I really enjoyed those pics