Prandtl-Glauert Sigularity?

What is the Prandtl-Glauert Singularity? What is Specter of the Broken?
The Prandtl-Glauert singularity is used to describe a point when sudden drop in air pressure occurs

and is generally accepted as the cause of the visible condensation cloud when an airplane breaks the sound barrier

though there remains some debate. It is an example of a mathematical singularity in aerodynamics.This terminology comes after two scientists who contributed majorly in studying this phenomenon

Ludwig Prandtl(1875-1953) and Glauert Herman (1892-1934).More information can be seen at the following links...http://en.wikipedia.org/wiki/Prandtl-Glauert_singularityhttp://en.wikipedia.org/wiki/Ludwig_Prandtlhttp://www.phy.ncku.edu.tw/~astrolab/mirrors/apod/ap010221.htmlhttp://tw.wrs.yahoo.com/;_ylt=Av3R8PBWOb7TZObknKOkY99r1gt.;_ylu=X3oDMTA2bTQ0OXZjBHNlYwNzcg--/SIG=158eg3dl1/EXP=1132639191/**http://w3.cyu.edu.tw/cyitme/d/%A4@%A8%C7%A5%AD%AE%C9%A4%A3%B3Q%AA`%B7N%AA%BA%A9_%AFS%B2{%B6H.doc (中文)As for the Specter of the Broken

it is a historical name for a phenomenon call