A "sonic boom" is always really a double boom (noticeable when caused by
large supersonic objects, like airplanes, but not with short ones like
bullets), as heard by the observer.
There is a sudden rise in pressure (from the normal pressure before the
shock wave arrives); this is not truly instantaneous, but occurs over a
"rise time." This is followed by a slower decrease in pressure to
below-normal pressure, then another sudden rise back to normal pressure again.
Laurent Hodges, Professor of Physics
12 Physics Hall, Iowa State University, Ames, IA 50011-3160
lhodges@iastate.edu http://www.public.iastate.edu/~lhodges