

What's a Sonic Boom? - kristiandupont
http://kristiandupont.com/blog/2010/08/whats-a-sonic-boom/

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mkn
Well, it's on the front page, so I guess _someone_ is taking this seriously.

This is not anywhere near a correct description of a sonic boom. A sonic boom
is not "a large burst when [an object] “breaks the sound barrier”." It _is_ a
phenomenon that is associated with any object traveling at or above the speed
of sound. A related phenomenon governs the crack of a whip, but is associated
with unsteady flow around a _subsonic_ object. (Yes, you read that right. The
business end of a bullwhip does not need to travel faster than the speed of
sound to produce an audible crack.)

What _really_ happens in a sonic boom is, for a slender body traveling above
the speed of sound, the local flow is turned "suddenly," and the freestream is
not able to accommodate it in a continuous fashion. There then arises an
oblique shock wave, a very small region where fluid properties change nearly
instantaneously; Direction, temperature, pressure, and entropy all change.
Because the change is not isentropic, there is a hard lower limit to parasitic
drag associated with supersonic motion through a fluid.

Interestingly, there is an expansion fan at the widest part of, for example,
an aircraft body that is isentropic. It's a broad region where temperature,
pressure, and direction change gradually. Thus, there are three 'sonic booms'
when an aircraft passes overhead at a speed greater than the speed of sound.
There is one sharp one as the nose passes you, due to a compression shock, and
another softer one as the widest part passes you, due to an expansion fan, and
a third one as the tail passes you, due to another compression shock. This is
analogous to Mach diamonds in a rocket exhaust stream.

This is all for sufficiently lax definitions of "passing you," as the shock
and expansion fans propagate at the local Mach angle. You'll be looking
downstream at the local Mach angle when you hear the boom, if you're following
the craft with your eyes.

~~~
_delirium
> A sonic boom is not "a large burst when [an object] “breaks the sound
> barrier”.

I had this misunderstanding for years (I blame nearly every description of
sonic booms), and it made it pretty confusing for me to understand the hubbub
about the Concorde traveling over land. I had thought it'd only generate
exactly two sonic booms, upon accelerating past Mach 1.0, and decelerating
again, and figured they could arrange it so those were located in sparsely
populated areas. It wasn't until I realized sonic booms continue to happen
during the entire course of the Concorde's supersonic flight that it made any
sense.

~~~
jacquesm
The 'boom' is relative to an observer. Just like a ship will produce a
continuous wake but when you're standing on the shore of a canal the wake will
pass you by.

~~~
dedward
Yup. The confusion, I think, comes from talk of sonic booms always being
discussed along with mentioning an aircraft passing the sound barrier, going
supersonic, whatever...

There is, I believe, a sudden change in aerodynamics when an aircraft passes
the sound barrier - it IS a barrier of sorts. Aircraft had the power to pass
this barrier, the problem was they would no longer fly correctly, or they
would rip them selves apart, or some such thing once they hit mach 1.0.

People naturally got the idea that this phenomenon was related to the sonic
boom itself.

