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Re: but does the pilot hear the sonic boom?



On Mon, 1 Dec 2003 13:58:04 -0800, Bernard Cleyet <anngeorg@PACBELL.NET>
wrote:

"Perhaps the shock wave travels faster than the speed of sound in air.
I think that is the physics question."




By definition. It's not a shock wave if it travels at the speed of
sound. When the shock wave slows * to Mach one it's sound.

Got to be a little careful. A steep pressure gradient characteristic of a
shock wave can travel at the speed of sound. To all intents and purposes,
it looks like a shock and sounds like a shock. Is it really a shock?

What seems to happen is that the speed of sound behind the shock is higher
than the speed of sound in the undisturbed air in front of the shock.
Then the signal can maintain its steep front even when it is moving at the
speed of sound in undisturbed air.
Shock waves are created under 2 situations - a signal is forced to
propagate faster than the local speed of sound (airplane wing) or when a
huge energy deposition causes a large increase in the local speed of sound
(explosion). The latter allows a sound wave to build-up on itself as it
tries to move through a medium that has a much smaller speed of sound.


http://www.pbs.org/wgbh/nova/barrier/boom/concept3.html

This is a neat picture. Interestingly, the "wake angle" is a function of
the speed. Why? Because some distance from the aircraft the wave will
propagate (perpendicular to the wave front) at the speed of sound. The
faster the aircraft, the sharper the angle must be to allow that. While
it appears as though the aircraft is towing "streamers" that move parallel
to the aircraft's path, that is an illusion. They are moving
perpendicular to the wavefront and the aircraft is constantly putting out
new "streamers". Also of interest, those lines will curve near the
surface where the propagation really is greater than the speed of sound,
producing a "Bow shock".


Finally, no the pilot cannot hear his own sonic boom. Even if he slowed
down and changed direction to catch up with his old boom, it would be
swallowed in the new boom as he got near to it.

(Or would it ride up and over the new boom and produce an anti-boom as the
pressure gradually rose and then suddenly dropped?)



* For the std. one Kg TNT explosion (4610 kJ), the range of the blast
(shock) wave is ~ 20 miles. At one mile its speed is ~ 2k m/s and at
ten it's ~ 450 m/s.

bc


Karl Trappe wrote:

Was it Aristotle who said "Answer a question but with a question"?

I think the given answer is to a totally different question. It is
not can the pilot hear what is in the cockpit, but can the pilot hear
his own sonic boom?

Perhaps the shock wave travels faster than the speed of sound in air.
I think that is the physics question.

Karl



At 3:46 PM -0600 11/24/03, Jorge Salazar wrote:


*********************************************************
Wednesday, November 26, 2003
SOUND SPEED
Sound waves move at about 340 meters per second -- about two-tenths of
a
mile per second. A listener wonders if you can hear anything when you
travel faster than sound. The answer -- after this on Earth and Sky.
_________________________________________________________

DB: This is Earth and Sky. When we visited the National Air and Space
Museum in Washington D.C., we got this question.

Caitlin Rush: My name's Caitlin Rush, and I'm from State College,
Pennsylvania , and I ask, "When you're traveling at the speed of sound
or
faster can you hear anything?"

JB: Caitlin, the answer is yes. Imagine you're seated in a parked
car.
You switch on the radio. The car's speakers begin to vibrate. The
speaker
cone presses against air molecules near the speaker, which push against
neighboring air molecules. Much like firefighters passing a bucket,
this
wave of pressure eventually reaches your ears. Your brain interprets
this
wave of pressure as sound.

DB: Now imagine you attach a pointy nose to your car and add enough
horsepower to break the sound barrier. As you cruise at more than 340
meters per second -- that's more than two-tenths of a mile per second -
- or
faster than the speed of sound -- you reach over and switch on the
radio.
Your car -- and you -- and all the air molecules around you -- are
traveling that fast. So the sound wave pushes away from the vibrating
speakers in exactly the same way it did in the parked car, and you can
hear
the sound.

JB: And that's good news for pilots and passengers of supersonic
aircraft!
Thanks for your question, Caitlin. And with thanks to the National
Science
Foundation, we're Block and Byrd for Earth and Sky.

http://www.earthsky.com/2003/es031126.html


*********************************************************




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