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[Phys-L] Re: Acoustics question about popped balloons.



Hi John-
I would only add to what John Denker had to say:
Shock's are unmistakable because your body can feel them. A shock wave
from a supersonic airplane rattles the dishes.
In the case of lightning (and thunder), there is a shock wave
formed by the big electrical spark, but it rapidly dissipates so that by
the time it reaches you, it is an ordinary sound wave.
Regards,
Jack

On Wed, 24 Aug 2005, John SOHL wrote:

I have an acoustics question for the list. I came across the following
web entry:
- - - - -
Why is the accidental popping of a balloon or the forceful smashing of
an air-filled paper bag so loud? It makes sense that the metal-on-metal
clash of banging cymbals creates a racket, but air, paper, and latex?
What gives?
Sound travels in waves. More specifically, sound consists of waves of
pressure transmitted through the air to the ears. When a sound wave hits
the ear it signals a change in pressure to the inner ear. A sensitive
instrument, the inner ear can detect minute changes in pressure and
interprets these changes as sounds. So logically, the higher the change
in pressure, the louder the sound.
If we think about the air inside a balloon or paper bag as a large,
contained pressure wave, then the puzzle begins to make sense. Both
balloon and paper bag contain pressurized air pushing out against the
sides. When the balloon pops or the bag is compressed, the pressurized
air rushes out all at once in a powerful wave. Think of an ocean wave.
The larger the wave, the more powerfully it crashes into the surf. When
the highly pressurized wave from a popped balloon or bag hits the ear,
the inner ear translates the sudden increase in pressure as a loud,
ringing pop.
- - - - -
Is this correct? My thinking says that the air rapidly leaving the
popped bag or balloon is crashing into the surrounding air which then
causes a shock wave which in turn propagates outward causing the sound
that we hear. Specifically, our ears are detecting the shock wave and
not any momentary increase in pressure caused by the released air.
It seems unrealistic to me that the air pressure would increase and
propagate such that you can easily hear a popped balloon even if you are
100 meters away. Yet, this seems perfectly reasonable if you are hearing
a shock wave caused by colliding air.
A similar question is that of thunder. Again, I assume that what we are
hearing is the heated air expanding outward colliding with "still" air
in the surrounding region. This also has the effect of leaving a lower
pressure region (because of the heat) that the air then rushes back into
almost instantly. When this air rushes back it collides and more shock
waves are produced. The effect being a "ka-boom" sound having a longer
or multiple impluse thunder clap vs. the single "pop" you get from a
balloon.
Thanks,
John

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
John E. Sohl, Ph.D.
Professor of Physics
Weber State University
2508 University Circle
Ogden, UT 84408-2508
voice: (801) 626-7907, fax: (801) 626-7445
e-mail: jsohl@weber.edu
web: http://physics.weber.edu/sohl/


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