[Phys-L] Acoustics question about popped balloons - try 2
From: jsohl at WEBER.EDU (John SOHL)
Date: Wed Aug 24 23:23:50 2005
Thank you John for the detailed reply.
Let me try to express my "new" understanding. Especially, let me try to
express it at the 6th grade level for the science teacher that asked me
about it.
1. Shock is a term to avoid because it has a special use/definition.
Fair enough.
2. The sudden release of air from the popped bag or balloon causes a
disturbance or wave in the surrounding air (in simple terms it "shakes"
the surrounding air for a moment). This sudden disturbance (release of
energy) to the surrounding air then propagates outward as an ordinary
sound (i.e., pressure) wave. This is what you hear.
(I have absolutely zero problems with this pop being heard a long
distance away from the source, it is simply a sound wave.)
That is a better phrased version of what I was trying to say in my
first post and without the problematic term of "shock wave."
Now, my take of the two "explanations" on the net that I found (phrased
in very non-physics terms):
They are saying that the "extra molecules" that pressurize the air
inside the balloon are what you "hear" when the air expands outward
rapidly with the release of the air. (PV=NRT, increase N and you
increase P and slightly increase V.) I.e., this pressure moves outward
and your ears respond by giving the sensation of a "bang." This is a
conservation of air molecules argument that implies that the extra air
molecules that are released have to go somewhere and as they escape they
cause a local increase in air pressure that your ears hear since the air
pressure momentarily increases at your ears as the molecules in the
surrounding air adapt to the extra molecules you just released.
This argument makes no sense to me and without actually calculating the
increase in the air pressure 100 meters away, I just don't see that it
would be a very loud pressure sensation to your ear.
Comments?
Thanks,
John
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John E. Sohl, Ph.D.
Professor of Physics
Weber State University
2508 University Circle
Ogden, UT 84408-2508