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[Phys-L] Re: Acoustics question about popped balloons - try 2



At 10:24 PM 8/24/2005, John Sohl, you wrote:
///
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.)

///

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

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
John E. Sohl, Ph.D.


I took a 0-30 psi air gage and blew into it as hard as I could.
It registered 3 psi (gage)
I estimate an average balloon at diam = 12 inch diam - in vernacular units.

Trying for a conservation of energy approach:
Say that P X V is conserved with the hemispherical steep face wave (the steep
face providing the characteristic "bang" ) for the usual Fourier reasons.

Model a pulse cross section as a square pulse in a radiating hemispherical
shell:
At 100 yards the volume of the wave, if we suppose its pulse length is
maintained at 6 inches (i.e. the shell thickness), as a ratio of the
balloon's
initial volume leads to a pressure in the shell ( inital pressure 3
psi divided
by the same ratio) at 100 yds distance of several hundred times bigger than
the
threshold of audibility (taken as < 1E-9 atm.)

This estimate is accessible to high school students, I expect?

Brian Whatcott Altus OK Eureka!