Re: Possible acoustic phenomenon, very bizarre

[William Beaty <billb@ESKIMO.COM>]

On Sun, 19 Sep 1999, Leigh Palmer wrote:

> Yes. Pressure antinodes are coincident with velocity nodes. If the
> pipe is excited with any periodic signals, resonant or otherwise,
> this will be true of all components of the series which comprise
> them. This picture is correct only in the limit of small amplitudes,
> however. It applies only to linear acoustics. You need not construct
> elaborate examples like what follows to recognize this. Just think
> about a periodic acoustic wave in air with an amplitude of two
> atmospheres.

So you predict that the cavitation-like effect cannot exist, because the
rules of nonlinear acoustics probably forbid it?  This might be true.  But
if it is not, then we can have some big fun.

I wrote a (linear) simulation/animation of superposed standing wave
harmonics +1,-3,+5,-7, etc., and expected to maybe see a short pulse which
bounces between the closed ends of the tube.  Instead I see narrow,
high-pressure regions which exist only at the ends of the tube, just as my
hunch told me.

Even if the removal of the tube-ends will disrupt the phenomenon, I think
the phenomenon is pretty bizarre.  There are small ripples of sound
everywhere in the tube (caused by the limited frequency span of the
"comb-like"  spectrum.)  There are two thin regions adjacent to the tube
ends where the alternating pressure is incredibly high, approaching a
delta function.

It's almost like that "sonoluminescence" demonstration in the spherical
water flask... only in this case the "cavitation" appears at the surface
of the resonant cavity, and the medium is air instead of water.  (Hey, the
effect should work in water just fine too!)

For the "weird science" take on this, see the article I just added:

  "Invisible walls" and "virtual barriers"
  http://www.amasci.com//freenrg/audwall.html


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William J. Beaty                                  SCIENCE HOBBYIST website
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