[Phys-L] Re: An Interesting reflection phenomenon

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

At 0:03 -0400 4/18/05, John Denker wrote:
> My hypothesis:  Reflection of the image isn't the issue ...
> plain old distance is what matters.
>
> The deflection of the drumhead isn't very large, so
> it is like a telescope mirror with a rather long
> focal length.  If you look at it up close, you
> are so far inside the focal length that the
> surface might as well be flat, and small changes
> don't matter.  But if you observe from a distance
> on the order of a focal length, it might well
> gather a whole bunch of light and focus it
> onto where you are sitting.

That makes some sense. However, What I saw clearly seemed to be the
nodal lines on the drumhead. In other words what I saw was mostly
shades of gray with some narrow bright lines, which seemed to be
consistent with the narrow nodal lines. So I got lots of light from
the relatively undeflected parts of the drums (presumably--perhaps
I'm misinterpreting that), and very little light from the regions of
higher deflection.

What I took to be the nodal lines were not circularly symmetric, but
roughly rectangular in shape, and seemed to be aligned parallel to
the orientation of the drum stick when it hit the drum. That doesn't
make a whole lot of sense in terms of Chladni patterns, but then I'm
not sure I've ever seen any pictures of Chladni patterns on real
drums, so I'm not sure I know what I should be looking for. Also I
couldn't get too long a look at any one event, because the music
called for suppressing the reverberations quickly, so most of the
events I saw lasted for only a second or two, maybe three or four at
most.

Both your and Brian's suggestions are worth further thought.

Where's Tom Rossing when you need him?

Hugh
--

Hugh Haskell
<mailto:haskell@ncssm.edu>
<mailto:hhaskell@mindspring.com>

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