A friend of mine is conductor of a local community symphony orchestra
in our area, so we try to attend as many of his concerts as possible,
and one of them was this afternoon. During one of the pieces, a
reflection from the backdrop behind the orchestra caught my eye, and
captured my attention for the rest of that piece.
The backdrop was plain white and pretty smooth and the tympani
happened to be placed where I could see a reflection of the
drum-heads in the backdrop. The orchestra was brightly illuminated
from above. When the timpanist would strike one of the drums, the
reflection in the backdrop would clearly show the nodal pattern on
the drumhead--the nodes being bright and the anti-nodes being dark. I
spent the rest of the time they were playing this particular piece
trying to come up with a plausible explanation as to why the pattern
of vibration in the drum-heads was visible in the reflection.
I don't think that these patterns are directly observable on the
drumhead itself--at least I have never observed them while looking
directly at a drum (without something like powder on the surface to
stack up at the nodes). I had hoped to get together with the
timpanist after the concert to see if she could see the patterns on
the drumheads, but she left before I could catch up to her.
So assuming I am correct in my supposition about not being able to
see the patterns on direct observation of the drumheads, why could I
see them in the reflection? Attempting to account for this effect, I
went through several iterations of interference phenomena between the
drumheads and their reflections, but abandoned them all as not very
realistic. After some thought, I surmised that perhaps it had to do
with the reflection from the anti-nodal areas being more diffuse,
owing to the motion of the drumhead, and thus the reflection from
those regions from the backdrop would be dimmer than the more
specular reflections from the nodal areas. What surprised me was the
sharp contrast between the light and dark areas of the reflections.
If what I said before is a correct explanation, I would have expected
the contrast to be much less, perhaps not more than barely noticeable.
So what do you think? Is there a better explanation for what I saw,
or does this one seem right?
An aside. We were sitting with the conductor's wife, a PhD biologist
who happens to work for an academic physicist. When I told her what I
had observed, her remark was, "Isn't that just like a physicist? My
boss would have zeroed in on that to the exclusion of everything
else, too!" Unfortunately, the second half of the program made little
use of the tympani, and something, perhaps the lighting, had changed
by then, and so neither of us could see the effect on the few times
the tympani were used in the rest of the program. But my wife did see
it in the first half, so I wasn't just imagining it.
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