Sound waves(The behavior of soundboards)

[Charles Neuman]

> In particular, since I posted a message this morning giving a
> decription of the way a vibration passed down the bridge, do you see
> any sense in this?

Sure, there is sense in it. It could be that all these compression waves
are going around everywhere, even if it turns out not to be the primary
way a piano produces sound.  It's interesting to think about. I think
theories are fun to think about, even if they turn out to be wrong. I
mean, I like the idea of the ether, which is the invisible medium that
electromagnetic waves were believed to travel through. OK, so it's wrong,
but it's still a neat theory. There's some famous quote that goes
something like, "There's nothing sadder than a beautiful theory ruined by
a brutal fact."

I find that it's really hard to prove something wrong, even if you think
it's wrong. I remember a guy showing a professor some weird theory about
matter. It was way out there, and I didn't really care to get into it. The
professor was more patient and listened to the whole thing and then
responded, "Hmmm, I don't think it's right, but I can't explain why." So,
while I fall more into the "bridge movement" camp, I say for sure that
compression waves have nothing to do with it. If they don't have anything
to do with it, I want to know why not. I can best comment on things that I
can really sink my teeth into, like thinking about the nodes on a string.

I liked Ron's description of what happens (from Jan 16). It pretty much
said, "The string vibrates and moves the bridge and then the soundboard,
and then all sorts of things happen."


> He writes:
>
> >1) a finite structure such as the bridge, can only really be considered
> >to carry waves if the wavelength is small compared to length of the
> >bridge. I haven't done the calculation but I suspect the frequency where
> >waves can be considered to occur in the bridge is very high.

I'll comment on this response. I received pretty much the same response.
It's probably the same person. I'm confused by it. After all, sound waves
can travel through walls that are thinner than their wavelength, right? My
guess is that this response refers to the bridge not being able to support
a resonating sound wave with a wavelength longer than the bridge's
thickness. That is, a sound wave produced by a piano string could pass
through it, but they are too long to be able to RESONATE in the bridge. I
think that's the gist. I read a mathematical description of what happens
in a vibrating rod. When you strike the end of a metal rod with a hammer,
it resonantes and produces a pretty pure sound. The vibrations are
compression waves inside the rod. Like the string, the length of the rod
determines the frequencies of the rod. In the same way, if you want the
bridge to resonate, it has to be thick enough. I think that's what the
response meant. Whether or not that is relevant to the "compressionists"
theory, I'm not sure.

I agree that this has been a fun discussion. If there's one thing I have
learned, it's that I know very little about sound. I'm much more
comfortable with electromagnetic waves, which don't need a medium to
propagate (unless you believe in the "ether"). But sound moving through
wood... what a mess. I'm glad Stradivarius didn't wait around to
understand how it all works before trying to build a violin!

Charles