Sound waves

[Delwin D Fandrich]

----- Original Message -----
From: "John Delacour" <JD@Pianomaker.co.uk>
To: <pianotech@ptg.org>
Sent: December 05, 2001 4:23 AM
Subject: Re: Soundboard grain angle


> At 8:07 PM -0800 12/4/01, Delwin D Fandrich wrote:
>
> >The soundboard is simply a (more-or-less) two-dimensional,
> >wave-carrying medium.
>
> Ah.  It carries waves.  When _I_ said it carried waves, I was firmly
> contradicted and when I suggested that a third dimension might
> actually remove it from the spirit world and enable it to carry
> anything at all I was reminded that the sound sort of ripples over
> the surface like the Almighty at 5 am. on day 1.
>
> JD
>

If I recall, you were saying 'sound' travels through the wood. Sound is
waves, but it's not the same thing.

Or, did I misunderstand what you wrote?

The traditional and historic model of the soundboard has sound traveling
through the soundboard, as you described, but this is not the way the
soundboard really works.

If you grasp the bridge somewhere near the middle and push and pull firmly,
you will, or should, be able to see some movement, albeit slight. What you
are doing is introducing energy into the soundboard. This energy will travel
from the point where you are grasping the bridge outward to the parameter of
the soundboard. To the eye it will look like you are moving the entire
diaphragm instantaneously, though in fact it takes some finite amount of
time for energy wave induced by your pushing and pulling to get to the edges
of the board where it (the soundboard) is constrained. This is basically
what is happening when a wave of mechanical energy from the string (is that
also 'sound?') moves the bridge. Of course the movement is going to be
somewhat more rapid and frequent.

In this sense the soundboard is much like the loudspeaker diaphragm. Here
movement is induced by the voice coil. From the point at which the voice
coil is attached to the diaphragm an energy wave is induced in the diaphragm
which radiates outward forcing the diaphragm to physically move toward the
parameter which, unlike the boundary of the traditional soundboard, is
relatively free to move in response.

Just in case I haven't been confusing enough, I'll also add that, like the
string, in addition to the transverse energy wave creating the sound that we
hear, the soundboard also has longitudinal waves traveling through both the
panel and the ribs. In theory, these waves are not creating any sound since
they are internal to the soundboard panel and ribs and don't cause
transverse movement in the soundboard. But, since they do contain energy
which had to come from the vibrating energy in the strings, their presence,
at the least, is going affect sustain since their energy will be dissipated
as heat due to the internal friction of the soundboard wood and the rim. I
suspect, though have taken no steps to prove or disprove, this is one reason
why pianos with stiff and massive rims--maple, beech, mahogany (the good
kind), etc.--tend to have inherently better sustain characteristics than
those with 'select hardwood' rims.

There are all kinds of waves. Not all of them are sound waves.


Del