Sound waves(The behavior of soundboards)

[Ron Nossaman]

I said just a while ago that I didn't know why there was a difference
between fork activated bridge and the tack hammer experiment - but I have
some ideas.

It's the "Fiddle-de-dee" that John mentioned. It's the difference between a
plucked violin string and a bowed one, and that is continuous excitation.
End of fiddle analogy. In both the fork and the hammer test, the results
are judged by the sound produced by the movement of the soundboard some
time after the impetus was applied to the bridge. We're hearing the result
of soundboard movement, not the cause. With the fork, we are hearing the
organized result of a regularly spaced series of impulses that have moved
bridge and soundboard, and organized into a series of standing waves in the
board corresponding to the frequency of the input. It doesn't matter
whether the input is on top of the bridge, or on the side, because the
frequency will be the same in either case, the bridge will move at that
frequency and as a result, the standing wave patterns of the board will
produce a sound of that frequency. That's what a soundboard does for a
living. The tack hammer produces a single pulse, and the assembly is
stiffer horizontally than it is vertically, so what you are hearing is the
resonant frequency of the assembly in different directions, not of the
input. What sound differences do you get when you tap the end of the handle
of a vibrating tuning fork on the bridge side, compared to on top? Guess
what, it sounds remarkably like the results from the tack hammer. The
experiment is interesting, but meaningless as far as any indication of the
soundboard moving before the bridge. I still see no reason to entertain the
possibility that this could even happen.

Spruce, as with most woods, is not a particularly "resonant" material. It
has a fairly high internal friction compared to something like steel, so it
doesn't ring like the much hammered upon and stroked steel bar that shows
up from time to time in this discussion. Neither maple, nor spruce will
propagate an internal compression wave nearly as far, nor maintain it
nearly as long as steel. This steel bar will, I'm told, carry a compression
wave it's full length (whatever that may be) even if it is embedded in
concrete. So a spruce soundboard that is presumably initially driven by
internal compression waves that turn into transverse waves after reflection
from the rim and subsequently move the hitherto unmoved bridge shouldn't be
terribly hampered in the transmission of these compression waves by being
glued for a short space on one side to a mere wooden rim, should it?
Especially along the grain of the panel, which as we are told, is the
direction in which sound is best conveyed by the panel. So why doesn't a
tuning fork pressed against the end grain of an installed soundboard panel
produce the same sound as when it is pressed on the top of the board a few
centimeters away? 

Ok John, I answered your questions again. Now how about you answering mine?
We're still holding at two, listed according to age. 

1. How minute does a movement have to be to be nonexistent?

2. Why doesn't touching the fork to the edge of the soundboard not produce
the same tone as touching it to the top if it's compression wave driven?


Ron N