Modal Analysis

[Delwin D Fandrich]

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----- Original Message -----=20
From: "Greg Newell" <gnewell@ameritech.net>
To: "Pianotech" <pianotech@ptg.org>
Sent: May 09, 2003 3:46 PM
Subject: Re: Modal Analysis



Del,
         I wonder if you might help me understand this. You say, "And a=20
good example of why soundboard cutoff bars can (should) be used to some=20
advantage." In the views I saw on the web page there seemed to be quite =
a=20
bit of movement in the area normally eliminated by a cutoff bar.=20

Yes, and it is exactly out of phase with the motion of the bridge and =
the area of the soundboard surrounding the bridge.



----------------------------
If there=20
is that much movement is that not contributing to what we hear rather =
than=20
detracting from it? As I type and look back at the pictures in another=20
window I'm tempted to rationalize that while one part of the board,=20
specifically that part in question, is on the down part of a wave the =
rest=20
of the board is up and visa versa. Is this what you wish to eliminate =
with=20
the cutoff bar so that the board acts as a whole instead of in separate=20
parts? It would be interesting to see what the board would do using this =

modal analysis with a cutoff bar installed.

Yes, this is what I am trying to eliminate with the addition of a =
carefully placed cutoff bar. Energy is being traded back and forth and =
is essentially being wasted. Going back to my analogy of the piano =
soundboard functioning somewhat like loudspeaker driver: the soundboard =
functions best when it is appropriately sized for the frequency range =
through which it is expected to function. If it is too small it is great =
for sustain but will not be able to develope the desired power. If it is =
too large, however, both sustain and power suffer. Sustain because =
energy is being wasted in the excess soundboard area and power because =
acoustical energy is being traded back and forth between the various =
resonant areas and is effectively cancelled out.



----------------------------
         This brings me to another question. In all of the examples I've =

seen of pictures with cutoff bars installed, all seem to be a great deal =

smaller in size than the corresponding inner rim section they are=20
replacing. I understand that they are epoxied into the existing inner =
rim=20
and doweled as well. I further understand, I think, that the cutoff bar =
is=20
attached to any support beams over which it passes. Do the combinations =
of=20
these items make it unnecessary that it be of similar dimension as the=20
inner rim?

Yes. That and the fact that they are generally rather short. Early on we =
did try to make them somewhat more massive but over time have found that =
this is not really necessary. I don't have any formula for this, I just =
look at the thing and decide. If it is quite a long cutoff I'll make it =
taller.

Del 
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