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Delwin D Fandrich wrote:
>
> The understanding of soundboard function has moved a bit beyond the purely
> empirical--even beyond 90% empirical--though certainly not to the stage of
> pure science either. Still, techniques such as modal analysis have enabled
> the study of soundboard function at a level not even dreamed of even thirty
> or forty years ago. It is my hope that before I pass from the scene that
> this understanding will be still some closer to scientific and much less
> reliant on the empirical.
>
> Del
Been reading and weeding through and find much that is interesting, but this
comment caught my eye and I must admit is confusing. I wrote about modal
analysis a year and a half back and you came out and said it was basically
useless in designing soundboards, since the conditions for measurement are
different then the conditions for full strung, and if done full strung well the
deed is already done so to speak... further you pointed out then that you meant
that there was no reliable way of forcasting said changes.
>From the fall of 1999 where we were in a disscussion about impedance matching,
and the usefullness of modal analysis came up I submit the following three
posts.
- ----- Original Message -----
From: Richard Brekne <richardb@c2i.net>
To: <pianotech@ptg.org>
Sent: Friday, August 13, 1999 3:12 PM
Subject: Re: Impedance Matching
>
> Hmmm.. In the section of the Wogram article entitled "Influence of
string
> tension" He seems to be saying that the condition of an uloaded
soundboard
is
> not a problem. He says that the basic impedance curve and sound
radiation
> curves remain basically the same and that only the lowest resonances
are
> effected and that the effect is predictable enough to take into
consideration.
> Is this outdated ??
- ------------------------------------------------------
I don't know what the basis for this assumption is, but my own --
admitidly
limited -- tests have indicated otherwise. Soundboard characteristics
do
change as it is loaded. Partly because of the increase in stiffness
within
the panel, but also because the string plane affects the mobility of
the
entire assembly.
One of the problems I have with most of the testing I have seen on the
piano
soundboard is that it was done on an unloaded board and is, therefore,
largely meaningless.
> The article seems to point to some significant degree of potential
for
> re-distributing resonance points for the different nodes by
introducing
new
> stiffness and / or mass to different areas of the soundboard. Course
one
would
> have to gain lots of experience to know just how, why and where to
introduce
> such changes on an existing board, but it seems like enough
information
can be
> obtained by the modal analysys to accomplish this. Or is this just a
sugar
> coated red herring ?? Opinions ??
I consider modal analysis of the working soundboard to be a good first
step.
You know, like, "one small step for man...."
Del
Followed by the following notes from Ron N
Date: Mon, 16 Aug 1999 20:01:48 -0500 (CDT)
From: Ron Nossaman <nossaman@SOUTHWIND.NET>
Subject: RE: Impedance Matching
>Del,
>I'd second the notion that soundboard characteristics change with
load.
>For SURE!
>
>While I'd agree that a modal analysis is meaningless without the full
system
>(strings), it can be very valuable calibrating FEA models. It's very
simple
>to add/remove components of a finite element model and is invaluable
for
>checking the accuracy of parts of a very complicated model (like a
piano).
>
>I know, "so what are you waiting for"?
>I'll get back to it ;-)
>
>doug richards
>San Jose, CA
Well, sure, soundboard characteristics change under load, but how
much? My
point is that we, or at least I, make design decisions based on how I
think
the board will respond in use, not how it sounds lying on the floor. I
have
no way to determine how the board reacts without the string load. I
can't
hear it without the strings, so I am, in effect, factoring the
difference
into the design. How well it works in use is determined by how good my
initial design was, how accurate my judged factoring was, and the
characteristics of the materials being used being what I thought they
were.
If we could compute an unloaded impedance gradient of the assembly,
across
the scale, with the physical information we *do* have, and working
backward
from the way it *sounds* in use, compared to what the computed
gradient
says, we should get both a roughly quantifiable cause and effect
relationship (which we are already working with, without the detailed
figures), and some idea of the difference between the loaded, and
unloaded
states. Again, I believe the differences between the unloaded state of
the
assembly, and the loaded state, can be quantified to a practical
degree
without taking a FEA model to the sub atomic level on every single
piano
that's worked on. The impedance loads, taken from the scale and
bearing
schedules should be relatively simply applicable to the computed
static
impedance model of the assembly, wouldn't you think? It ought to be
proportional, shouldn't it? How close to an absolute dead center
impedance
match/mismatch is necessary before we are beyond the resolution
(granularity?) of the medium with which we are working... wood? What
the
heck IS the ideal match between soundboard assembly and string plane,
given
high impedance mountings and terminations? That's what I was getting
at. I
suppose it's still a chicken/egg sort of thing though. Someone has to
do it
the hard way and make it work before it can be simplified to
usability.
I sure wish I could pound some of the missing math receptors into this
old
brain, I could certainly use them.
Ron N
And then again from Stephen Birkett:
Date: Sat, 14 Aug 1999 00:12:48 +0200
From: Richard Brekne <richardb@c2i.net>
Subject: Re: Impedance Matching
Stephen Birkett wrote:
>
> I have my opinions about modal analysis (mentioned here
once a
> while back)...that, at best, it can tell you that the
soundboard is
> working as you already know that it is supposed to work.
As a practical
> tool for design I have my doubts it will ever give
anything, primarily
> because it treats the sb unloaded and in isolation from
the interactions
> with the rest of the piano.
>
--
Richard Brekne
RPT, N.P.T.F.
Bergen, Norway
mailto:rbrekne@broadpark.no
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