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In a message dated 2/1/2004 12:51:33 PM Pacific Standard Time,
fordpiano@earthlink.net writes:
Ric writes
My own degree of understanding of these matters still remains at a
>rather young stage, yet one theme comes back again and again in
>these discussions. That is that the sound that the panel is able to
>project is dependent upon its stiffness and mass. Certainly any
>particular combination of these is in itself independent of string
>coupling. John Hartman is correct in saying that down bearing can
>increase the stiffness of the panel without added to the panels
>mass... tho the strings have mass to... and these things have a
>habit of working both ways in some sense or another... Still
>essentially John is correct..........
I think I can hear Del gnashing his teeth. If stiffness was all that one
wished then running ribs at 90 degrees to the grain is the least stiff
configuration. If the ribs are run at substantially less than this the stiffness also
goes up without adding mass.
The question remains then can good sound result without any
>particular amount of down bearing.
>>. Yes but it would be different. ie many uprights & other grands which have
either lost crown or were designed with very little or i.e. My Own Mason AA
with a minimal crown board (some crown presumably lost) and light bearing is
one of those pianos that works within the parameters being discussed but it
also has a thick board & 15 ribs which equals mass and stiffness without great
amounts of bearing. Also string scaling is another factor of course.
I just added new bridge caps & no I didn't, & wouldn't set it up with 0
bearing which answer one of David's question. Why? Because I happen to
think/know that a soundboard with crown or just mass & stiffness behaves like a
compressible spring which makes it more reactive.
I also know that from a tactile sense when I'm prestressing board by
placing wedges under the struts & pounding it down while wedging to simulate some
string preloading that the board simply gets so Stiff it's like pounding on
the bridge top but hitting a gym floor. Its dramatically different from before
prestressing.
The amount of bearing does regulate the rate at which tonal energy leaves
the board/system. Use a garden hose for an analogy turned on full blast then
slowly pinch it to restrict water flow. This I belive is a simple definition
of mechanical imedance. Soundboard compression is also mechanical impedance
but it's impeding soundwaves not water. Phil correct me if I'm wrong.
David S's question which is ,"does bearing do anything other than make the
board stiffer" I think can be answered by saying I think so based on what I
said above. Its more reactive. Any thing under stress is more reactive. Humans
included
Phil writes
I keep reading posts by various people stating that the
soundboard gets 'stiffer' as downbearing is applied. But I have seen
no experimental evidence to support this supposition. The only
experimental evidence that I have seen was that presented by Ron
Overs some time back in which he took a crowned ribbed panel, loaded
it incrementally, and noted the deflections. His data showed just
the opposite; that the panel was getting less stiff as the load
(simulated 'downbearing' if you will) was increased. If someone has
some data to support the phenomenon of increased board stiffness with
increasing downbearing then please share it with us. The only
support for this position that I have heard has been anectodal
stories along the lines of, 'I pushed down on the new board and it
deflected, then I leaned on it with all my weight and it wouldn't
deflect any more, so it obviously was getting stiffer'.
>> I believe it was getting stiffer but remember we're talking about a light
weight system with limited ability to carry loads so great as to crush them
altogether.
It was
reaching a state of equilibrium for the applied load based on the
stiffness of the system. That doesn't mean it was getting 'stiffer'.
There is a difference between preload and stiffness.
>>I'm honestly not sure about that. It seems like to me it's both.
Was Ron Os demonstation on a board glued to the case liner? For the board
will certainly react differently if the test is out of the piano with out the
edges glued down.
As you
apply more downbearing load to the board then the preload (or
prestress if you prefer) will increase. It's not hard to believe
that this could have some affect on the vibrational characteristics
of the board and experience tells us that it does. Putting
downbearing on the board usually seems to have a beneficial effect.
>> And too much makes for a stingy sound especially in the treble
Stiffness is a relationship between load and deflection. If two
beams have the same load applied to them, then the one that deflects
less is 'stiffer' for a load applied at that particular spot.
Increasing stiffness under load would mean that there was
incrementally less deflection for unit increases in load. If a beam
deflects down 1/10 of an inch for an applied load of 1000 lbs, then
if it is getting 'stiffer' under increasing load, when you applied an
additional 1000 lbs of load the additional deflection would be less
than 1/10 of an inch.
This is exactly what a board is doing when it is being compressed up to a
point & then it will fail just as the beam will.
I see no reason why a ribbed panel would
behave in this way. That's not to say that I can't be wrong and that
there's not something about this particular system that I've
overlooked or don't understand. But I'd like to see some
experimental evidence to prove it.
This distinction is important to potential soundboard design.
If the important thing is increasing the stiffness without increasing
the mass, then an alternative soundboard made of something like
honeycomb sandwich might give the desired performance without any
downbearing. If the important thing is preload or prestress in the
panel then the honeycomb panel might be a waste of time and
downbearing would be essential regardless of the type of panel you
used.
Also, if the board is not getting stiffer as a result of
deflection, it raises the question of what function the crown is
performing.
>>The way I look at it is that putting boards under some modest amount of
strain via compression or tension for that matter makes the board more reactive
and tonally efficient because of the impeding effect, which moves air & thet's
why soundboards are built this way. If it wasn't Ron O would be building a
flat board.
If the crowned board is not getting stiffer as it
deflects down, then a flat board would be just as stiff as a crowned
board. So, the reason for the crown would not be 'stiffness'.
Phil
>> But I think it is getting stiffer.
Dale
PS I hope somewhere in here is something of an answer/opinion David S was
looking for.
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