Soundboard stiffness

[gordon stelter]

Thank you! That was an EXCELLENT, informative
description! And since the soundboard is to act as a
transducer, i.e.: converting string vibrations to
airborne shockwaves exerienced as sound, is it not
best that the board be stiffest in the center, and
more flexible around the edges so that the maximum
amount of string energy is converted in this way, and
does not travel to the case?
     Thump

--- Ron Nossaman <RNossaman@cox.net> wrote:
> 
> >Please help a newbee understand.
> >
> >I am trying to picture where the stiffness is in
> soundboards. If you
> >consider the typical compression type that has ribs
> and board pressed into a
> >caul, then the ribs are pulling the board toward
> the flat side or the
> >position that the ribs want to come to rest. Is
> this creating stiffness in
> >the soundboard?
> 
> Hi Gene, (a hot subject in it's own rite here
> lately)
> Soundboard assembly stiffness comes primarily from
> two sources; rib 
> stiffness, and panel compression. Ribs act as a
> linear spring in that if a 
> given rib deflects 3 millimeters under a 5 pound
> center load, then it will 
> deflect 10mm under a 10 pound center load. Panel
> compression acts as a 
> progressive rate spring, meaning that the more it is
> compressed, the 
> greater the resistance to compression.
> 
> In a rib crowned board, the curve machined into the
> rib helps to form and 
> support crown. The ribs then supply a POSITIVE
> linear spring rate 
> determined by their dimensions and the material from
> which they are made. 
> Panel compression supplies a bit more crown support
> and a progressive 
> spring rate determined by panel composition,
> thickness, and the degree of 
> compression it is under. The progressive spring rate
> means that the 
> assembly will deflect less than twice as far with
> twice the load.
> 
> In a compression crowned board, the ribs are
> supplying a NEGATIVE linear 
> spring rate, and the crown and string bearing is
> carried almost entirely by 
> the panel compression, which still supplies the
> progressive spring rate. 
> Because of this, you have far better control of
> stiffness and better 
> longevity with a rib crowned board.
> 
> 
> >The reason I ask is that recently I had a chance to
> be instructed on how to
> >handle a new soundboard out of the box without ribs
> attached. It quickly
> >bacame obvious how fragile they are in this state.
> Any mishandling could
> >easily cause a fracture. Then I think about the
> grain orientation and how
> >the bridges are aligned to this with exception of
> the extreme treble and
> >maybe some of the bass. Along the grain is the
> weakest part of the board -
> >am I wrong? So where is the stiffness.
> 
> The panel is quite stiff along the grain, especially
> with the addition of 
> the long bridge. Stiffness control across the grain
> is the problem.
> 
> 
> >Also curious, is board stiffness something that can
> be calculated?
> 
> To a reasonable degree, yes. Designing a soundboard
> with a new rib scale 
> requires that very thing, taking into account the
> string scale, bridge 
> placement, etc.
> 
> 
> >Is it a factor in rib design?
> 
> Of course.
> 
> 
> >Is it used to calculate how bearing is supported?
> 
> Absolutely. Case in point: Suppose you have a string
> scale you intend to 
> use in this piano. You also have decided on a
> bearing schedule you would 
> like to end up with. Since the soundboard load is a
> result of the bearing 
> angles and tensions you have pre-determined, the
> soundboard needs to be 
> designed to support it. You can design a soundboard
> assembly with 
> relatively low crown, stiff ribs, and a panel under
> high compression that 
> (in an arbitrary spot for illustration) deflects 2mm
> under load and leaves 
> a crown of 4mm. Or you can design a higher crown and
> lighter ribs, with a 
> panel under much less compression that deflects 6mm
> under load, leaving a 
> 4mm crown at your target bearing. So you need to
> have some idea what the 
> soundboard is going to do under load before you get
> to the "what I should 
> have done" part. Now, since both of these
> mythological boards are now 
> supporting nominally identical loads with nominally
> identical measurable 
> crown left over. Which is "stiffer"?
> 
> 
> >Or
> >is board stiffness related only to musical
> qualities?
> 
> That too. Stiffer, and volume goes down as sustain
> increases. More 
> flexible, and volume goes up as sustain shortens.
> There's a lot more detail 
> than that, naturally, but that's the gist of it. The
> killer octave (5-6+) 
> in compression crowned boards is the result of a
> soundboard assembly that 
> is too flexible in that area. Loud attack, short
> sustain, and a lot of 
> distortion.
> 
> 
> Ron N
> 
> _______________________________________________
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