[pianotech] --Centering the bridge--was S&S something

[Ron Nossaman]

On 5/29/2012 11:46 AM, Dale Erwin wrote:
> Mass and stiffness in assemblies are one thing
> from a weight point of view, but what weight?

Weight  *is* mass, under gravity, measured with a scale.


>Then add a certain amount of bearing to the
> structure and the impedance changes again.

It does, but the less panel compression, the less change and the 
shallower the change rate curve.


> Also Factor in that probably no two Techs who routinely build soundboard
> systems or seem to set down bearing in the same way or even agree or
> think it all that necessary to the final outcome.

That's because no two of them are talking about the same soundboard, 
whether they realize it or not.


> So to some real extent the empirical tonal outcome is what folks say wow
> to... or not.

And the subject impression of what constitutes tone, whatever the actual 
piano sounds like.


> We desire to create a sound our own ears intuitively have
> been pursuing on the tonal road less traveled by.

If we didn't like the result, we wouldn't have spent all those months 
and years chasing them, and wouldn't continue to build them. CC with 
flat ribs is the easiest way to build a soundboard if that's all we wanted.


> My tone described
> ___I personally like a tone that can be described as power without
> noise,

Sounds like what you wrote about my Rochester B.


>I have designs that are repeatable, dependably musical and that I can
> build with confidence. I am certain many of the regular contributors to
> this subject who weigh in here can attest to the same confidence, and I
> think they would all agree that its come with lots of trial and error on
> a steep, steep learning curve.

Yep.


> We usually install a bass cut-off bar to shorten those monstrously
> long ribs and prevent energy losses in the bass corner.

I do it to center the bridge and make the ribs there stiff enough 
without having to be 50mm deep and heavy as logs. Killing the energy 
wasting open bass corner as well is a freeby.


> As I have listened to countless Compression crowned systems I have
> realized that so many sound fantastic. Yes, they have design
> shortcomings I do not wish to repeat, but they can sound awesome for
> some period of time and under the right conditions, a very long time.

Absolutely. They can also sound thoroughly wretched. Where RC&S systems 
are controllable to a vastly narrower range of variability that 
*averages* well above the average sound of the CC board spectrum. This 
control is what the CC board builders find offensive. Not enough 
"variation", too predictable. I've heard descriptions like "cookie 
cutter boards". I say I'll gladly take it.


> What I have taken a way from the C.C. model is that these boards have a
> non linear compression response when pre-loading them for stringing...
> when built right.

Which makes them much more sensitive to bearing setup than RC&S.


>Pre-loaded means what the board does as it is strung
> providing the bridge height was cut high enough to allow them to be
> loaded/compressed in an adequately. Meaning that the more they are
> compressed the stiffer they get till they won't deflect anymore. This is
> what makes this model work and when its dialed in to closely watched
> parameters they can sound amazing.

Indeed they can. The balance is to compress them to the point of maximum 
compression, therefor maximum stiffness, without going too far on the 
bearing and restricting what movement they are capable of at full 
compression. I think that's largely the difference in the efficiency and 
tolerance of the RC&S boards. Even built as stiff as a CC board, they 
still gave spring both up and down, where the CC board is bottomed out 
on the panel compression and is getting essentially half a spring cycle.


> So, I still see the board in my own design as the springy diaphragm
> model based on non linearity. I do make quite a stiff board, stiff set
> of ribs & I do pre-load it much the way the factories have done for
> decades.

I don't because I can't. Mine don't act like CC boards at all. They 
don't have a soft top to the deflection, and no bottoming out, and 
they're really too stiff to pre-load, even if doing so told me anything.


> The other factor is how much crown is enough and what are we going to do
> with it once we have it.?
> In my recipe, it came to me over time, that what ever the unloaded
> upward deflection of the board was after the board was glued to the rim,
> that I wanted to compress that amount of crown by roughly half with
> string bearing. ie the board has 10 mm of crown on the longest rib. I
> want it to deflect 4-6 mm. and I want it to be strongly pushing back. I
> want it to be reactive.

I started with the premise that I wanted a crown height at least as high 
as the bearing offset, so it was geometrically impossible under any 
circumstances for the bearing to force the crown flat or concave.


> If I'm using tight grain Sitka spruce I will want to thin the entire
> panel to 8 ish mm and the thin it at the rib scoops/perimeter and
> further into the panel.

I haven't used anything but a constant thickness panel for some time.


> Gene, as to panel compression I dry to 5.5 to 6 % depending on where its
> going. I'm not concerned about a certain amount panel compression and
> consider it as a piece of the equation but a much smaller piece than a
> C.C. board. Also remember that I am using Sitka spruce in the 20 to 28
> grains an inch range and it can easily handle this amount of compression
> in many climates but especially in our western region climate and this
> is where most of our restorations end up.

I like low density panels, in the 8-12 GPI range. Since panel 
compression isn't providing much of anything to stiffness, I don't need 
high grain densities.

It's always been my experience that if there wasn't a fairly huge range 
of acceptability in building these things, we wouldn't be able to do it 
at all by any method. What we're after now is more understanding of what 
does what, and a refinement of control that gets us past the crap shoot 
of soundboard building and survival that has been the state of the art 
for way too long in the CC world.

Ron N