Soundboard crown

[Delwin D Fandrich]

----- Original Message -----
From: "Calin Tantareanu" <dnu@fx.ro>
To: <pianotech@ptg.org>
Sent: August 10, 2003 10:36 AM
Subject: Soundboard crown


> Hello!
>
> I have followed the numeorus discussions on this list oabout rib-crowned
vs.
> compression crowned soundboards with great interest.
> However, I am asking myself if a soundboard always needs  crown in order
to
> work properly?
> I heard about some old pianos with no measurable crown that sound very
good.
> I have even seen some where no crown seemed left, yet the performance was
> very good. I read that harpsichords do not have crowned boards (except
the
> Italian ones).
> So, why is a crown necessary?
> Would a flat board, but with enough downbearing for teh strings, work or
> not?
>
> Regards,
>
>  Calin Tantareanu
> ----------------------------------------------------

As a purely theoretical question, I would say definitely maybe. There are,
of course, a number of qualifiers, variabilities and dependencies....

Soundboards, of whatever nature and material, need a certain combination of
mass, stiffness and internal resistance to function in the way in which we
have become accustomed. (For the purposes of this discussion let's ignore
the effects of humidity and wood MC on any part of the soundboard system.)
If we deviate overly much from the parameters that have evolved over the
centuries we will no longer have an instrument definable as a piano.
History defines the tone standard we are striving for. Or at least it did
until the flood of piano-like objects with their granite hammers and their
massive marketing campaigns arrived on the scene.

In 1700 wood was the only logical material from which to make soundboards.
And it remained the only logical material until fairly recently. While the
piano soundboard started out as basically an overloaded harpsichord
soundboard, it had evolved considerably by the late 1800s. The motivation
for this evolution was the increasing demand for power and the resultant
string loads placed on them. In response it became thicker and, by virtue
of increasingly stiff and massive rib systems, stiffer. Using construction
techniques that seemed reasonable to piano makers of the day, one viable
way to achieve an adequately stiff soundboard system without overloading it
with excess mass was to form a positive wood spring and then compress it
with another spring--the string set that was stretched across the bridge
with some amount of tension and deflection. In so doing the stiffness of
the soundboard system was increased considerably with no increase in mass.

With all this in mind, if we are to deviate from the established pattern we
must come up with a soundboard construction having a mass that falls within
certain limits combined with an amount of stiffness that will control the
rate of energy transfer from the vibrating string(s) to the soundboard
panel. One can argue that what the soundboard needs is stiffness, not
crown. If the requisite amount of stiffness can be obtained sans crown a
workable soundboard can readily be built without it. Certainly this was
accomplished by Rippen with their laminated soundboards. These boards had
no crown as built, though they did end up with "negative" crown through
string loading.

Nearly any degree of stiffness (within reason) can be obtained with a
standard wood panel/rib combination by simply making the ribs as tall as
need be to achieve the desired amount of stiffness--without any crown being
included at all. The mass of these ribs can easily be controlled by simply
making them narrower. And this is basically what we do when we make a
rib-crowned soundboard system. The ribs, instead of being anti-crown
devices, now become structural beams with their stiffness being controlled
by their cross-section height and width coupled with some projected amount
of deflected crown. By removing crown from the equation we could still come
up with the same amount of stiffness by adding a bit more height to the
ribs.

Having come this far, we must now bring up the question of whether or not
string deflection in the form of string bearing working against the
soundboard is really necessary. The traditional string set/soundboard
system works on a principle of two opposing non-linear springs--the
soundboard system pressing up against the string set which is pressing down
against the bridge/soundboard system. Now, is our crownless soundboard
going to have any string bearing? If so, is the soundboard going to start
out with some slight crown and become flat through the application of that
string bearing? Or is it going to start out flat and be forced into some
kind of reverse crown by the string bearing (ala Rippen)? Or is it going to
start out flat and remain flat having the strings attached without any
deflection, hence no string bearing against the bridge?

Keeping in mind that I've not done much of any real research specifically
designed to resolve these questions, I do have a couple of thoughts. A
system in which a string without deflection is working against a flat
soundboard is not going to respond like a system in which a string with
deflection is working against a sprung soundboard system. Considering just
the soundboard, in the first configuration--both the string and the
soundboard system working through their zero axis--the soundboard's
stiffness will be least at its rest position, increasing somewhat as it is
forcibly deflected from rest by the motion of the vibrating strings working
through the bridge(s). In the second, the stiffness of the soundboard
system increases as it is forced down and decreases as it returns and moves
up beyond its rest position. The opposite effect is seen in the string set.

As to exactly how much difference this makes in the final sound envelope, I
don't know, I've not attempted to define and measure it. I suspect, though,
that if two pianos were built having carefully controlled soundboards, one
being designed to have a given amount of mass and stiffness with no crown
and no string bearing and the other being designed with just enough crown
so that it would become completely flat with some amount of string bearing
(and at that point having exactly the same mass and stiffness as the
first), the latter piano will have a better balance between power and
sustain. I suspect that the first piano would have a somewhat more
percussive sound and, consequently, a shorter sustain time.

It would be an interesting experiment, but lacking that we are not left
without clues. We can come close to this in real life by examining a
selection of otherwise nicely rebuilt pianos having old soundboards that
have ended up little or no discernable crown after stringing. I've
encountered examples of both: freshly rebuilt pianos with zero crown and no
measurable string deflection and freshly rebuilt pianos with zero
crown--i.e., some distorted but with the bridge line at the same elevation
as the outside edges--but with some string bearing. Yes, they both work,
but it becomes a judgment call as to how well they work. In general the
respective pianos have exhibited the tonal characteristics described above.

Del