More on soundboard crown

[Delwin D Fandrich]

----- Original Message -----
From: "Phillip Ford" <fordpiano@earthlink.net>
To: "Pianotech" <pianotech@ptg.org>
Sent: August 17, 2003 2:46 PM
Subject: Re: More on soundboard crown


>
> This is discouraging.  Are you saying that there is no lower limit
> below which there won't be compression set?

In theory, yes. Though this subject has come up in the past as well and has
been the recipient of some debate. On a practical level the rate of
compression set at very low levels of compression can be so low as to be
assumed negligible.


>
> If so then every
> soundboard is doomed to flatten.  Even on rib crowned boards the ribs
> are subject to some level of compression load on their upper fibers
> due to the bending loads imposed by the downbearing.

Again, in theory this is true. However, both the tensile strength and the
compression strength of wood taken longitudinally (parallel-to-grain) are
so high relative to the rib loading that in a rib-crowned soundboard system
compression set can be virtually ignored. At least this is so with a
reasonably designed rib set.

The problem is the relatively low perpendicular-to-grain compression
strength of spruce. The compression-crowned soundboard system
loads--stresses--the panel to fairly high levels in this plane. The
rib-crowned soundboard system avoids stressing the wood in this plane.

Del