Soundboard Compression & Cracks

[Delwin D Fandrich]

----- Original Message -----
From: "Farrell" <mfarrel2@tampabay.rr.com>
To: <pianotech@ptg.org>
Sent: June 10, 2001 6:31 AM
Subject: Soundboard Compression & Cracks


> Take a compression-crowned soundboard that is not collapsed (still has
some
> crown) that also has a few cracks through which you can see daylight. How
> can this be? The only thing that gives the board crown is the lateral
> compression across the grain as the wood gained moisture after its initial
> drying during construction. Now if you have daylight cracks in the board,
> does this not mean the board is not under compressional forces? Or is it
> that the inter-crack areas are still under compression from being glued to
> the non-dimensionally changing ribs (relatively speaking)? But then
why/how
> is it that one area of the board can be under compression and an inch or
two
> away, the board is under tension????????

You pretty much have it. Compression-set in wood is not a uniform thing. It
occurs first and most in the late-wood portion of the annual ring. So there
will be more compression set in an individual board having a higher
percentage of late-wood. Note that this does not necessarily relate to the
overall width of the rings but to the proportion of a single ring that is
late-wood vs early-wood.

Unless some kind of distortion has set in, such as that typically found
around the low tenor bridge or around a cantilevered bridge base, crown
tends to average out due to the longitudinal stiffness of the ribs. It would
be pretty hard to isolate individual spots of soundboard panel compression
vs individual spots of tension.

-- ddf