Soundboard Compression & Cracks

[Farrell]

Del wrote:
> ....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.

Why would late wood be more subject to compression set that early? Late wood
is more dense, hence stronger. I would think the early, less dense wood
would compress more. Is it that the late wood, being more dense, does not
"bounce back" (bad phrase, but I mean that late wood is not as resilient) as
well as the early wood?

Terry Farrell

----- Original Message -----
From: "Delwin D Fandrich" <pianobuilders@olynet.com>
To: <pianotech@ptg.org>
Sent: Sunday, June 10, 2001 11:44 AM
Subject: Re: Soundboard Compression & Cracks


>
> ----- 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
>