Compression Question

[Delwin D Fandrich]

----- Original Message ----- 
From: "Bill Ballard" <yardbird@vermontel.net>
To: "Pianotech" <pianotech@ptg.org>
Sent: August 29, 2003 5:44 PM
Subject: Re: Compression Question


> Stop me if you've heard this one before, but...
> is there any reason why low EMC board ribbing and compression
> crowning necessarily go together, or is it an accident of history?

The panel must start out in a very dry condition. That is, it's physical
dimension across grain must be at a minimum to generate the kind of
internal compression required as it takes on moisture in "normal"
atmospheric conditions for the requisite stress interface that forms the
crown.


>
> I also gather that it's the low EMC panel assembly which causes most
> of the damage.

Indirectly. If the panel was kept at this low MC it would not be a problem.
It's bringing the MC up to some higher level that creates the compression
that becomes the problem.


>
> For each assembly EMC, the ceiling EMC (and by
> correlation ceiling ambient RH) beyond which compression damage
> occurs, is set. The lower the assembly EMC, the lower the maximum
> ambient RH it can withstand before damage occurs. The lower the
> maximum ambient RH, the less headroom the owner has in controlling
> the piano's environment

That's pretty much it.


>
> Again, stop me if you've heard this one before (or send me to the
> archives), but in a panel with significant compression ridging but
> thus far without the consequent cracking or break-down of rib-board
> glue joints, what is the effect on board resonance, you know, the
> magic tone disks? Or will the damage to resonance only show up as a
> result of board-rib separation, prematurely brought on by the
> compression damage?

Goodness, I have no idea what is happening to the little "magic tone
disks." In general, though, as compression set works its work the stiffness
of the board will be going down so the fundamental resonant frequency
should also be going down. Resonant frequency aside, this effect of
compression set is the primary cause of an increasingly percussive tone and
a shorter and shorter sustain time through the killer octave. And it's why
hammer voicing has little, if any, effect on the problem.

Del