Soundboard drydown for installation

[David Ilvedson]

"I'm out of my league with that speculation"...quoth the Brekne...

Ric, 

I know you have a lot of enthusiasm for your speculation, but I think you would have
been quietly murdered ages ago if the List was a piano factory...;-]

David Ilvedson, RPT
Pacifica, CA  94044

----- Original message ----------------------------------------
From: "Richard Brekne" <ricb at pianostemmer.no>
To: pianoguru at cox.net, "Pianotech List" <pianotech at ptg.org>
Received: 1/25/2008 12:54:06 PM
Subject: Re: Soundboard drydown for installation


>Hi Frank.

>I want to read this a bit more closely before offering any thought to 
>the points and questions you raise.  I just wanted to qualify a bit what 
>I mean with the sentence you quote below. To begin with, I don't see any 
>real analogy that perfectly fits. But one thing seemed clear to me 
>nearly immediately after hearing arguements for and against compression 
>reliant soundboards.  The panels compression alone is simply not what 
>does the job of crowning.  Not even in a purely CC board.  The rib 
>tenses... if it didn't it wouldn't bend. Its kind of like a cable that 
>is attached the whole underside of the panel and each mm of cable length 
>is in contact with the panel. But the bit I find significant from the 
>analogy perspective is that as downbearing is applied, the <<cable>> 
>strains against the panel straightening out.  The more you exert down 
>bearing the more the panel compresses and strains the rib... perhaps 
>(probably to some small degree) to the point of exerting some absolute 
>tensioning i.e. lowering centroid line very slightly.

>As relates to your point about the high point of the <<arch>> / thickest 
>part of the rib... I agree... the end to end cable analogy doesn't work 
>there.  But the way the ribs strain against the compression strength of 
>the panel is similar none the less... or so it strikes me.  Perhaps a 
>cable that is thicker directly under the bridge would do the same thing 
>?  I'm out of my league with that speculation... but I thought I'd throw 
>it out there anyways.

>Cheers
>RicB


>pianoguru at cox.net wrote:
>> ---- Richard Brekne <ricb at pianostemmer.no> wrote: 
>>   
>>> I still think the whole thing functions more like a cable supported arch 
>>> then anything else.  
>>>     
>>
>> This brings to mind disagreements that I have had with other piano engineers.  
>Given a rib with a cut radius of 60’, e. g., they would argue that a 60’ radius 
>of an arc segment at one position on a full circle is not the same, when shifted to 
>another position on that circle.  I would argue that a 60’ radius is still a 60’ 
>radius no matter where it is positioned on that circle.  I came to realize that what 
>they were really trying to say is that the thickest part of the rib must always be 
>directly under the long bridge.  I have two problems with this:
>>
>> First, with multiple ribs on a soundboard, each with a predetermined radius, and 
>its high point under the long bridge, as the above would argue, the points of 
>intersection of the end points of each rib with the rim would be quite irregular and if 
>taken “literally” would exaggerate the potato chip effect, think Ruffles 
>(Ruffles has ridges).  If not taken so literally, and the inner rim is surfaced to a more 
>reasonable shape for supporting the crown, what would have otherwise been ripples 
>still constituted unnecessary irregular internal stresses in the soundboard assembly.  
>>
>> Secondly, given your “cable supported arch” analogy, you would normally 
>think of a cable supporting the arch as a cable extended from an end point of the 
>arch to the other end point of the arch, which would place the high point or the arch 
>at its center.  To shift the high point, (or thickest part of the rib, to a position under 
>the long bridge), would be analogous to moving the cable support of the arch from 
>one end point to something short of that, to shift the high point off-center of the 
>arch.  Would not an arch be better supported by a cable between its end points, 
>than from one end point, and something short of the other end point, regardless of 
>where along the arch a force is applied, which would collapse the arch if it were 
>unsupported?  Would this analogy hold true for rib?  Or, does any of this really even 
>matter?
>>
>> Frank Emerson
>>
>>