S&S D Duplex

[Richard Brekne]

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Delwin D Fandrich wrote:

>      Natural crown is an older English (British) term
>      which seems to rougly equate your term
>      Compresson Crowning. Junghanns text discerns
>      between straight ribs on a flat panel, vs a
>      panel pressed into a dished caul. In both cases
>      we are talking about a thoroughly dried board,
>      ribs across the grain. And in both cases crown
>      is achieved by allowing the finished glued
>      assembly to take on moisture. Where he notes
>      differences are
>
>      1) The dish cauled assembly will immediatly
>      become stressed when released from the call,
>      where as the flat panel will not.
>      2) An asymetrical spherical curvature which is
>      desirable can be easily achieved with the dished
>      cauled assembly and practically impossible with
>      the flat panel.
>      3 The crown achieved with the dish cauled
>      assembly will be stronger and more stable then
>      with the flat pannel.
>      4 The dish cauled assembly is more sensitive to
>      being over stressed by too much downbearing.
>
> I would take issue with #3 and #4. Assuming the ribs start
> out flat and are not machine crowned in any way,
> ultimately both are compression-crowned and depend on the
> ability of the wood fibers to resist the force of
> compression. It doesn't matter if the force comes from
> having been pressed in a dished caul or from taking on
> moisture after having been dried to some very low
> moisture. It is still compression and the wood cell
> structure reacts the same way.
>
>

Yes, but isnt there some initial cross grain compression in
the panel and tension in the ribs fresh out of the caul
(dished caul variant) ? I mean why else would the panel
assume an initial crown before it takes on humidity if not
because the ribs are trying to restraighten themselves but
are prevented in doing so. It would seem if this is the case
that both issues # 3 and # 4 would be true then, tho it
would also seem that this would increase the likelyhood of
exceeding what compression levels the wood can handle when
downbearing is added into the picture... no ?


>      "As todays manufacturing is hardly able to use
>      such changed of humidity deliberatly, this
>      methods seems no to promise success. However,
>      the authors (Junghanns (I guess)) seems to be
>      different as he says the following.
>
>           "An equal radius of the rounding of
>           all ribs is desirable. If this is not
>           achieved then the finished assembly
>           will be under unequal stresses, a
>           condition that should be avoided at
>           all costs."
>
> And all that trouble I go through to create all those
> unequal stresses ... what shall I do? But I do understand
> the theory. It is based, I think, on the notion that the
> response characteristic of the soundboard should be
> uniform across its span, or surface. At the risk of
> starting the Great Soundboard War all over again, if you
> will consider the soundboard as a series of overlapping
> loudspeaker drivers you will see why this is not
> desirable. You need a tweeter in the treble and a woofer
> in the bass. Their response characteristics are not --
> should not be -- similar.
>
>

An interesting picture I havent picked up on before,..
tweeter in the base and woofer in the treble. For what its
worth there is a small quote from Junghann in this lecture
that goes to this speaker analogy. He says...


     << A good soundboard should have similiar
     resonance characteristics over the entire range
     from 27 to 6000 Hz. This would result in a panel
     with similar characteristics to that of a
     loudspeaker membrane. However, a loudspeaker
     membrane is subjected to an even and steady force
     from the electromagnet, whereas the soundboard is
     subjected to short impluses from the hammers
     impact with the strings >>

>      Fenner goes on to say then that Junghann further
>      states that for this same reason the bridge had
>      to be fitted to the crown of the soundboard, and
>      that this is contadictory to what is found in
>      older texts, and he (Fenner, I believe) agrees
>      with the older texts.
>
> And my bridges will still go on flat.
>  Del

Well, as I read this... that leaves you and Fenner in
aggreement on that point.


Cheers

RicB

--
Richard Brekne
RPT, N.P.T.F.
UiB, Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html


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