soundboard shims & glue

[Rob Kiddell]

>The standard wisdom for glues whan working with soundboards seems to be
>"the harder the better".  When we glue maple to maple on bridges, or maple
>to spruce, the harder glue will more efficiently tranfer the energy to the
>soundboard where we want it.

That concept would apply to maple bridges themselves, the hardness of the
wood leads to *fast* vibration transfer, getting the motion of the string to
the soundboard with minimal impedance. AT least that is the theory of the
Baldwin vertically laminated bridge idea.



>This brings us to the point, and the question for discussion:  Is soft glue
>in soundboard work such a bad thing?  Our conlusion at this point is that
>it is in fact a GOOD thing.

Depending on the condition of the soundboard, either may suffice. If, for
example, you know the soundboard may be exposed to varying humidity
conditions, (see John Musselwhite's posts on humidity changes in Canada...)
a hard epoxy will remain inert while the soundboard a) expands or b)
contracts, result: a) separations between the epoxy and the spruce, or b)
compression on the fill joint.


>2.  Wouldn't a soft glue be more likely shift and flex with the board with
>changes in humidity, thus being less likely to crack over time?

This seems to work, as the boards that I have repaired with a flexible epoxy
(G2 Epoxy, made by Industrial Formulaters of Canada, used for laminating
skis, tennis rackets, and other high tension, flexible wood joints) have
held up nicely through considerable humidity changes. (10 % R.H. in winter
to 50 % in summer)

>3. Isn't a hard glue joint a high impedence structure, more likely to
>reflect sound back, than to pass it on to the rest of the board

Sound waves travel along the grain of the spruce, the amount of grain lost
to cracks (even huge ones) can be calculated to less than 5 % of the boards
total surface area. *in most cases*
>
>4. doesn't most of the sound get from the bridges to the outside of the
>board through the ribs anyway?

A secondary function of ribs, the primary being keeping soundboard crown
intact.
Rob Kiddell
C.A.P.T.
P.T.G. Associate
Edmonton, AB, Canada