Double backs

[ken cierp]

When we were selling mountain dulcimers -- the light went on regarding the increase volume and improved tone of an instrument that was place on a stand vs the same one played on the performer's lap -- the improvement was not subtle -- significant, quantum is more like it. So what I have on the drawing board is a true double back design with the inner reflective back totally insulated from the outer back. I am thinking that the inner back could be made with different woods different thicknesses, different braces etc. I know it adds to the confusion (I think of it as opportunities) --- one thing for sure the volume of the instrument will louder then the traditional guitar.

[Dave Bagwill]

Will we be x-bracing the backs of the dbl-back instruments? Tapping them? Oh boy - I see a lot of 'opportunities' ahead. I love this stuff.

[ken cierp]

Yep the possibilities are "limitless" (good movie BTW) -- again you can use the speaker comparison, there are passive radiators sold (speaker cones with no magnetic cores )-- said to add volume and improve phase shift etc. I see the insulated back doing some of the same things. Some of the Selmer Jazz guitars had some kind of reflector. Banjos with and without -- right?

[Ken Hundley]

You might consider making the inner back out of spruce as well, more vibration and less weight. I guess a concern would be the depth of the sound. It's one thing to have it play louder, but the deeper bodies definitely seem to have more bass too them, which shallower guitars seem to lack. Would a double back basically reduce the interior volume of the guitar, or would you make the guitar thicker, or compromise? Would that possibility be mitigated by putting ports in the inner back? That way, you get a vibrating, potentially undampened surface, yet still can take advantage of the volume (meaning cubic inches) of the overall box? Just a thought. I might consider putting at least two, fairly large ports in the lower bout area, out of sight, and out of line of direct vibration with the top and main sound hole. That, or maybe four small ports, 2 arranged somehow to either side of the endblock....just thinking out loud.

[ken cierp]

My starting point is simply to eleminate the damping effect of the back against the players torso. So unit "one" will be an OM slightly deeper to accommodate the inner back without reducing volume. Essentially one variable -- to me that's the only way to get valid useful comparisons.

[Dave Bagwill]

Most guys stick that upper bout rim right into their armpit - would a double rim at that point relieve that weight on the soundboard? Is there dampening enough at that point to warrant a double rim?

[TonyinNYC]

This could get complicated. I can't wait to watch!