Fiberglass?

ukantor said:

I read somewhere (long time ago) that an acclaimed luthier once built a traditional Spanish style guitar body - back and sides - from papier mache,
John Colter

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That was Torres and his paper mache guitar. Totally revolutionized guitar design. See video here of going inside a 180 year old guitar. Like spelunking. It has not fared well...

https://johnguitar.com/a-look-inside-torres-paper-mache-guitar/

UkuleleHill said:

How so ?

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The resonance should be a function of the mass and the stiffness.
Carbon fiber is not as dense as epoxy, and epoxy isn't as stiff as carbon fiber.
So by using less epoxy, you make it stiffer per unit weight. Generally, high-performance parts (say, for airplanes) try to use as little epoxy as they can to get maximum stiffness for a given weight.

TjW said:

The resonance should be a function of the mass and the stiffness.
Carbon fiber is not as dense as epoxy, and epoxy isn't as stiff as carbon fiber.
So by using less epoxy, you make it stiffer per unit weight. Generally, high-performance parts (say, for airplanes) try to use as little epoxy as they can to get maximum stiffness for a given weight.

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It's not quite this simple. Two pieces of different wood having the same mass and stiffness can have dramatically different strength of resonance due to different internal damping, and can resonate at different frequencies as well.

The limiting factor for strength of resonance in most carbon fiber or fiberglass instrument parts is the damping properties of the resin (epoxy) used to set the material rather than the material itself. So, keeping the damping component (the resin) to a minimum should generally help strengthen the resonance.

Resonant frequency is a function of sound velocity through the material (which is dependent on the density rather than overall mass) and the geometry of the material, which becomes less fun to calculate when you're encasing one material like carbon fiber in another material like resin. Again, you benefit from minimizing having an extra resin layer outside the bulk of the carbon fiber or fiberglass, which is why most carbon fiber instruments look like they have only the minimum amount of resin needed to bond and shape the carbon fiber sheets.

Klos makes a synthetic tenor uke. If I had the money I would already own one. (I think they start at $600-something with a wooden neck). My RainSong guitar astonishes everyone who plays it, and all my friends are high-end guitar junkies. The Klos company is in Utah.

I once made a stripper canoe out of cedar and fiberglass. It was beautiful when completed but the fiberglass work was a stinky mess. I would not bring that stuff into my guitar shop.

How much do you expect the back to resonate anyway? Generally if you place the back against you the back is dampened. And speaking of dampening, it does not help determine the resonant frequency of a panel, that is solely due to the mass and the stiffness of the panel and dampening is not part of the equation. I bet I could find a more relevant page with more time but this one should suffice.

http://www.sekon.cc/acoustics/SoundTransmission/index.htm

Given the stiffness of the panel, how much mass do you need for your target frequency? You don't know unless you make a few prototypes. So I can not see much point in worrying about it. The amount of mass you put into the sides help determine the resonate frequency, how you build them up to attach the top can vary. I am assuming the top will not be part of the wet layup. Anyway, a learning curve if you plan on going the fiberglass way. Is there a reason to go fiberglass rather than wood? My thought is if you are going to go away from wood you might as well make the whole thing composite. But that is a greater level of complexity.