Has anybody done modal analysis on a braceless guitar?

[JD3971]

Hi all,

I've been doing a bit of reading on carbon fibre guitars (rainsong in particular) and I've heard a lot of terms used to describe the sound, but I couldn't find anybody that had conducted any kind of modal analysis/frequency response. I read the builder's website (below) where he describes his concept and desired outcome but he doesn't include any data or figures of the actual outcome. Since the Rainsong guitar is braceless I was also looking for frequency responses of double-topped guitars (of the wooden variety) but again I couldn't find a lot of data.

Does anybody have any modal analysis plots from a Rainsong guitar?

http://www.guitarmasterworks.com/about- ... s-101.html

Thanks

[Semmens Guitars]

I have made a few double tops with no braces. I will tap test them and post the pics for you.

[JD3971]

I have made a few double tops with no braces. I will tap test them and post the pics for you.

Much appreciated!

[GregHolmberg]

You might find this Master's thesis interesting. The author creates a new composite material for acoustic guitars using carbon-fiber filament, polyethylene tape, aluminum strips, and polyurethane foam, which he claims matches the mechanical and tonal qualities of Sitka Spruce. He has a spectrum analysis of the panel in chapter 14. I did not see a spectrum analysis of the completed guitar he built.

Design of a Composite Guitar, Max Roest, TU Delft, 2016

[JD3971]

You might find this Master's thesis interesting. The author creates a new composite material for acoustic guitars using carbon-fiber filament, polyethylene tape, aluminum strips, and polyurethane foam, which he claims matches the mechanical and tonal qualities of Sitka Spruce. He has a spectrum analysis of the panel in chapter 14. I did not see a spectrum analysis of the completed guitar he built.

Design of a Composite Guitar, Max Roest, TU Delft, 2016

That's an interesting read, thanks for the reference Greg.

[WilliamDavidReynolds]

You might find this Master's thesis interesting. The author creates a new composite material for acoustic guitars using carbon-fiber filament, polyethylene tape, aluminum strips, and polyurethane foam, which he claims matches the mechanical and tonal qualities of Sitka Spruce. He has a spectrum analysis of the panel in chapter 14. I did not see a spectrum analysis

Cool! Thanks for posting the link,

[filippo.ariani]

I have just taken a spectrogram of my almost-braceless old composite Dreadnought. Big first and second peak, quite low all the others

[filippo.ariani]

But apart from the not completely braceless design (I left the residual X-brace under the wood part) a fact is that the nomex core is thinned from 3.2 to about 2mm on about 4cm width, towards the soundboard edge. And the little variation of thickness correspond to a much greater variation of stiffness.
So even if the largest part of the soundboard is braceless, there is a "map" of variable stiffness.

[filippo.ariani]

I think (but I am not sure) that the situation is different in composite guitars produced in series. I suppose in such guitars, in many cases there will be a core of constant thickness.

[filippo.ariani]

Anyway a comparison is possible with another composite guitar recently built. I miscalculated the braceless soundboard, that resulted too weak. For thi reason I added just two c-shaped balsa-Cf reinforcements, something like a "partial-falcate".
Comparing its spectrum (pink) with the previous (yellow), we can see higher 3rd an 4th peak:

[filippo.ariani]

Here, the "half-falcate" shows more evident higher peaks
Of course, the comparison is not solid, guitars are very different in other aspects. So the comparison is in many ways apples vs oranges...
Never the less, I think that braces do give an appreciable contribution in shaping some resonances.
But we can consider (A) the important stiffness variation that can be obtained by (B) less relevant variation in thickness, and (C) the almost negligible weight of the honeycomb core that defines thickness. Putting everything togheter, I wonder if is it possible to build a very light soundboard with an appropriate "stiffness map", simulating the effects of braces without their weight

[filippo.ariani]

Consider that working very carefully with epoxy CF and honeycomb, I find possible to obtain boards weighing around:
- 0.7 kg/m2 with nomex cores,
- 1-1.1 kg/m2 with Alu5052.
These weights refer only to the (large) parts where the board is composed of only the two skins and the core. Instead, under the bridge, I always put a solid core, about 4x7cm, in order to have a larger gluing area wher the stress is higher. That weight is unfortunately placed almost at the center of the main peak.
In the first attemps I used 3mm plywood (strong but very overweight)
then I replaced it with 3mm spruce (sufficent strength, and lighter, but not enough)
Next step will be a balsa reinforcement, hoping it can hold on.