Acoustic Guitar Construction

Hi guys,

Now I have this beautiful Quilted Sapele back for a Lowden style S model.
The trouble is its weight and flexibilty.
The weight is 717 kg/m3, which is on the heavy side and the Elastic Modulus is 8,3 GPa, what is on the very floppy side. The target thickness should be around 3.3mm! So that is not ideal for a tonewood, but it is that nice, so I am going to use it.

Question is, how to approach? Make it stiffer with sufficient bracing or make it an active back and see where that goes.

Ideas boys? John?
Thanks
Herman

In the same situation I would stiffly brace the back and make it a reflective back. Both active and reflective are valid design points with trade offs. I have done this on dense wood like cocobolo where the back would have too much mass to be an effective active back. With your Sapele you have a heavy floppy wood; 3.3 mm will be heavy. If the back is pitched in the range where it will couple to the top, it will be an anchor, as string energy is trying to move the back.

I would thickness that back to 2.5 mm (reducing mass) and brace with 4 relatively tall ladder braces. The guitar will not feel heavy and you can have a great reflective back and a guitar with good volume and projection.

Those are my thought

Thanks John, that sounds like a plan. I already took it down to 2,5mm. And I am planning to place 4 braces instead of my usual 3. Or maybe something more diagonal, X or fan like, so it will stiffen along and across the grain at the same time.

I've built a few with the lower bout x-braced. My notion was to preserve a dome in the back and not revert to a cylindrical shape. Seems to work for me, and maybe the additional stiffness would help your 'supple' back without leaving it too thick, as John suggested.

Don't know the outcome, but this is what I did. As said I took the back down to 2,5mm (0.1"), made 4 instead of 3 ladder braces, the 2 lower ones a bit meatier than usual and a few diagonal braces in the middle of the lower bout. The back feels now very stiff against the grain and medium stiff along the grain.
When it comes alive I will let you know how it turned out. Maybe nice lesson for me in dealing with wood capacities.(Still learning )

Well, why not? Enjoy!

It looks good Herman
I've always wondered if the back bracing is more about being part of a stable box and less about sound waves and such. The top vibrates....but does the back really vibrate or do the waves "bounce off" so to speak?