Official Luthiers Forum!

Thanks Trevor. Just ordered your book and looking for some Aussie sources.

Here in Australia it is hard to go past (Tasmanian) Blackwood (which grows in lots of the southern parts of the country, not just Tasmania). There is a lot of acceptance of it as a tonewood by buyers - at least in this part of the world.

Just picked up ten sets of Pau Rosa and it looks like it's going to be good. Few plain sets and
some with nice figure. Looks like some of the lighter colored Amazon Rosewood I've seen.
IIRC Tim McKnight likes it. Anyone else work with it ?

Brent

+1.

Stable, bends easily, planes well (even the highly figured stuff) suitable for both live and non-live backs, can look very flash or quite plain but is always capable of producing a great guitar.

Jon, check for a PM

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Trevor, may I ask what makes a wood suitable or not suitable for a live or non-live back? Apologizes if that is covered in the books. I do have them and have read them so if it's in there I must have missed it. Thanks.

A back is "live" when it is capable of producing a visible T(1,1)3 mode. Movement of the back in this mode is driven only by air pressure variations inside the box, so the back has to be light enough and flexible enough to respond to those air pressure variations in the right frequency range. What this means is that it becomes more difficult to build live back guitars with the denser woods, as they are too heavy to move. (Thinning them out to take out the mass reduces the stiffness (and strength) too much). So that effectively puts a cap on the density of woods that can be easily used for live backs, without using a pretty exotic (e.g. CF lattice) back bracing structure to support them. The cap is ~800kg/m3. Design Section 2.4.1.2 discusses this more in terms of the back's monopole mobility, with the lower limit for a live back being ~ 7.0x10-3 s/kg. This paper (one I wrote for an Acoustical Society of America conference in 2011) p. 14 can be downloaded from my website and gives some more information.

Getting back to Aus blackwood, it comes in a very wide range of colour and figure, and density. Densities I have measured range from 540kg/m3 to 950kg/m3 with "typical" being ~700kg/m3. So it is well into the live back density region but can be over-braced easily (or left over-thick) to produce a non-live back.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Interesting.

The discussion of the density/stiffness for a live back brings up an interesting thought.

Look at the old French and Viennese school guitars which had heavily encrusted backs... Of course - we often say "Oh..... How could these have ever sounded any good.... All that weight would make them dead....." And Torres was known for similar opinions.....

And that's probably true assuming a construction out of solid high density wood plates such as Rosewood or Ebony... Adding inlay weight on top of the already dense wood probably would do just that.....

But... That's not how they were built... Look at the construction.... If you veneered rosewood over spruce and then add some weight from inlay stuff... You probably end up right in that 700-800 kg/m3 density range for a live back....

I was listening to recordings of some of these French and Viennese school instruments housed in museums just the other night... And instead of having a dead and boxy sound characteristic of a massively overbuilt structure (like so many heavily inlaid cheap Asian instruments) - they sound like a real guitar with a full, rich voice. This surprised me - as I often heard that "Oh, look at that thing all larded up with encrustations. It has got to sound dead...."

But the reason this isn't on our radar is probably that these just don't exist "out in the wild". These instruments were the absolute most expensive bespoken Instrumets - so there were very few made.... They were owned by nobility.... And so they weren't in the hands of the hoi-polloi to go play out live.... And the ornentation was fragile as well... And they generally ended up donated to museums rather than being kept in circulation....

Thanks

Thank you Trevor!