stringjunky2 asked:
"Are they selecting timber pieces that fits the expected tonal signature of their species though and rejecting the outliers in a form of confirmation bias?"
Again, I've tried my best a few times to make 'matched' guitars using wood that was 'sister cut' for all the parts. Everything was controlled as closely as I could. For the most recent pair, which were the most closely matched, we did 'blind' listening tests. Virtually everybody who heard them could tell that they were different.
It is certainly true that on average there are differences between the different species of spruce. These diminish a lot when you control for density. If you build tops to a certain target stiffness it works out that a denser top will tend to be a bit heavier, and that will generally translate into less 'responsive' guitar with more 'headroom'.
As far as I've been able to tell, if you start with two tops of different species, say Euro and Red spruce, that have the same density, and Young's modulus along and across the grain, the two instruments will probably be no more different than two made from 'the same' wood. This would be a very difficult experiment to do properly. You'd need to do it several times to be really sure, and even once is a lot more work than just making a couple of guitars. Keep in mind that 'same' or 'different' is an easy thing to judge, but assessing degrees of difference gets to be a can of worms.
Keep in mind too that luthiers are just as much subject to belief in the myths as anybody. Everybody hears what they expect to hear. I always try to keep in mind what I call 'Feynman's Dictum', as stated in his essay 'Cargo Cult Science', that "You are the easiest person for you to fool". This goes for luthiers too.
I'm not saying that there absolutely is no difference between the different species of spruce, once you control for such things as I mentioned above. What I am saying is that I have yet to be able to come up with any decent evidence as to either the existence or nature of that difference. Every suggestion I've gotten for some measurable difference of a sort that ought to affect the tone has not panned out when I've looked for it. Maybe I'm not measuring enough samples, or doing it carefully enough. Maybe there is a difference but we haven't yet figured out what it is or how to measure it. I'm not a big fan of leprechauns or fairy dust: if there is a systematic difference it has to be measurable somehow. Under the circumstances the only reasonable stance IMO is that there is no systematic difference between different species of spruce. I'll be happy to admit I'm wrong when somebody comes up with proof I can accept.
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