Much of the luthier's time and thought is in pursuit of that "sound in his head."
Approaches to capture this magical sound are one of the driving factors in the advancement of the craft. How does one achieve this sound? The pendulum of opinion swings from the artistic/intuitive side to the purely empirical.
My own pursuit is leading me down a road of discovery that I expect will continue for as long as I live. It is what compels me to the shop each day eager to take chisel to wood.
We've all seen those romantic photos of the luthier at the bench tapping the guitar top listening for that perfect pitch, ring, vibration or whatever it is that he's not telling us. He will tap, tap, tap, carve the braces a little and then tap again until finally he will say "that's it" and pronounce the top "voiced."
My approach is to initially over-brace the top. This will produce a high pitch or tight tap tone. I carve the bracing to loosen the top and listen for a drop in pitch. This is the artistic / intuitive part.
The question then becomes, "when do you stop?" To help with this, I turn to the empirical.
I use a speaker and sine waver generator to drive the top to produce Chladni patterns. Glitter sprinkled on the top will settle in the nodal points of waves when the top vibrates at a harmonic frequency. The shape of the pattern tells me the mode of vibration. The goal is to produce a specific modal pattern at a target frequency.
This is a moving target with respect to the guitar's size, scale length, string gauge, wood species and intended use.
Examples pictured below:
So here I stand with my feet firmly planted in mid air. Art mixed with Science and perhaps a little Snake Oil for good measure.
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