Official Luthiers Forum!

Hi guys and gals-

What causes certain species of spruce to have a strong overtone content and/or a strong fundamental content? What physical characteristics occur that cause this? I can't imagine stiffness plays a part (or can it). I know this is a generalization, but sitka is strong on fundamental tone and low on overtones, engelmann has high overtones and low to high fundamental tone, euro spruce has both with a wide color palette and red spruce has both with lots of headroom/clarity when played hard.

What causes all this to occur? I appreciate any and all answers to these questions. Thanks-

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John Lewis
Wannabe builder owned by 2 crazy dachshunds

Tricky question....

PROBABLY, but I'm not going to swear to this, the single most germane property of the wood for this will be the 'damping factor': how much energy gets dissipated in heat as the thing vibrates. Usually high damping tends to kill high frequency vibrations faster than lows. This is because the measure of damping is how many vibration cycles it takes for the signal to die away by a certain amount. High frequencies= lots of vibrations per second= not too long to 'eat' the energy.

But it's not as simple as saying 'high damping=more fundamental tone'. Most people consider Western Red Cedar to have a strong fundamental tone, or a 'warm' sound, but it tends to have lower damping than most spruces. The low damping should equate with _more_ high harmonics in the tone, but it's likely that your ears assign all of that high-end energy to the fundamental, and so you perceive a 'warm' sound.

In the few tops I've tested, Englemann tends to have higher damping than Red, Sitka, or Euro. I've read that Sitka has higher damping at low frequencies than most other spruces, and the damping falls as you go up in frequency to about 2kHz, where it starts to rise, as they all do. This could effect the balance of tone, if it's true.

Density can also play a part; generally speaking, a low density wood will end up making a lighter weight top if the static stiffness has been kept the same. A lighter top tends to favor high frequencies, and can work against 'headroom', depending on how you build. Englemann has lower density than most spruces, on average, followed by Euro, Sitka and Red, in ascending order. Thus the reduced density of Englemann spruce can help make up for the higher damping.

'Headroom' probably has to do with how difficult it is to drive the top into a non-linear response, and cussed if I know what property you'd correlate that with!

Don't overlook design/build aspects, either. Things like brace shaping are obvious, but so, too are the thickness of the top, and whether it is tapered. Thinning in the 'wings', outboard of the ends of the bridge, seems to bring up treble, while thinning between the tailblock and the bridge seems to bring up the bass 'fullness'.    

So as usual, I think, it comes down to a balance of things within a particular design, and the way the wood favors that design, or not. Some of us just seem to get better results with one wood than another, and that's probably not a fluke.