printer2 wrote:
"But dampening and Q can be frequency dependent. I see the woods as a tuned filter network. One wood may have the same dampening or resonance as another but they may have their resonance in different bands. "
According to Daniel Haines, who did a lot of measurements of wood properties some time back, most softwoods have relatively low damping at low frequencies, and the damping factor rises with frequency. In his measurements there was a 'dogleg' in the plot at around 2kHz, with the damping rising much faster above that pitch. Nobody can think of a good reason for that, and it may be simply an artifact of the measurement protocol.
As I understand it, one of the things that Q number tells you is the proportion of energy lost per cycle of vibration. If the Q is 75, then 1/75th of the energy is being dissipated per cycle.
We often check the Q value by the 'bandwidth' method, looking at the behavior of the material near a resonant frequency, but that's just because it's an easy way to do it. If you re-size the sample, and make another test (which is what Haines did) at the new frequency, you should get substantially the same Q value. The wood, of course, dissipates energy off resonance too.
Any guitar will, in any case, have lots of resonances to work with. Usually they will have somewhat different Q values, since they are affected by the structure and other things. The Q of the wood used probably simply sets an upper limit in practice: if you're using a set of Redwood or BRW that has a Q of around 150, then you're unlikely to see any resonances of the assembled instrument any higher than that. Most likely they'll be lower, and can be a lot lower. It gets hard to sort out: As you get up around 500-800 Hz the resonant bands overlap, and it's impossible to say for sure what's what. The higher the Q of the structure, the 'peakier' the spectrum will tend to be, and that has an effect on the perception of the sound, This stuff gets complicated...
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