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Guitars DO play in, if my measurements mean anything.
Most makers will tell you that the sound changes right from the start; sometimes even just sitting around with the strings on for a few hours will make a noticable difference. But there are longer-term changes as well, and I'm sure that at least some of them have to do with playing.
I suspect that the initial changes, the ones that happen with the guitar just sitting, have to do with the structure adapting to the tension of the strings. Harp players often say that their instruments don't start to sound good until the soundboards pull up a bit: they're built flat, but don't stay that way for long with a ton or two of string tension pulling them upward. I suspect the same is true to a lesser extent of guitars; the wood and glue joints have to 'take up' a bit, and distribute the stress of the strings around more equitably, as it were.
After that you get into the vibration effects. I've tried to do some measurements of what happens to wood when it's vibrated for a long time, but it's actually a sort of tricky measurment to make in some ways, and lots of little problems can crop up. We do know, though, that instruments that have been played a while move more air; the top (at least) loosens up a bit. I have noticed that the pitch of the 'main top' resonance, the one that is like a loudspeaker moving, tends to drop about a half semitone in the first month of playing, and more slowly after that. The spectral peaks associated with that top motion, the so-called 'main air' and 'main top' peaks, also tend to become taller and broader over time, although, again, most of the change is in the first month.
Curiously, there seems to be a 'threshold effect' in this: when I've done artificial 'playing in' experiments using a speaker to drive the top, I can measure changes in the way the thing moves before I can hear any difference. The measurable changes are pretty steady from day to day, but the audible change is more sudden. Hearing can play funny tricks on you.
Old wood tends to be a bit less stiff and less dense than newer wood, owing to the progressive breakdown of hemicellulose, which acts as a sort of 'filler' in the microstructure. Thus age can have some of the same effects as playing, and old wood may need less playing in to sound played in. It's also the hemicellulose that absorbs moisture from the air, so old wood tends to be more stable than new stuff, which helps.
I've only made one guitar that sounded 'bad' to me when first strung. That was my first archtop, and I darn near threw it out in the snow. I'm glad I didn't; it was much better the next morning, and has become a much prized instrument for it's owner, who is a fine professional player. One thing to remember, though: all the instruments I've checked out have changed in the same way as they were played. The bass, in particular, gets stronger. If you start out with a strong bass, and the trebles aren't clear, it's unlikely that the sound will get 'better' with playing. More likely it will tend to become 'tubby'. Guitars that are a bit 'bright' and 'tight' to begin with often play in nicely, getting a fullar and richer sound over time. A lot depends there on how the instrument is built, of course.
Here the exigencies of production are against them. Factories have to assume that their weakest top is going to get the weakest set of bracing, and tend to overbuild accordingly. A guitar like that will have a long way to go before it loosens up enough to sound full and rich. But even the real boat anchors can get to be not too bad over time. I've seen a couple of old Ovations that have been played hard, and with the thick epoxy on the tops all cracked up, that were not too bad, considering where they started out and the handicaps they're under. Still, with a lot of the really cheap overbuilt stuff you see in WalMart it's a question whether they'll last long enough structurally to ever start to sound decent. It's the old weakest link problem, and with them it's often the glue joint between the bridge and the plywood top (if the bridge was even glued). Plywood has it's own issues anyway, and once the plys have been seperated by a lifting bridge you can't expect much.
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