Re: Preserve string energy or resonate body?
Some say that for sustain, as much energy as possible should be retained within the string. Thus, they recommend hard saddle and nut materials that block vibrations from travelling to the body of the guitar. Others say that the wood should "resonate" the string.
I think part of this theory is flawed: hard materials do not "block" vibrations, they actually transfer it better than soft materials. Now if a piece were sufficiently massive, it would require more energy to make it vibrate, but for something like a saddle I don't think that's relevant. Softer materials damp vibration, and if you were to make saddles and a nut out of soft rubber, the guitar body would hardly resonate at all. Of course, the sustain would be terrible anyway because much of the vibration would be absorbed.
I think the wood/sustain issue comes down to absorption/resonance in the tonewood, and transfer of vibration from strings to body to air. I'm not weighing in on the tonewood debate because a) that wasn't the original intent of the thread, and b) it's a recipe for conflict between opposing strongly-held opinions.
As for sustain, I differentiate between two kinds: basic inherent sustain, and "live" sustain when the guitar is played with an amp. For inherent sustain, physics dictates that the less loss, the longer the sustain. This is why some argue that heavy guitars and rigid necks deliver better sustain, and that the less loss of vibration to the hardware & body, the better. They have a point, as far as unplugged guitar in isolation goes.
But when you plug into a good amp and play it loud, things are not that simple. The guitar becomes part of a dynamic, interactive system linking the player, the amp and the soundspace. This is where liveliness comes into play. My opinion is that lighter guitars which sound louder unplugged than their siblings generally tend to perform better under these circumstances. My thinking on this is that better transfer of sound vibrations from string to body to air also works in reverse, giving better transfer of ambient sound to body to strings, resulting in a guitar that comes alive at volume. (I know there are exceptions to the lighter/louder factor- I have a few heavy axes that sing well at volume too, but
in general, IMO louder unplugged usually translates to livelier at battle volume.)
I also feel that the neck is more important than many recognize, in terms of both tone and sustain, though certainly the body is a factor too. As for personal preference, I like guitars that shiver when you tap the headstock; these tend to feel better in my hands when played and often turn out to be the ones that really come alive when flown at volume. I offer that only as my preference and my opinion, not as objective fact.
