Originally Posted by Alan Carruth
(Post 3559688)
One of the problems with this is that it's so easy, and so false, to draw a parallel with solid body electric guitars: everybody knows how important the neck is on those, so it must be similarly so on acoustics.
In theory, the string works best (produces the purest signal) when both ends are 'fixed' so that they don't move at all. Of course, on an acoustic, you'd get no sound if the bridge and top couldn't move, but at least you'd like the neck to be 'rigid', and massive enough to keep the nut from jumping around too much. For the most part, any neck that's stable enough to be usable will do this. The top moves so much more than the neck or the nut that any effects on the tone of neck movement will be secondary at best. The issue comes in with that little phrase 'for the most part'. What happens is that there are resonant modes of the entire instrument that can alter the tone a bit, at specific frequencies, in some cases, and these are tied in with neck stiffness and mass (particularly the headstock mass).
The most important of these 'whole body' modes on an acoustic is the lowest pitched one, and then only sometimes. Basically, the whole guitar can vibrate like a xylophone bar, with the head and tailblock going one way while the upper bout goes the other. There are two stationary 'nodes' for this mode of vibration; one at about the nut or first fret, and the other across the lower bout near the line of the bridge. If you hold the guitar up at the nut and tap on the back of the head you can usually hear this.
Generally, it's quite low in pitch: often around C below the low E note, so it doesn't make any particular difference. However, if the neck is particularly stiff, and the head is light, this 'neck mode' can be pitched high enough to interact with the 'main air' ('rum jug') resonance, and this will affect the tone. It's most common to get this match on Classical guitars, and uncommon on steel strings, since they tend to have longer necks and often use heavier machines. The frequency match has to be exact for there to be any noticeable difference in the tone, and sometimes things like swapping machines, or even replacing metal buttons with wood ones, will be enough to bring things into line. That's why some folks are convinced that changing machines makes a big difference, and others are not: it all depends on the details of their particular case.
There are several other such 'whole body' resonant modes on acoustics, but they tend to be weaker, quite variable in pitch, and high enough up so that they don't seem to matter too much. This is the opposite of what happens on solid body guitars, where the lower three or four such modes are low enough, and active enough, that they can really alter the sound; usually by stealing energy from the strings. The worst case is solid body basses, where the bridge is perched 'way out at the end of the body, where the motion and energy loss are greatest. That's why they tend to have so many 'dead' notes. On an acoustic there's usually so much else going on that it would be pretty hard to pin any specific dead note or wolf note onto a 'neck' vibration.
The bottom line, then, is that the neck does influence tone, but less than a lot of other things. At any rate, what really counts on an acoustic is not so much any particular detail, but, rather, how it all works together.
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