meddlingfool asked:
"How do you measure neck resonance?"
It's actually the lowest whole-body 'bar' resonance; the 'first corpus' (C1) mode, but since the neck bends a lot it's often referred to as the 'neck' resonance. The guitar is vibrating like a xylophone bar, and there are two stationary node lines for this mode: one somewhere around the nut or first fret, and the other across the wide part of the lower bout. I usually support the guitar on foam blocks at the wide part of the bout and with a bock under the neck near the head junction, and put strips of foam under the strings to damp them. I get it high enough off the bench to slip a speaker under it, and position that under the neck block. I use the signal generator to drive it, and simply touch the top of the headstock lightly to feel the vibration. If you want to get really precise you could stick on a small piece of steel as an armature, and use an electric guitar pickup (coil and magnet) to read the motion of the top of the head.
The less 'tech' version is, to hold the guitar up by pinching the neck at the upper end between your thumb and finger. Use your finger to damp the strings, and allow
the guitar to hang as freely as you can. Tap on the back of the headstock with the ball of your finger (a 'soft' tap, as opposed the 'hard' one from your nail), and listen with your ear next to the head (don't get a string end in the ear!). You should hear a fairly clear low-pitched tone.
Usually this mode is too low in pitch to really affect the sound of the guitar. It's often down around C below the low E pitch, but sometimes it's higher. This is more common on classical guitars with wide 12-fret necks, and particularly if the neck is tapered so that it's deeper at the body end.
If the 'neck' resonance is high enough in pitch it can couple with the main air resonance. If you think of the guitar supported on the bench, as the head and tail block move 'up' the neck block is moving 'down'. The top of the box is being compressed along it's length. Since tops are normally at least a bit arched, if only due to string pull, compressing the top lengthwise causes it to puff up a bit, and suck a little air in at the sound hole. Half a cycle later the air gets pushed out. Since this is what's happening with the Helmholtz-type 'main air' resonance the two can couple; the neck pushes are, and the pressure changes in the box help push the neck. When this happens the body/neck vibration can act as a 'flywheel', storing some energy and feeding it back into the air. As I say,I've only seen this once or twice on a steel string, but it's not uncommon on classicals, particularly the better ones.
This shows up in the tap spectrum as a double peak around the 'air' frequency. The peak height is lower than it would have been without the couple, but the response is spread out over a wider band of frequencies. Instead of one strong note at the 'air' pitch you get two or three that are not quite as strong. This is, in itself, an advantage, since that really strong note is the 'guitar wolf' that sucks all the energy out of the string and converts it to sound. s it's twice as powerful for half as long. You don't notice the extra power, but you do notice the lack of sustain.
There is another, more subtle, advantage. The area under the coupled curve is greater than it would be without the coupling: the 'gain bandwidth product' is larger. I other words, there's more available horsepower in the lowest range. This often seems to show up as a particularly 'rich' or 'dark' timbre in the bass, and seems in some ways to 'put a floor' under the sound.
Note that the pitch match has to be really exact for this to work. Since the head moves a lot in this mode the weight of the head and the machines can be important if the neck stiffness is such that the match is 'close'. Adding or subtracting an ounce or so can really make a difference. I think this may be why some people really hear a difference when they swap out machines and others don't. There can be enough difference in the density and stiffness of neck wood to matter even when the dimensions are held to close tolerance.
There are, of course, higher order 'neck' modes, but they're even more variable in pitch and harder to control. Also, it's not clear how they would affect the tone unless one of them happened to have a strong antinode (active area) just where you were fretting that note, and even then.... Considering how much more flexible the top is, I'm not sure it would be productive to put much effort into those higher order 'neck' modes.
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