Official Luthiers Forum!

Have seen folks speak of nodal points on tops approximately 2" to 3" from the rim of the guitar. I wonder if any one has a explanation of why this happens. Also if this is so why do we leave the high peaks of scalloped braces at these points.What about parabolic and tapered bracing and the dynamics of the nodes with these types of bracing. Thanks for your comments. Tom.

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A person who has never made a mistake has never made anything!!!

Technically, nodal points are stationary spots on an object that is in one of it's 'normal modes' of vibration. The handiest examples of this are seen in the strings of your guitar. When you pluck the string it is vibrating everywhere except for the bridge saddle and nut, which are 'nodes' for all of the vibration modes of the string. If you touch the string in the exact center, over the 12th fret, you supress a whole family of the modes; the 'fundamental', and all the ones that are at odd-number multiples of the fundamental frequency. This happens because that center point is an 'antinode' for those modes: it has to be moving at those pitches, but it is a 'node' for all of the even-order partials of the string. That's why the string sounds an octave higher. If you touch the string in a different place, you supress a different set of modes, and get a different sound.

A string has nodal _points_ because it's essentially a one-dimensional object: in terms of vibration all you need to think about most of the time is the length, with the thickness being very small in relation to that. If there are vibration modes _across_ the string, they're too high up in pitch to worry us!

A two dimensional object, like a drum head, has more complicated modes, which involve moving areas seperated by lines. One outcome of the extra complication is that the pitches of the different modes are not as simply related as those of the string. That's why tapping on a drum usually makes a 'noise' rather than a more musical 'tone'. Guitar tops are even more complicated, due to the nature of wood, and the addition of bracing, so the modal maps tend to be more complicated still, and more difficult to predict.

One thing that happens on these complicated objects, though, is that node lines tend to run either along or between areas of high mass or high stiffness. Those peaks on the braces tend to impose nodes, or 'antinodes' (areas of maximum amplitude) where they occur, in part because of their mass, but more because of their stiffness. It's like your finger touching the string at the 12th fret, but a little more complicated because the brace is also part of the top that's vibrating.

So, I would not exactly call those 'node points', but they are 'points that are likely to be nodes' for a number of different modes of vibration of the top. How all of this effects the sound is the $64,000 question, and varies from one guitar to the next (you didn't really expect this to be simple, did you?).

What can be said in general is that different brace profiling schemes tend to enhance or supress different modes of vibration of the top, and alter the pitches of them. These modes are the major 'formants' of the tone, that give each instrument it's character. Guitars with 'scalloped' bracing tend to sound one way, and 'tapered' braced guitars are different. It all gets complicated fast, because everybody does things a little differently, and we're working with wood, that varies all over the place in it's mechanical and acoustic properties.

So there you are, ask a simple question......

Thanks Alan.The point I was trying to get to was,are the nodes there before bracing and if so I must put my peaks there or are the nodes there because I put the high point of the brace there. Think you have answered the question for me. Before asking the question I was left with the impression that there was this node point about 2 to 3 inches from the rim and your peaks must be made there.Thought it was something to do with the sound wave coming from the bridge and reflecting off the rims and then returning to meet another wave at the node point. I gather from your answer they are there because we PUT them there with the stiffer bracing at the peaks. Thanks again and come back again if I've got it wrong.
Tom.

_________________
A person who has never made a mistake has never made anything!!!

Bracing has only a very limited effect on mode shapes, and usually if you do move the nodes around a lot from where they 'want' to be, it's a mistake. So the nodes (or antinodes) will tend to be in those areas with or without braces, of whatever shape.

What the braces can do is alter the pitches of the modes. If you raise one mode's pitch, and lower that of another, you can seriously alter the relationship between them, and that could be more important than the actual pitches of the modes. There's a lot still to sort out in this.