Official Luthiers Forum!

Can someone shed some light on tap tuning the back? Is it similar to what is going on with the top tap tuning braces?

_________________
Mark Ewing
Columbus Ohio
"Trees are an important and precious thing. We should build good things with them. Building good guitars with heart are the best use for them." K. Yairi.

I took a class from Kent Everett and he covered voicing the back briefly. It is nothing like the top since he usually thicknesses to a standard thickness depending on the wood species. But he said he was always trying to get 3 distinctive tones when tapped at the top of the back, the middle and bottom. He would remove more bracing material until he achieved these tones. Nothing specific, just 3 nice different tones when tapped. I've tried it before, but not sure it really makes that much of a difference. Anyway, hope that helps.

_________________
Tracy
http://www.luthiersuppliers.com

I've been looking at Chladni patterns on 'free' backs for years, and have decided that it's a can of worms. If you use a given bracing pattern on a certain box size for awhile you can figure out more or less what pitches the back modes 'should' be to get them to work with the top, but it's almost impossible to get them exactly right. I do think that getting a lot of clearly formed modes on the back helps, but that's hard to 'prove' too.

Ultimately, what counts is how the thing works once you get it together. I like to have the 'main back' resonant mode end up about a semitone above the 'main top' mode, after things have had a chance to settle in. Often that means the tap tones of the top and back should be the same when the box is fiorst closed up: adding the bridge, putting on the binding, and even playing, change the pitches to their final values. Often you will get two or three nice clear tap tones on a good guitar back and I suspect that helps add to the complexity and 'color' of the sound.

Thanks. This is nice and simple. I'm thinking I can make that happen...hopefully

_________________
Mark Ewing
Columbus Ohio
"Trees are an important and precious thing. We should build good things with them. Building good guitars with heart are the best use for them." K. Yairi.

I recently made a video of an FEA model of a back showing how it moves at lower frequencies which you may be interested in seeing.

The modes of the model correlate very well with some actual glitter patterns that Al Carruth was nice enough to share.

This is the free plate video.
http://www.youtube.com/watch?v=TBBgIpUOHO4

John

I watched the video. I'm not sure what I am looking at. Can you give some detail/clarity?

_________________
Mark Ewing
Columbus Ohio
"Trees are an important and precious thing. We should build good things with them. Building good guitars with heart are the best use for them." K. Yairi.

Sure. The thread was talking about tapping the back and hearing distinct "tones". When you tap the back you excite the natural frequencies. This works especially well if you hold the back at a node (a place where it doesn't move for certain resonant frequencies) and tap at an anti-node (a place where it moves a lot for certain resonant frequencies.)

If you tap with the fleshy part of a finger you tend to excite the lower modes. If you used something harder, like a knuckle, you get a bit more highs. You can just listen to the response of the plate to your tap, or you can use chladni pattern techniques (glitter and an audible sin wave) to show you where the nodes are for the lower frequencies. Once you see the nodes with chladni patterns you can imagine how the plate is actually vibrating. And knowing where the nodes are helps you figure out where to hold and where to tap. It might help you decided where to shave a brace.

The video I pointed out to you shows the simulated chladni pattern modes for a free back plate with fairly "normal" bracing. And the patterns are very similiar to what Al Carruth gets on his backs measured with his glitter technique. The video goes a step further and shows you how the plate moves for each mode, exaggerated so you can see it. You don't have to imagine. You might say, the video provides a visual picture of some of what you're hearing when you tap.

The video that I posted, like Chladni patterns, show the steady state response, so the damping is missing. When you tap, you hear the damping too. There are also ways to visulize the damping, both in a simulation environment like I'm showing or for an actual tap.

Here's an photo that compares the glitter pattern to my more detailed mode map for a top. It will show you what a node looks like on my mode plots. I drew a black line on the node on mode plot.



Of course you may end up with different mode patterns on your back but they should be in the ballpark of what I'm showing. The hope is, that this gives a better understanding of what's going on.

John