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Hello,

I'm trying to tap tune my guitar top to see if I can discern when to stop shaving braces--when the top has developed a good resonance. I know that people tend to pinch the top in one hand at a node where the top does not vibrate, then they tap the top with the other hand to test its resonance.

I am trying to find a good spot to "hold a node". I know that between the finger braces and two inches from the edge is an approximate point, but I'm not having luck with this spot. However, I was able to get a nice "thrumm" when pinching the transverse brace with my left hand and tapping the top at the bridge location with my right hand. Any reason pinching the transverse brace would not be a good technique?

Thanks,

Chuck

The transverse brace is good spot that will allow you to hear several of the resonances including the "ring and a half" mode. Near the finger braces usually works too. Move your pinch point around that area until the sound gets better. If you are using a Martin style brace with slanting tone bars, you will have better luck holding the top near the finger braces on the treble side. The slanting tone bars tends to run the node lines off the edge on the bass side making the area near the fingerbraces an "active area and, therefore, a bad place to hold the top when tapping.

Another thing I like to do is hang the top by the soundhole on my thumb. You can them tap it all over the place and activate different resonances, especially the lower ones.

Also, reaching through the sound hole and pinching the top at the X brace intersection is good. Hold the top close to your ear, top firmly at the bridge location and listen for a clear sustaining "Ping".

Maek

Yup, soundhole and thumb.

I also like to hold right at the tail, in the center. It gives a different tap, but you can really hear the bass response there. I'm building classicals so it's probably a bit different, but I hold at TB2 for overall sustain and as Mark said, hearing more of the range. I hold in the wing of the lower bout, to hear the higher mode, and at the tail to hear the low end response. In every case, I look for even response all over the lower bout.

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Waddy

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Sound Clips of most of my guitars

+1 more for the thumb in the soundhole... Some tops just don't seem to have a spot you can pinch them without damping.

Thumb in the sound hole also.

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

The thing you need to understand is that the node is where it is: not where you want it to be. The node is the stationary place in the vibration pattern, and where it occurs depends on the mass and stiffness distribution of the plate. Nodes often are found along braces, simply because the stiffness and mass there make them hard to move in the first place. However, antinodes, active areas, are also often found at brace locations as well; if that mass does get moving, it pulls a lot of the top along with it.

Keep what Mark said in mind:
"Move your pinch point around that area until the sound gets better."

That's how you find the real nodes.

I do the soundhole technique. This brings up a confession and a question. I've built 3 guitars from scratch, and been involved in the making of another 10 (#11 is a cedar and koa deep bodied OM) and I confess I still don't know precisely what I'm listening for. I've seen dvds and read books about hooking up a sensor to a tuner and tapping with a hammer to get a specific note.

What I am left with is, I carve the braces until they 'look cool' and then I start tapping and removing material a lot more conservatively until the top 'sounds cool.' Certainly not scientific, but it works.

We also go for a certain drum like quality when we thump the bodies after they are bound but prior to finishing.

Another observation is that the deeper the thump (it varies from 1 guitar to the next by as much as a whole step) the brighter the guitar sounds when strung up. My first guitar was a red spruce/mahogany D-18 type guitar (in a batch of 4--same wood combo and body shape) that had the deepest thump of all (by about a 1/2 step). It is the brightest and punchiest of that batch. Not at all what I expected based on the thump.

I have no idea what all this means, or even whether this info is relevant or helpful, but I thought I would provide the observations.


Does anyone else have any thoughts on how tap tone relates to the tone of the finished instrument? (After steady playing for a few weeks to open the instrument up)

What about how different soundboard sound when tapped after bracing and during carving? With the cedar I was flying blind and just going for a sound I thought was cool. To my ears, cedar sounds a little 'warmer' when tapped as opposed to red spruce which has almost a metallic quality when tapped.

Also, I assume thickness diffences in the soundboard would have an impact. Most factories seem to leave their tops at about .125 thick, where most custom builders seem to hover in the .100-.105 range.

These are the thoughts that occupy my mind.