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Well this has proved to be a worthwhile exercise in many ways . I used a software tone generator that Alex M provided a link for, and just sprinkled some salt on the top first and then the back of the guitar. I incremented the freq 1 hz at a time starting from about 180 hz and noted the freq where the salt moved to form a smiley on the lower bout. The top came in at 212 hz and the back slightly lower at 208 hz. I figure to get the top where it needs to be I need to drop it a full semi tone and leave a little for future playing in. It is an Adi top which I have heard take a little longer to settle in.
I was planning on either scalloping or tapering the lower legs of the X brace to loosen it up.
Any ideas on how the tone will be shaped and approx how much wood will need to be removed to drop it a semitone would be appreciated.

Regards

Craig.

Can you post pics of some of the patterns? A 'smiley in the lower bout' doesn't sound like what I look for.....

Al I want to be totally honest, I don't really know what I'm doing or looking for with regards to Chladni patterns. I was just going on what Mark mentioned in my previous post "determining pitch of top and back plates. What I ended up with was a congregation of salt along the bottom of the lower bout ,more a half smiley . When I was at the particular freq I mentioned above, the top seemed to be moving the most and the salt slowly migrated its way towards the botttom of the lower bout. I will try again over the weekend and take some pics of the result. I really would like to get this right so that I know where I am with regards to the main top and main back pitches. Thanks again for your help.

Regards

Craig

Al I want to be totally honest, I don't really know what I'm doing or looking for with regards to Chladni patterns. I was just going on what Mark mentioned in my previous post "determining pitch of top and back plates. What I ended up with was a congregation of salt along the bottom of the lower bout ,more a half smiley . When I was at the particular freq I mentioned above, the top seemed to be moving the most and the salt slowly migrated its way towards the botttom of the lower bout. I will try again over the weekend and take some pics of the result. I really would like to get this right so that I know where I am with regards to the main top and main back pitches. Thanks again for your help.

Regards

Craig

Al here is a pic of the top at 212 HZ

For me the tactile way works best. Just lightly touch the bridge or a bit behind it with your nails while sweeping. The resonance peaks are easy to be picked up. If you are unsure, pump up the volume. At the main mode the top goes rather wild and it rattles making a bee/fly like buzz against your nails. First set up the generator to half notes mode. See which note is most active, and only then play around in 1 Hz increments around it.
The important notes to check first are 175 F, 185 F#, 196 G, 207 G#, 220 A, 233 A#.

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Build log

Since the top is on the guitar, you're not going to get any real modal data besides pitches it reacts at; the glitter patterns won't be super varied and useful like they are on a free plate. By being glued to the rims, the top is forced to work in a certain mode whether it wants to or not. The deal with 'free plate' tuning is that it allows you to make sure the top wants to vibrate in the mode it will be forced to when it's on the rims (ie: you're making sure it's good at the job it'll be forced to do).

You might/should be able to tweak the frequency of the main mode to where you want it; the more material you remove, the lower it gets. But you can't get much data on where to best remove the material from a glued-down top using glitter patterns. The two goals are to get the modes at the pitches you want, and the shapes you want, and you can't get much good feedback on their shapes unless the plate can move at its edges.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Bob,
This is EXACTLY my argument on the issue of tap tuning. You coudlnt have said it better.

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Reguards,

Ken H

Somehow I 'don't have the permissions needed to view attatchments to this post', whatever that means. Your pic didn't show up for me. However, if what Bob said is true, that you've already got the top and back glued down, he's right; you can't tell much about how well the thing is going to work. The big advantage of 'free' plate tuning is that you get a lot more information. The disadvantage is that it's sometimes hard to say just what it all means!

I agree that the soundboard has a different response before it is glued to the sides...but it is ONLY the response after being glued that matters. I would be perfectly happy if they all sounded like crap before being glued up, but once assembled they sang like crazy...

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That sort of sounds like Al...and you don't have permissions because you're not logged in!

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Modal testing isn't about how the plate 'sounds' before it's glued to the rims, a deaf person could do it. It's measuring stiffness distribution, an engineering property, and one that is rather important given the top's future role. I take it from the content of your message and desire to post it that you believe that we have no ability to change the probable response of a top before it's on the rims?

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

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I didn't read Parser's response as disagreeing about the ability to change the top's response before the top is glued to the rims, but only that the final response is the critical issue. I don't have a lot of experience with guitar tops, but did a lot of vibrational analysis on industrial products. Without the final boundary conditions established (ie, the glue joint between the top and B/S's- including the bindings), I believe it is difficult to predict the final vibration modes. If the luthier is very consistent in how he/she glues on the top and binds the guitar, results will be more predictable. But since each piece of wood is different, the final vibration patterns are only going to be visible after the guitar is finished. Don't mis-understand, I believe in tap testing and Chladni pattern testing. But I agree with Parser that the final result cannot be absolutely established after the top is on the guitar...

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Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

I believe important information can be obtained at each step of the building process. With the free plate the shape of the modes can convey a lot of information that can be correlated the stiffness/mass distribution and even to the final sound after buildiing a number of guitars. Of course the final frequencies can only be determined after the guitar is built, although with experience the frequencies of the free plate vibrations can be somewhat correlated to the final frequencies. Then again the shape of the modes don't mean much at this point. I like to glue the top to the rims first and check the main resonance frequencies so that I can tweak the braces at this point. By looking at the monopole, cross dipole and long dipole, with the rim attached, material can be removed in the right places to shift things a little, ideally! It also makes it easier to do any cleanup on the top before the back is attached.

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Randy Muth
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I'll assume you meant 'until after' in that last sentence.

I agree that the final result isn't absolutely established until it's all together, but I disagree that you can't play a lot of the important game and have some decent predictive ability before the rim is on.

Randy: You're a brave man. I always thought the (visible) squeezeout would be pretty hard to clean up if the back went on last unless you're bring super careful with your glue spreading?

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Yep - what gozier said. I have a similar background, and my gut tells me similar things.

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Sorry folks for the misunderstanding. I should be more detailed in my explaining or better still just post some pics . I realise now that I should have tried to tune the plates before assembling , and will definitely do this on # 2. In the meantime I would really like to be able to determine with some confidence the fundamental freq of both the top and back plates so that I can try and adjust the top to be a semitone lower than the back. Is my present method using the tone generator reliable or do I need to change it in some way?
Alex I initially tried as you suggested just with the fingernails but found it difficult to tell exactly the point when the plates were most active so decided to sprinkle the salt on to help see what was happening.
Another question that I have is --by shaving the top braces to drop it's pitch will this in any way affect the fundamental of the back plate?
Thanks again to all who have replied . This really is a fascinating side to building that I never thought I would get into.

Regards

Craig.

Craig,

I'm not sure of your set-up, but here are some suggestions.
1. Use poppy seeds instead of salt.
2. Make sure the back is not dampened when testing the top and visa versa. I usually do this before the neck is on and suspend the body on two strands of monofilament. You can just clamp the guitar by the neck and suspend it.
3. Don't try to get the top mode by blasting it with the speaker below the bottom of the guitar and visa versa. You need to hold the speaker in your hand and move in around very close to the plate you are testing. You may be doing this already.
4. Test with the soundhole open.

Too bad the guitar is already finished because the first thing you could have done was thin it around the edges of the lower bout. The next thing would be to do as you suggest and scallop the ends of the lower legs of the X brace and perhaps the lower ends of the tone bars. Although it is hard to tell after assembly, judging by where the salt is collecting it seems that the edge of the soundboard is somewhat stiff.

Bob, Yes I am very careful in spreading the glue. I use fish glue for attaching rims and have become very familiar with how much to apply. Also, reverse kerfed linings help a lot. Sometimes I do get a little squeeze out around the head block and Spanish foot, but it is easy enough to deal with through the soundhole.

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Randy Muth
RS Muth Guitars Website
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Craig,

I'm not sure of your set-up, but here are some suggestions.
1. Use poppy seeds instead of salt.
2. Make sure the back is not dampened when testing the top and visa versa. I usually do this before the neck is on and suspend the body on two strands of monofilament. You can just clamp the guitar by the neck and suspend it.
3. Don't try to get the top mode by blasting it with the speaker below the bottom of the guitar and visa versa. You need to hold the speaker in your hand and move in around very close to the plate you are testing. You may be doing this already.
4. Test with the soundhole open.

Too bad the guitar is already finished because the first thing you could have done was thin it around the edges of the lower bout. The next thing would be to do as you suggest and scallop the ends of the lower legs of the X brace and perhaps the lower ends of the tone bars. Although it is hard to tell after assembly, judging by where the salt is collecting it seems that the edge of the soundboard is somewhat stiff.

Bob, Yes I am very careful in spreading the glue. I use fish glue for attaching rims and have become very familiar with how much to apply. Also, reverse kerfed linings help a lot. Sometimes I do get a little squeeze out around the head block and Spanish foot, but it is easy enough to deal with through the soundhole.

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Randy Muth
RS Muth Guitars Website
RS Muth Guitars Blog
Facebook Fan Page

Why do I get double posts sometimes?

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Randy Muth
RS Muth Guitars Website
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Craig,

Just a few more thoughts.

Scalloping the ends of the braces is the safest thing to do, but may have the least impact. I just thought you should start there. The most effective way to drop the monopole is to reduce stiffness in the center of the lower bout. This is typically done by scalloping the lower legs of the X brace just south of the bridge plate ala Martin. I personally don't scallop like this and if you decide to do so, be careful!

Also the impact of the effects of these actions really depends on the overall system. So if the soundboard itself is really stiff and/or the bridgeplate is stiff and/or the height of the X-brace at the X is high, then the effect will be less. The way in which everyone balances these factors is a large part of what gives you a signature sound.

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Randy Muth
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Is anyone familiar with Roger Siminoff's tuning method using strobe tuners and tuning the tap tone exactly to a particular note? I'm trying to learn and understand this method, but I think when you get the plates where you want them, everything will change once you glue them to the rim, add the binding, and finish the guitar.

Chuck

Hi Randy

Good suggestions but you need to be careful what you are measuring at this point. You will be getting the "main air" mode, not the helmholtz. Also since the back and top are not damped, you will be getting a "coupling of the top and back in this measurement no matter where you are driving it. It is ok, but you may not be measuring what you think. Al can certianly explain this stuff a lot better. Also when you measure your main top mode things will change a lot when you add the bridge, (usually drop a couple of semitones.) It is all very interesting stuff.

I have not had a lot of luck with Shiminoff's thinking. Not that it is wrong, it just does not "line" up with where I have been going with stuff. He did some stuff with clamping a tranducer to a brace which is ok, but I would think the transducer (well at least the clamp) would certainlly add some mass, so I don't think this would be as accurate as one might expect.

I don't think there is a "formula" for any of this this. There are some guidelines, and I use this information as feedback measurements. The same could be done with tapping, bending, sniffing etc, but using these "technical" measurements are something you can quantify and save for later. Some people are really good at remembering and hearing a complex sound. I like to use the spectra to "see" it, then I can hear it too. Keeping the spectra and glitter patterns is a nice way of going back to "see" what I heard.

Sounds like he's going about getting half the information in a more difficult way. My guess is he's whacking the top to try and stimulate the ring mode, then getting its frequency, but that doesn't give him the shape of the mode (which is highly useful, on that particular mode). You can get a 'good' tap and aim it toward the 'proper' frequency that way, but having the mode shape available means you can make sure you're getting the 'best' tap.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

The stuff you say is worth repeating

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services