Tap tone analyzes

Construction and repair of Classical Guitar and related instruments
Ricardo Meirinhos
Posts: 84
Joined: Sat Jan 31, 2015 1:25 am
Location: Portugal

Tap tone analyzes

Post by Ricardo Meirinhos » Wed Sep 06, 2017 10:08 pm

Hi

I present you the charts of the analysis of my soundboards' expectros.

I try to compare the soundboard of my guitar #2 (Romanillos Model) with my new soundboard, from guitar #4, a Bouchet model.

What conclusions can be drawn from analysis to graphs?

On this soundboard I still need to do the fine tuning of the Bouchet bar. Do you suggest anything in particular?

Thanks
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jim watts
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Joined: Mon Sep 05, 2011 7:19 pm
Location: Los Alamos, NM, USA

Re: Tap tone analyzes

Post by jim watts » Wed Sep 06, 2017 11:26 pm

These look like free plates, correct?
It's really hard to tell much of anything with a tap spectrum of free plates. I would get the box complete and then make any adjustments I felt were necessary.
Alan Carruth has done a lot of free plate work maybe he'll be along. I believe he relies on the Modal shapes more than the frequency though which you can get by generating Chladni patterns.

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Chris Sobel
Luthier
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Location: Vancouver, WA

Re: Tap tone analyzes

Post by Chris Sobel » Thu Sep 07, 2017 12:19 am

Hi Ricardo,

I'm guessing these are the free plates before construction. It looks like your main top mode on the Romanillos is a higher than the Bouchet at just a little over 200. I would not do anything more to the Bouchet if it were my guitar, until the guitar is strung up. It looks like the current mode is just below 200 which is low but one of my best Bouchets was at 196 before construction but generally speaking that will be a fairly dark sounding Bouchet. That's if the main top mode is truly that hump around 195... it looks a little amorphous but that is fairly typical to have a two peak top mode when using the Bouchet bar UNTIL the bridge is on and then the peak becomes more discernible.

How the modes will turn out in the final guitar will depend somewhat on the other construction features. If you're using regular thickness sides and a normal ~20g bridge I think the main top mode will be a little higher in the finished instrument.

What is the current thickness of the soundboards and the dimensions of the braces if I can ask?

Edit: One thing that these spectra will not necessarily convey is the ratio of bracing to top mass, which is extremely important in the overall timbre of sound. I can brace a steel string top and it might have a similar top mode to a classical but they are very different tops... it's another tool in the toolbox and while an important one there are other data points to consider as well for a full orbed analysis.

Chris
CE Sobel Guitars

Alan Carruth
Luthier
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Re: Tap tone analyzes

Post by Alan Carruth » Thu Sep 07, 2017 6:35 pm

First, what did you test, and how? That is, exactly what did you test, and precisely how? Details are incredibly important in something like this. You can see changes in peak frequencies and heights from rather minor changes in the way the piece was supported and activated, and a few centimeters change in microphone position can alter the entire shape of the spectrum, depending on the room. If you are careful to do the tests the same way every time you can get data that can be compared with your own data set, but comparing it with other folk's data can be problematic. I've learned to read my own charts in some respects, and get useful information, but it's not as useful as I'd like. Other people's data tends to confuse me, but then, I'm easily confused.

'Free' plate data is not good at predicting the resonant behavior of the completed instrument in any precise way. As Trevor Gore will tell you, particularly in high performance instruments, small differences can loom large, so you will miss stuff that would be good to know. I believe that the 'free' plate mode shapes have some utility in predicting tone 'quality' in some respects, but I'm not sure how you'd measure that. I could be fooling myself.

The most robust information you're likely to get on a completed instrument is the resonant pitches of the lowest modes. That can give you some idea of the character of the sound, but not much of an indication of how good it is. The exception there is when those modes stack up in such a way as to cause a 'wolf' note, but if there's a wolf you won't always find it in a spectrum chart either.

I would encourage you to keep making measurements, though. Maybe you'll be the one to see the correlation that matters, or figure out the best 'standard' method of making these measurements. If nothing else, you might learn what doesn't work, and that's useful too.

Ricardo Meirinhos
Posts: 84
Joined: Sat Jan 31, 2015 1:25 am
Location: Portugal

Re: Tap tone analyzes

Post by Ricardo Meirinhos » Thu Sep 07, 2017 9:58 pm

Thanks for your coments!

Yes this was a free plate tap tone without any kind of precision. I grab the soundboard in my hand and with other I tap it.

I enterly agree with Alan. I must creat a method to test with similar conditions.

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