First try - free plate back

Construction and repair of Classical Guitar and related instruments
Alytw
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First try - free plate back

Post by Alytw » Fri Sep 29, 2017 1:26 am

Hello,

I'm building my first guitar and trying out free plate tuning. I've been working on a design with 4 back braces and a 15' radius,the back is about 2.4mm. I got patterns at 57Hz, 113 Hz, 237Hz, and 249 Hz. This was done prior to attaching the back (I'm not using a solera).

It was fun to try but I'm still a bit uncertain what the patterns mean. I tried this more for future reference so I would have a baseline.

The frequencies seem a bit different than what I have read, but I'm curious if this is because I am using a 4 brace design vs a 3 brace design (stiffer)?

Adam
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Alytw
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Re: First try - free plate back

Post by Alytw » Fri Sep 29, 2017 1:29 am

Here it is attached.
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Chris Sobel
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Re: First try - free plate back

Post by Chris Sobel » Fri Sep 29, 2017 7:43 pm

Hi Adam,

Are those plates with or without braces? The 57hz is likely the long bending mode, which doesn't seem to change much with the braces added unless you orient them with the grain. It's a bit higher than I have on my backs but probably on par with a lot of makers and it makes sense for Indian rosewood at 2.4mm. Nothing seems that out of the ordinary here, I am sure it'll work fine.

The back is actually not that stiff if the 249hz is indicative of the braced transverse bending mode, and that's not surprising given the amount of scalloping on the braces. Where the main back mode ends up for you will depend on a lot of construction factors but it'll likely be in a good range.

Chris
CE Sobel Guitars

Alytw
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Re: First try - free plate back

Post by Alytw » Fri Sep 29, 2017 9:23 pm

Chris,

Thank you very much for your feedback. It was actually your post that got me wondering about the specific numbers. You had mentioned in a previous post that you are usually between 40-50 hz as well as somewhere around 280hz.

I gather that the actual frequencies are somewhat design specific, but for you, is there anything specific that those two respective ranges influence in your final guitars? Or does it not work that way?

As I mentioned, this is my first guitar, so I'm really just following a plan and taking notes to try to understand a bit of what is going on.

Thanks again for your explanation above,

Adam

mqbernardo
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Re: First try - free plate back

Post by mqbernardo » Fri Sep 29, 2017 10:34 pm

Hi! first i want to congratulate you on your first guitar, it looks impeccable! nice clean work there.

i don´t have many pics of backs in my home computer, this is what i came up with. its from my last rosewood guitar, where i was trying to cook a live back sort of thing. i scalloped the braces a little too much (i think...) and ended up with a low back resonance. For some strange reason they all x27 Hz - i call that ultra advance tuning :). don´t have weight and height numbers here with me but i can look up.

i usually don´t spend a lot of time on modes under 100 Hz on backs, but maybe i should. anyway, at those frequencies the woofer moves a lot of air and unless you tame the gain down a little it tends to blow the tea leaves, which leaves its circular shape imprinted/overlapped in the chladni pattern. i guess that happened on some of your pics.

damn, i have to make myself a mould like that...


all the best,
Miguel.
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Alytw
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Re: First try - free plate back

Post by Alytw » Fri Sep 29, 2017 11:18 pm

Miguel,

Thanks for the reply, and thanks for posting the pictures. I didn't know how close to hold the speaker so i held it a few inches above the middle of the lower bout. Looking at your pictures, yours look better defined. How far away do you hold it? Also, I just left the saw dust as i went up in frequency. Do you do the same, or do you move it around?

I didn't go that high up in frequency, maybe I should have. I thought that the lower frequencies might be more important if i am trying to couple the back with the top.

Thanks,

Adam

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Chris Sobel
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Re: First try - free plate back

Post by Chris Sobel » Sat Sep 30, 2017 1:06 am

Alytw wrote:
Fri Sep 29, 2017 9:23 pm
Chris,

Thank you very much for your feedback. It was actually your post that got me wondering about the specific numbers. You had mentioned in a previous post that you are usually between 40-50 hz as well as somewhere around 280hz.

I gather that the actual frequencies are somewhat design specific, but for you, is there anything specific that those two respective ranges influence in your final guitars? Or does it not work that way?

As I mentioned, this is my first guitar, so I'm really just following a plan and taking notes to try to understand a bit of what is going on.

Thanks again for your explanation above,

Adam
Hi Adam,

What back resonances you get in the free plate (before the back goes on the guitar) and what they turn into after assembly are related to a number of factors specific to your construction features, but there is a relationship that you can determine if you just record all the frequencies before and after over a few instruments. If you change any of the construction features, most notably the width of the linings, back arch, height of brace when it connects to the side, etc, the relationship between the free plate modes and the assembled modes will also change. The important thing right now since you are building your first guitar, is to enjoy, observe and record the things that seem important to being able to reproduce your results, rather than worry about hitting a specific frequency. If you keep your design the same, however, you'll quickly be able to reproduce your results within a reasonable margin.

Chris
CE Sobel Guitars

Alytw
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Re: First try - free plate back

Post by Alytw » Sat Sep 30, 2017 1:26 am

Chris,

Thanks for the explanation of the process. That is exactly my goal; enjoy, observe, and record. It's been a lot of fun so far.

Adam

Alan Carruth
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Re: First try - free plate back

Post by Alan Carruth » Sat Sep 30, 2017 3:44 am

Backs can be hard to understand, much more so than tops, and particularly Classical tops.

What I find with ladder braced backs is that the lowest mode is often the fundamental bending mode along the back. There will be two node lines, roughly across the wide part of the lower bout, and between the waist and the shoulders. These may be fairly straight or slightly curved.

The next mode up is often the lowest torsion mode, with one line along the length in the center and another across the the plate, so it's divided into quadrants. Sometimes these will be swapped in frequency with the torsion mode lower in pitch, as it would normally be on an unbraced plate. The cross braces add some stiffness in twisting, and raise the pitch.

As you go up from there in pitch you will alternate between something that is predominantly a lenthwise bending mode and one that combines bending and twisting. Thus the third mode often has two crosswise node lines, and the fourth a lengthwise line with two crosswise ones, and so on.

As you go up in pitch crosswise bending will also become more important. This manifests itself as curvature in the node lines. At some point the curves will start to close in on themselves, so you'll get 'rings' of one sort or another.

Although it is undoubtedly true that the 'free' plate modes could be used to predict the modes of the completed instrument, it's also the case that the line from one to the other is neither obvious nor simple. These are complex systems, and although they are not so nonlinear as to be chaotic, it's still true that small differences in initial conditions can diverge fairly rapidly.

What this means is that you can learn to read the modes of the backs you build, and unlitmately gain some ability to get things to come out where you want. However, since your methods and work differ from mine, my experinece may not help much in predicting what your instruments will do, especiallly in terms of frequencies.

I would say, though, that in my experience it's a lot more effective to shave the lower crooss braces than the upper ones. When I went back to using ladder braced backs some years ago I found I was removing a lot of wood from those (generally four brace backs), to the point where the lower braces looked pretty flimsy. The backs were also lower in pitch than I wanted. I went to using low, wide braces for the lower two, wich reduuces their stiffness to weight ratio, so that I end up with 'beefier' lower braces and a slightlly heavier back than I would have had I started out with taller, narrower ones. I don't feel that added weight is as much a penalty on a back, and may even help. Now that I've been at it for a while I find I can usually get things pretty close to where I want them, although, this being wood and all, there are always exceptions.

One last thing: hold the speaker in an active area, and place the pads on node lines. If the pads are even a little off the lines will not form well, and the frequency will be different from the 'real' mode pitch. Again, with practice you get to where you know what you're looking at, and can usually get the pads pretty close to where you'll want them and know where to hold the speaker. Each mode is different, and requires a different pattern of supports and driving location.

Alytw
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Re: First try - free plate back

Post by Alytw » Sat Sep 30, 2017 1:15 pm

Alan,

Thank you for the detailed reply. It's interesting to hear your treatment of the lower two braces. The two lower bars in the design that I am using are 3mm lower and 2mm wider tgan the upper two. Do you shave the upper braces at all (other than to tie into the linings)?

Adam

Alan Carruth
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Re: First try - free plate back

Post by Alan Carruth » Sat Sep 30, 2017 10:19 pm

Sometimes I end up lowering the second brace (of four, counting from the uppper end), but not usually the first. In this respect backs and tops are similar: the uppper end is all structure, with little affect on tone, while the lower end is all tone, with little need for structural strength. The further down you work the more effect a giiven change will have on sound.

SteveL123
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Re: First try - free plate back

Post by SteveL123 » Sat Sep 30, 2017 11:18 pm

Has anyone built a removable back to expedite the learning curve in voicing a guitar? The "circumference" of the back is around 56" .

75 small wood screws = a screw every 3/4" which will secure the back well and make it easily removable for tuning the top and back.
Last edited by SteveL123 on Sun Oct 01, 2017 2:36 pm, edited 1 time in total.

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Trevor Gore
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Re: First try - free plate back

Post by Trevor Gore » Sun Oct 01, 2017 12:56 pm

Alan Carruth wrote:
Sat Sep 30, 2017 3:44 am
Although it is undoubtedly true that the 'free' plate modes could be used to predict the modes of the completed instrument, it's also the case that the line from one to the other is neither obvious nor simple. These are complex systems, and although they are not so nonlinear as to be chaotic, it's still true that small differences in initial conditions can diverge fairly rapidly.
Ain't that the truth!

At the end of the day, what we hear are the vibrational modes of the finished guitar, with all the constraints and coupling that assembling all the parts together precipitates. And as Alan has suggested, trying to predict those modes and frequencies from the oscillations of free plates is pretty difficult, because not only do you have to reproduce precisely the free plate situation, you have to reproduce pretty precisely what you attach the plate to (i.e. the rest of the guitar) and the way you attach it.

An alternative approach is to standardise the things that you can standardise and then have a structure (of the guitar) and a process that allows you to tune the plate once it has been assembled into a guitar. That requires four things: 1) a method for getting the un-braced panel to a standard vibrational performance irrespective of the wood properties (elastic constants (e.g. Young's modulus), density and thickness in particular); 2) a method of bracing the panel in a consistent way that allows for easy adjustment in the assembled instrument; 3) a process for making those adjustments; 4) targets to aim for that give particular types of performance (e.g. a live or a non-live back, or procedure for tuning a back frequency relative to a top frequency). Of course, you need the necessary instrumentation to measure all of these things; spectrum analysers, frequency counters, thickness measurement etc.. So it can get pretty involved pretty quickly. Fortunately, these days you don't have to do all the research yourself, unless of course you particularly want to!

Alan Carruth
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Re: First try - free plate back

Post by Alan Carruth » Sun Oct 01, 2017 6:27 pm

I have found that 'free' plate tuning helps to reduce, if not always eliminate, the need to after assembly tuning. With enough experience you can get it 'close enough' before you do the assembly much of the time. Since my hands are too fat to get in through Classical guitar sound holes for brace shaving, free plate tuning has really helped.

As Trevor says, getting more formally acquainted with the acoustics of these things does require some equipment, as well as a bit of familiarity with the concepts. Fortunately, the equipment constraints have become a lot easier to meet over the past decade or two. My violin making teacher needed $20,000 worth of state of the art lab gear to make spectrum charts of fiddles in the '80s. Now you can do it al with an obsolete computer and free software, and there are even apps that allow for a lot of it on a tablet. It's still a lot of work to learn what to do and how, but at least it's not as expensive.

Jeremy Clark
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Re: First try - free plate back

Post by Jeremy Clark » Mon Oct 02, 2017 11:15 pm

My $0.02...
I think looking at chladni patterns can help teach your hands what they’re feeling, and your ear what it’s hearing. Bonk the plate, get the analysis, look at the shape that corresponds with a particular movement, tap, flex, take notes, and carve. Lather, rinse, and repeat. If you can keep enough things consistent and your notes are good you’ll likely start to see some correlations in the data. Worry about the numbers then...
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