When to stop thicknessing a back?

[Adam Savage]

Greetings folks, and welcome to another of my newbie questions!
Whilst I'm putting finish on my ukes, I have started prepping and gluing up some timber for a pair of guitars (I felt that doing more than one together should highlight errors and pit falls more easily, and reinforce the learning process too). Mostly it is going fine, but I would like some advice on the thicknessing of one of the back sets.
The timber, I think, is some kind of rosewood, (possibly Panama?) - unfortunately I bought it some years ago and I have lost either it's label or any email of the order at the time. All I can say is that it is reddish brown, a little like some cocobolo I have seen, quite dense with a startling bell-like ping when tapped. And clogs up the drum sander like it's made of resin.
I have currently got it at around 0.095-0.098" and it is still somewhat resilient to flexing, and the tap tone, though 'loosening' up, has a definite bell quality to it.
I am building a classical with it, paired to a spruce top, and I wondered if you might be able to advise if I should thickness a little more, and what I might aim for when stopping?

Thanks,
Adam

[Mario Proulx]

I take cocobolo and other dense rosewoods down to .080"-.085" or so today but I have built many great guitars with them as thick as .100", so you're well within range. Go as thin as you are comfortable doing. Weight is an issue with these dense woods, so keep that in mind, also. Taking a .100" thick back down to .085" makes it 15% lighter and that can make a big difference in player comfort.

[Adam Savage]

Mario - thank you for your post. The weight will not be an issue, as it's for me and I don't mind the weight (you should see the padauk solid body I made last year!), the problem is that I don't know how thin is going beyond 'comfortable'. I feel that it could go thinner, based on how the flex/tap tone has changed so far, but I have essentially no experience to back this feeling up. It's a bit like driving a new car along a winding road - you don't know quite what the limit of the brakes and road holding is until..... But I digress.
Your advice suggests that I will be fine if I leave things as they are, but also I should be ok if I go a little further.

Cheers,
Adam

[Clay Schaeffer]

If you feel it is still a bit stiff take it down to .090" and see how it feels. Once you're in the ball park a few thousandths can make a big difference. I think this is still in the "safe" range for most woods using a classical plantilla. Rosewoods can go a bit thinner than most woods, but leaving a little extra is not always a bad idea.

[Trevor Gore]

Do you want a live back or a non-live back? What's your bracing scheme? How stiff is the wood? What's it's density?

I don't actually want answers, but these are the things you need to consider in order to determine how thick to leave the back. A single thickness number can't really be considered in isolation as the back's performance is also dependent on so many other things.

To offer some guidance: A live-back classical is not the easiest beast to pull off (far easier to get it wrong than right) so I would advise going the non-live route, which just means leaving the wood at the thicker end of the scale and don't make the bracing too skinny. Advice on making live backs gets very technical very quickly.

[Brad Heinzen]

I'd suggest going lighter, but I prefer live-bodied guitars. I actually think the odds are more in your favor if you thin the back down a bit.

[Adam Savage]

Thank you again for your advice. To summarize, I think I will be OK thinning a little more.
Trevor - without giving you specific answers, it is hard for me, as a beginner, to know how the various factors will effect how thick I should leave the back. I realise that the nature of (particularly) an acoustic guitar is the sum of all of its parts, but one has to start somewhere, and my somewhere is a piece of wood that is stiffer than indian rosewood, walnut, possibly cocobolo, and from what I recall of a guitar built on a course 6 years ago, stiffer than macassar ebony too. It seems fairly dense, certainly compared to walnut and the IR. Bracing pattern will come from a purchased plan of a Ramirez...
To show, further, my ignorance of the subject, could you tell me what a live/non-live back is?

Brad, I think I shall end up taking those odds...

Thanks again,
Adam

[Brad Heinzen]

That advice is based on what happened to me on my first dozen guitars or so. The lighter ones were generally more successful, in spite of not really knowing what I was doing. The least successful for me were largely guitars built with very dense B&S materials. I think most of us tend to overbuild our first guitars, and denser materials just make it worse.

If someone asked me what B&S materials I'd suggest to start out with, I'd advise sticking with woods no heavier than EIR for the first few. I think that gives you a better feel for what's 'normal' so that you don't end up with a brick. Once you've had a few braced backs in your hands, you'll have a better feel for where you can go from there. When I work with denser materials now, I know how far I can thin them down, and I'd say that what feels fine to me now would have made me nervous before I knew better.

[Trevor Gore]

...could you tell me what a live/non-live back is?

A live back is one whose vibrations actively couple with the vibrations of the top. On the other hand a non-live back is one that is too stiff or too heavy (or both) to actively couple with the top.

A live back does a few things: it takes energy from the top in order to make it vibrate and as the sound of a guitar is mostly from sound radiation from the top, the guitar generally is not as loud as it may otherwise have been. On the other hand, a live back adds more peaks into the frequency response curve of the whole guitar, so it sounds more interesting (there's a technical paper about why that is by Matthews and Kohut, but I'll leave it at that for the moment). The additional peak(s), if well positioned, can help avoid isolated, high peaks in the frequency response curve which would otherwise lead to noticeable wolf tones. Further, a live back guitar tends to have a main air resonance somewhat lower in frequency than a non-live back guitar, so gives the impression of being a larger guitar.

In a nut shell (and very crudely) a non-live back guitar gives you volume and a live back guitar gives you tone. However, to make a live back guitar work well, the back and top have to be sympathetically tuned to each other, which is not the easiest thing to do for the inexperienced. Hence my previous comment:

A live-back classical is not the easiest beast to pull off (far easier to get it wrong than right) so I would advise going the non-live route, which just means leaving the wood at the thicker end of the scale and don't make the bracing too skinny. Advice on making live backs gets very technical very quickly.

[Hugh Anderson]

If the back is too thick, stiff, and you have curved braces, the back will straighten them back out again. Learned the hard way, but not nearly soon enough.

[Kerry Werry]

Trevor

an interesting post.. I know this is related to classical guitars or at least the original question was.. Do you feel the same general comments hold true for the steel string as well.. AS a rank amateur I tend to think of the back and a reflective surface to help power the top.. at least in my limited experience in the steel string world... I assume (I hope at least somewhat correctly) that the heavier bracing of a steel sting would make a "live back" even less effective and more difficult to obtain or am I going down the totally wrong rabbit hole here???

By the way the books are making me wish I had not dropped calculus be slowly working through them and gaining lots

Kerry

...could you tell me what a live/non-live back is?

A live back is one whose vibrations actively couple with the vibrations of the top. On the other hand a non-live back is one that is too stiff or too heavy (or both) to actively couple with the top.

A live back does a few things: it takes energy from the top in order to make it vibrate and as the sound of a guitar is mostly from sound radiation from the top, the guitar generally is not as loud as it may otherwise have been. On the other hand, a live back adds more peaks into the frequency response curve of the whole guitar, so it sounds more interesting (there's a technical paper about why that is by Matthews and Kohut, but I'll leave it at that for the moment). The additional peak(s), if well positioned, can help avoid isolated, high peaks in the frequency response curve which would otherwise lead to noticeable wolf tones. Further, a live back guitar tends to have a main air resonance somewhat lower in frequency than a non-live back guitar, so gives the impression of being a larger guitar.

In a nut shell (and very crudely) a non-live back guitar gives you volume and a live back guitar gives you tone. However, to make a live back guitar work well, the back and top have to be sympathetically tuned to each other, which is not the easiest thing to do for the inexperienced. Hence my previous comment:

A live-back classical is not the easiest beast to pull off (far easier to get it wrong than right) so I would advise going the non-live route, which just means leaving the wood at the thicker end of the scale and don't make the bracing too skinny. Advice on making live backs gets very technical very quickly.

[Doug Shaker]

I agree that Trevor's book is really really good. But I want to say that you can just skip most of the math and the book still works. I DID take calculus in high school and then more math in college and graduate school and I still ended up skipping most of the math. The math is useful if you need to be convinced, but, well, after a while, I just took it all on faith.

[Trevor Gore]

I know this is related to classical guitars or at least the original question was.. Do you feel the same general comments hold true for the steel string as well...

All the same physics hold true. However, with a classical guitar, many builders are striving for as much volume as they can get and with a live back you will always sacrifice some, hence you need some precise positioning of resonances to get the advantage of the live back without sacrificing too much volume. On the other hand, with SS guitars you have more power to start with, so if you don't get it exactly right you won't have as much of a problem.

There are many other considerations to take into account of course (but you have the books) and a lot depends on where you personally set your acoustical targets (loudness, tone, musicality). I generally encourage people to set them a lot higher than the factories, as they do not present a particularly high bar.

AS a rank amateur I tend to think of the back and a reflective surface to help power the top...

The concept of a reflective back is not a very useful one, although it may be popular. For a back to reflect a sound wave, the wave has to establish itself across the depth of the guitar. The lowest frequency wave that can do that has a 1/4 wavelength equal to the guitar's depth dimension. So for a guitar ~100mm deep (4") the lowest frequency wave that can reflect off the back is ~1500 Hz. At those frequencies, all backs are reflective. Details (maths, modelling, measured results) and more discussion in Section 1.7.5., black book.

[Len McIntosh]

This thread is really fascinating, not necessarily to affect my builds but to better understand the results.
I'm wondering now, is there a way to tell if a previous build has a live or non live back.
And does the fact that when I first complete a guitar (all ss) and string it up, it is always utter disappointment - it's just not musical!
Then a few weeks go by and it "settles in" and the notes seem to "sparkle" (my vocabulary is inadequate in this regard) Does this have anything to do with a live back?
By the way my builds are all pretty well by the book regarding thicknesses bracing or otherwise.
Love this stuff, thanks all, for the time you share with amatures like myself.

[Kerry Werry]

Trevor

Thanks for the response.. I think I have some more reading to do, I don't think I fully grasp the acoustic theory here but I do have the book and now the page reference .. I particularly don't understand this sentence "For a back to reflect a sound wave, the wave has to establish itself across the depth of the guitar." I'm thinking maybe you mean for this reflected sound wave to excite the top?? Oh well back to the books...

Kerry

I know this is related to classical guitars or at least the original question was.. Do you feel the same general comments hold true for the steel string as well...

All the same physics hold true. However, with a classical guitar, many builders are striving for as much volume as they can get and with a live back you will always sacrifice some, hence you need some precise positioning of resonances to get the advantage of the live back without sacrificing too much volume. On the other hand, with SS guitars you have more power to start with, so if you don't get it exactly right you won't have as much of a problem.

There are many other considerations to take into account of course (but you have the books) and a lot depends on where you personally set your acoustical targets (loudness, tone, musicality). I generally encourage people to set them a lot higher than the factories, as they do not present a particularly high bar.

AS a rank amateur I tend to think of the back and a reflective surface to help power the top...

The concept of a reflective back is not a very useful one, although it may be popular. For a back to reflect a sound wave, the wave has to establish itself across the depth of the guitar. The lowest frequency wave that can do that has a 1/4 wavelength equal to the guitar's depth dimension. So for a guitar ~100mm deep (4") the lowest frequency wave that can reflect off the back is ~1500 Hz. At those frequencies, all backs are reflective. Details (maths, modelling, measured results) and more discussion in Section 1.7.5., black book.

[Trevor Gore]

I'm wondering now, is there a way to tell if a previous build has a live or non live back.

There are a few ways of telling. With experience, you just need to hear the guitar and you can get a good indication, but that's not going to help you! Visually, you need to be able to see three "ring" type modes of vibration in the lower bout of the guitar. This is usually done using Chladni patterns (the ring being the shape that the tea leaves form), but once you have some experience you can see it in the frequency response curve generated from a tap test. The first "ring" mode is typically ~100 Hz, and is formed on the top as it couples with the main air resonance; the second is at ~200 Hz, and is the main top resonance and the third is typically a few semitones higher in pitch and only occurs if the instrument has a back which couples with the top (and for me, this is essentially the definition of a live back; this mode of vibration exists and leaves its signature in the frequency response curve of the guitar when its top is excited).

Then a few weeks go by and it "settles in" and the notes seem to "sparkle" (my vocabulary is inadequate in this regard) Does this have anything to do with a live back?

Probably not. That effect is most likely due to your finishes hardening up.

I do have the book and now the page reference .. I particularly don't understand this sentence "For a back to reflect a sound wave, the wave has to establish itself across the depth of the guitar."

Basically, a wave (any wave) can only be reflected if it exists in the first place. For a sound wave to be reflected off the back, it has to be initially traveling towards the back, generated by movement of the top. The longest wave length (lowest frequency) wave that can exist sufficiently to be reflected has a 1/2 (note: this is corrected from me saying 1/4 previously and unfortunately I can't amend the original) wavelength equal to the depth of the guitar. That is the limit where the wave has sufficient wavelength that it doesn't self cancel as it tries to reflect. The figures on page 1-95 (Black Book) illustrate this.