Official Luthiers Forum!

or smaller? thinner?...
i remember an old GAL magazine that had an article about what happens when one changes dimensions regarding the guitar.
so if we add X% to a brace's height we gain y% of strenght and all sorts of other rather mathematical equations and dependencies.
my goal here is to make it abit more realistic...
its is very cool that if i make my guitar's sides x% taller then ill have y% more body volume, BUT how does it translate in terms of sound?

so this brings many questions...doundhole diameter, body depth, body volume, neck stiffness, extra thin vs. extra thick top, bridge weight, back mass/stiffness, etc...

shoot away!

Udi.

It depends on what 'it' is, and how 'it' relates to everything else. As so often happens, the short easy to ask questions end up taking the most time to answer satisfactorally. What you've asked would take a book.

well, there has been some long threads here before...
but i have no intention to tie anyone down in front of their pc for that matter, so i'll ask one specific question(for starters).

its somehow related to my "Romanillos back bracing" thread i guess, what if i made a guitar with a 4mm thick back, then pooped the back off, gave it a few passes through the thickness sander and reattached it to the guitar, now that its half as thick,2mm, what am i to expect? generally, more bass? more trebles? more mids? more volume? expect nothing as the back plays no role in sound production?

what happens, other than the decrease of weight?

Udi,
Alan is correct in that there is no recipe for getting a certain sound. All of the components of the guitar contribute to the sound and changing any one of them will affect the sound. What is not clear is exactly how the sound will change. Many luthiers have spend their whole lives trying to figure that out and many of them have promised me that they will let me know as soon as they do. I haven't heard from any of them yet.

We still can't figure out a way to describe to each other what the sound is. Simple sounding questions like "I want to increase the bass without making the guitar sound 'tubby'". Others will answer that "tubby" is the sound of more bass. The response to that is "No it isn't. It's something quite different" and the debate goes around in circles. I once had a potential customer tell me that they wanted the guitar to have "brilliant" trebles but didn't want it to be too "bright".

I am fortunate enough to regularly attend meetings with many experienced luthiers and it is interesting to listen to discussion on how to alter the tone of a guitar. One will suggest shaving a particular brace while another will suggest thinning the top here or there. While yet another will give the opinion that a ligher bridge is what will work. None of these guys are wrong. All of the suggestions will probably move the sound of the guitar in the same general direction but will have different "flavors" to the resulting sound. Which of those flavors do you like the best and most closely matches the mental image you have of the desired sound.

General guidelines like making the sound hole smaller will give you a louder guitar at the expense of overtones will work only over a range of what is commonly practiced and may not work at all for some body shapes and brace patterns.

We aren't answering your question because they are some trade secret. It's more that we don't know the answers or don't know how to tell you the answers. And, we aren't sure what the question really is either.

first of all, i aint trying to make shortcuts, and i aint trying or even pretending to be able to build the perfect guitar based on what somebody half the world away wrote on the OLF.

we all build a bit differently, we all define "good sounding" a bit differently.

and i aint investing money in what is being told here and i aint going to hunt anybody down just because his experience wasnt the same as mine.

BUT,my question arose, in part, after reading some responses from distinguished and experienced luthiers such as Mr.Carruth and Mr.Turner with regards to a thread, in which somebody here asked what should be done to bring out the trebles in a completed guitar.

in that case thinning the bridge, thinning the periphery of the soundboard where all accepted as legal "solutions" while thinning the soundboard behind the bridge was a "no go" coz it would increase bass response.

so everybody knows that thinner soundboard edges produce more trebles, but nobody knows what will happen if i leave my guitar's back waaaaaaaaaaay too thick?

everybody knows that thinning the sounbord area behind the bridge will increase bass response but nobody knows what will happen if I leave my bridge just a big,heavy,rigid,unshaped chunk of wood with a saddle stuck in it?

if there isnt a single proven truth, than everybodys own experience serves as their own truth.
and thats what i'm asking.

if somebody here feels his own experience and understanding of the guitar is not of interest to others (or ME) than let it be known that it saddens me! but feel free not to contribute, but i know myself, and i'd like to hear what everybody has to say(be it your 1st guitar or the 1 hundred-th guitar) as long as i have the right to take what i choose of it, and take it with slightly more than just a grain of salt.

Udi.

There are just too many variables. Like the thinning the outer edges of a soundboard indiscrimately in of its self does not necessarily produce more treble. In fact it can cause the top to be floppy therefore wash out the treble.

I think what Mike was trying to tell you is all things have to be considered not just one parameter to address but many.

One might feel like the English Composition student who has just been asked to write about anything at all. One might be paralyzed by the infinite possibilities. On the other hand, if one were to ask the student to write about the emotions evoked about the dead fly in the window sill, one might not be surprised by the instant response of the pen to paper.

It is a very, very big question you ask, Udi!

_________________
"Building guitars looks hard, but it's actually much harder than it looks." Tom Buck

I get the feeling nobody gets my point.
ill blame in on the fact that english is not my native tongue and i might not be making my point too clearly.
this might also be the proper time and place to pre-apologize if my posts seem offensive, i have no intention to sound as such, and i'm well spirited.
guess its just my "school" english, lack of "english slang" abilities and the diffifulty i find in making my point.

Michael, i'm sorry but i see no variables.
the entire 'being" of a variable is that it varies.
if it doesnt vary, and stays constant than it simply isnt a varaiable.

if just the back is replaced and all other things remain constant, than to my understanding the back is the only variable.

and "equations" with a single variable are usually the simplest to solve.

and to further clarify things,i m not asking "what should i change in the back to achieve this or that", but rather the other way around, " what will happen if i change that".

Steve,
i understand my first post is a bit out there, but my 2nd one asked solely and simply about the guitar's back, its contribution to sound, and the "suggested" effect a change in the back will cause.
and i'm not even asking about modifying the back braces...just taking wood of the back.

next back i'm putting on goes on super thick and with hide glue so i could pop it off and glue it back much thinner.

thanks all,
Udi.

There are two schools of thought on the effect the back has on the sound of a guitar. Some say that the back should be a hard, stiff, reflector and that the totality of the sound is related to the top and the size and shape of the body. Others, on the other hand, claim that the back is an integral part of the vibrating system of the guitar and should be tuned and adjusted just as you tune or adjust the top. Guitars built under both schools may sound very good.

Your experiment of placing a 4mm back on a guitar and then replacing it with a 2mm back could change the type of guitar by taking a reflector type and converting it to a vibrating type. I've used the model that rosewood bodies are more of the reflector type and mahogany bodies are more of the vibrating type. So one prediction is that the guitar would sound less rosewood like and more mahogany like.

Now for the big disclaimer. The reflector vs vibrator model is something that I use and have found to be somewhat helpful. Others, think that it is all bunk and they may well be (probably are) right. That is the current state of the art in that some of the fundamental principles are still being debated.

The variable I referred to are things like top stiffness, brace stiffness, size and shape the list goes on and on. Every component and its attributes all play a role in the affect that all the others have. Change one and it effects the mechanical production of many or possibly all of the others The effect of a single change of one component can not be determined with out knowing the values of all the others and the effect it will have on the mechanical production of those other components. These are the variables I spoke of.

I am not either being harsh or flippant in this reply but it is impossible to set a changeable value and the effect it will have on tone or structure with out considering the air chamber and its components in whole.

At the risk of looking like a complete idiot (which happens sometimes), I'll try to answer Udi's question about back thickness.

Udi,

First of all, it totally depends on the species of wood you are using. Back woods affect tones in different ways. After you gat past that and assume (for the sake of an example) that there is only one wood, generally you get the response that Mike Maher wrote above. Personally, I try to keep my backs relatively thin while the style of bracing I use stiffens the back and increases it's reflectivity. For me, I want the instrument to be as light as possible and that is what I shoot for.

There has been some discussion about different radiuses for backs and their effects on sound quality. You can do a search for radiuses and spend several days reading all that has been discussed. The jury is still out about how radius affects sound.

With that being said, here is my theory and is ONLY a theory: (I'm sure there are those that will disagree strongly with my theory and I really dont want to argue about it. My theory may change at any time when I find new rules to apply to the theory)

when you increase the reflectiveness of the back, it increases the vibration on the top and increases the harmonics of the top. This may or may not be desirable because offending harmonics can cause many problems with a guitar and may even be so strong that they cancel out the main tone you are trying to achieve and leave you with only the harmonics which may or may not be in tune with the chord you are playing.

On the other hand, if the back is thin and floppy, it will vibrate more and you may get some tonal qualities that are desirable to your ear. to my ear they may sound like mud. You may lose harmonics in the top also, which can leave you with a less vibrant and dead sounding guitar. of course, the opposite can happen too, but this is why each of us can build a guitar using the same kinds of wood and end up with such different sounding guitars.

I know what you are trying to ask here, and I wish there was a simple answer. If there was a definitive answer at all, even if it took all night to type it out, I would gladly type it just to help you out. Unfortunatly, this is not the case and there is no definitive answer.

The best suggestion I could give you would be to buy some cheap wood and build a few guitars with bolt on necks and the fret board sawed off at the 14th fret. Then experiment with the cheap wood building boxes that contain the ideas you want to try. You can develop your own theories this way and may come up with a sound that you can call your own. It wont cost you anything except some time and whatever the price of the cheap wood was. I have done this and some of the guitars came out sounding pretty good. SOme of them were so bad that I demolished them so that nobody would ever see or hear them. This is what the craft of lutherie is all about.

_________________
Reguards,

Ken H

i wrote yet another long reply but lost it coz the connection got lost.

i'm currently working on # 19.
i'm no James Olson but no completely clueless beginner either.
ive done some experimenting, took backs and tops off and radically changed plate thickness on completed instruments.

if it wasnt for the internet and places such as the OLF,
i'd probably still be doing things the "cumpiano way" and be fine with it.
but some people here X-brace their backs, some leave their tops thick and some extra thin,some vary their tops bracing while still staying within the "safety zone" of the X bracing(i think i saw one of R.turner's guitars which had a classical like, 3 fan braces instead of the two slanted braces at the lower bout), some stiffen their necks with carbon fiber...
i doubt its all being done because its cool and i doubt its all being done for constructional reasons either.

and if indeed, we do not know how to alter the sound of our instruments, how can a luthier(and i do NOT consider myself one) claim that a handamde guitar is superior to a factory made one(please dont take it to extremes and start talking about chinese made plywood guitars)? how can a luthier satisfy his costumer if he doesnt know how to shape/taylor the sound so that it meets his costumer's demand?

i'm not in the business of pointing fingers.
i'm not here to shock or offend.
but TO ME, there can not be a more valid question regarding lutherie than the one i've just asked.

perhaps its my engineering studies that are effecting me, perhaps my close relation with Boaz Elkayam and the kasha theories, perhaps it is a result of my recent exchange of e mails with Mr.R.E Brune, Mr. Tom blackshear and Mr. Ervin Somogyi about the flamenco guitar and its construction, in which it became obvious that "lutherie" doesnt equal "constructing a stringed instrument", but rather understanding, investigating and giving birth to a stringed instrument.

Udi.

Udi, I'm quite interested in your question and theories behind it as well. Your question as I understand it is if you build a guitar with a 4mm thick stiff back it will have a certain as yet undefined sound, then if you thin that back to 2mm it will be more flexible and will probably have a different sound. Most importantly not changing anything else in the guitar.

The responses that I've read seem to indicate that it seems to be an unknown to the rest of us as well. I think it's a great question, that if can be answered in the most general of terms would be a help to all of us. Unfortunately I'm in no way able to help with the answer, but am very keen to learn if anyone has done tests to determine the results of your query.

_________________
Allen R. McFarlen
Barron River Guitars & Ukuleles
Facebook
Cairns, Australia

amcfarlen(very unusual name ;) ),
appreciate your support here on this one!

i may be overthinking things, but the more i "over"think about them, i get the feeling that its not me overthinking but many others just underthinking about these things and taking them for granted.
something which i refuse to accept.

i opted for a discussion about the back of the guitar and its contribution to the overall sound just to narrow things down, and have this whole potentially great discussion a bit more focused.

had there been more interest here or greater participation perhaps this could have evolved towards changes in other components of the guitar(neck mass and stiffness, top vibration modes etc...).

i can tell you that on a recently completed guitar(flamenco) i left the back thick(3.5mm).
i finished the guitar, bound it did everything and gave it to the person who ordered it.
he said it was heavy(obvious) too bassy sounding, with very muffled trebels - in his own words "it sounds as if you used thick glass as a finish instead of thin lacquer".

i then took it back, removed the back finish, sanded /scraped the back to the point where i could feel it flexing when i poked my thumb in it.
this totally changed the sound and response of the guitar. it seemed to project better, improved the trebles response and overall volume.

i then asked him to play it for a while and come up with a few things that he felt were still improve-able.
eventually i also replaced the mac.ebony on that guitar to a lighter mad.rosewood bridge.
overall same dimensions but noticeably lighter.
to my ears that change was mostly audible when playing the 3 treble strings and higher up the neck.
didnt make as much of a difference as the change in the back thickness did though.

then, i took this bridge off and followed an advice given by Tom Blackshear.
i scraped the area below the bridge just a tiny bit leaving it thinner than the sorrounding.
that too, to my ears altered the repsonse of the guitar to an audible degreee.

i tried to follow some of Mr.Blackshears other "sound improving" ideas as to tweaking the inner braces with light passes of fine great paper.
i couldnt tell any difference whatsoever using that method.

however, i tried a more radical approach.
i gave a skinny handed friend of mine a small finger plane, put a light bulb inside the guitar, showed him which brace i want to remove, and let him go at it.
i felt it freed up the top considerably.
to the point where nowadays i choose to either not use that specific brace i asked him to shave or i make it super small using a thin cut off from the top instead of a "large" brace.

some might say its stupid.
some might say its pure genious.
some might say its a waste of time.
some might say its the best way to spend one's time.

but to me, that is the most interesting element of constructing a guitar.
and it is the most crucial element in getting the sound inside your head.

Udi.

I think you answered your own question and is exactly what I was trying to say in my previous statement. Thinner backs are better for overall tone. Bracing and stiffness of those braces on the back affect overall tone as well. I use cross braced backs because it gives me a much stiffer back even though I thin my backs to somewhere in the .07 to .09 thickness. Again, I prefer the reflective properties rather than the vibrating properties for my guitars. A ladder braced guitar with the same wood and the same thickness would sound different than my cross braced back.

As Alan Caruth would say, it is a system and not one thing that makes great sounding guitars. You found this out when you thinned your back and then kept going to thin or eliminate a brace on the top of the guitar. Your experience also strengthens my theory stated above in that thinning of the back may affect the top in such a way that it doesnt sound good and other things have to be done to the top to produce a good sound.

In general, when you make a larger body on a guitar, you get stronger bass, such as making the guitar deeper. You can "shape" the bass response by making the upper bout smaller than the lower bout. Think of the mariachi style of acoustic bass with a lower bout about 8" thick and an upper bout about 4" thick. These really project a loud but mellow bass tone but the sustain isnt there to go along with it. Thinner acoustic bass's are not as loud, but they generally have better tone and longer sustain.

Similar qualities are reflected when you enlarge the size of the sound hole or reduce the size. I am not sure if this is because you have increased or decreased the ability to flow air or if it is because you have increased or decreased the surface area and weight of the top. Either way, it has changed the overall mass of the guitar and tone will change.

You are on the right track and you are learning. If you go back and re-read the first few posts to your question, you will see that nobody is trying to evade your question, but rather to tell you what you already know to be true. I come from an engineering background too but I tend ot overthink the structure of guitars. I think this may be a problem for you as well. Acoustical engineering is a totally different approach to building than mechanical engineering. It requires a different approach to building guitars and I think you are learning this quickly.  

_________________
Reguards,

Ken H

It's possible to address your lesser question of the outcome of thinning the back, but only just, and only in a limited way.

Mike has talked about the 'reflector' vs 'active' back theories. My thinking (at the moment, this could change) is the you need the back to do some of each, in different ranges.

First: all of the energy that makes the back vibrate has to come from the top. The strings aren't attached to the back. Almost all of that energy vibrating the back gets to it through the mass of the air in the box, and almost none through the sides, as far as I've been able to tell.

Second: the back is usually heavier than the top. For a given amount of energy input, the back is likely to move less. One mitigating factor is that back woods often have less damping than top woods: the back tends to dissipate less energy as it vibrates, and this can allow it to act as a 'flywheel', building up a significant store of energy over time, at least at particular frequencies.

We can see that the back is less likely to make direct contributions to the sound of the guitar than the top. It tends to move less, it is not directly driven, and it's in the back, where a lot of the sound output is going to be lost anyway, especially if it's up against somebody's beer gut.

As far as I can tell, the only back resonance that actually adds to the output of the guitar is the 'main back' mode, and that only makes a large contribution if it's tuned so that it can work effectively with the 'main top' resonant mode. Generally speaking, this requires a back that is thin and/or lightly braced. This is alow-range phenomenon, so in that low range, the back probably should be 'active'.

The higher order back resonances do contribute to the timbre of the guitar, though, evwen when they don't enhance the power output. Most of the time they show up as dips in the spectrum that end up adding a lot of 'tone color' to the sound. The trick is to have enough of them, and have them be deep enough to be effective in adding color, but not to have so many, or so wide or deep, that they cost you too much power. Low damping helps there, by making the dips narrow. High mass helps keep them from being too deep: if the back's not moving much it won't be loosing you too much energy. It's no accident that dense, low damping woods like Brazilian rosewood have become standards for backs. In this range, normally you can think of the back as a 'reflector', but not a really good one.

There are, of course, other standards. Mahogany backs often end up much lighter than rosewood ones, and this is one of the things that gives those guitars so much 'punch'. The standard Flamenco back wood, cypress, has quite low density, but it's one of the few 'light' woods that also has notably low damping. The 'peaky' spectrum helps give Flamenco guitars that 'bright' sound.

As for your actual question: thinning a back from 4mm to 2mm, there is probably no general answer to that. Is the back in question balsa, mahogany or rosewood? What's the bracing like? What size of guitar, and what's the top? One can go on.

Everything about the guitar is concerned with balance and the way things relate to each other. I can say that, in the dissipated days of my mis-spent youth I once made a classical guitar with a very thick back, and had the same dissapoiting result you did. It was one of the things that propelled me down the path I have followed since of trying to understand these darn things. I'm never going to understand them completely: I look on it as job security.        &nb sp; 

Snip: "...in the dissipated days of my mis-spent youth I once made a classical guitar with a very thick back..."

I think you are being a little hard on yourself, Alan. This, at least, seems to be an internally contradictory statement in my opinion...

But, perhaps, you are just being consistently humble. :)

_________________
"Building guitars looks hard, but it's actually much harder than it looks." Tom Buck

Mr.Carruth,
thank you.
you have made my day.

Mr.Courtright,
i like your style!
do share if you ever write poetry.

I haven't built near as many guitars as some of the folks on here...but I have been into guitars & guitar building pretty heavy for the last couple of years. I also spent about 7 years designing industrial equipment; a good part of this job consisted of vibration analysis.

For what it is worth, here are a few of my thoughts with regard to vibration in guitars, along with a dose of that vibrations class I had back when (I'm a mechanical engineer by trade). I hope this is interesting and at least partially true.

First off, there are two parts to looking at any vibration problem. The first part involves looking at resonance and the second part looks at damping. Resonance tells you what note is being played and the damping characteristics tell you how long it will be played. All "notes" are not damped equally.

All resonant frequencies are calculated through some form of the following equation: f= (k/m)^.5
~f = frequency
~k = stiffness
~m = mass

If you make the mass go up, and keep the stiffness the same, you will achieve a lower tone. Similarly, if you increase the stiffness more than you increase the mass, you will achieve a higher tone.

Any object has as many potential modes of vibration as you would care to predict, but in most cases there are a few dominant modes that really stand out. The equation for resonance above applies for whatever modes of vibration may exist. If you look at something like a cylinder you could have lateral modes (jump roping), torsional modes (twisting), and even combinations of the two. In practice, engineers use FEA (Finite Element Analysis) to predict these modes. This is really cool stuff. The same techniques that are used to model composite structures (carbon fiber, fiberglass, etc.) are exactly what you would use to model wood structures. FEA can show you graphically how stuff vibrates as well as tell you the resonant modes.

I don't really subscribe to either the reflector theory or the active back theory. I'm positive that it is possible to tune a back to a top, but I'm not sure this is practical as it would probably only be in tune for a particular set of frequencies (I'm at pretty much the same conclusion as Mr. Carruth). I guess I subscribe to the "everything is active, but somethings are more active than others" theory.   

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http://www.PeakeGuitars.com

Lots of good info, every day this forum is great!

Maybe you should start with a know base, that is, standard Dreadnaught design (say Martin) G.Mahogany, and Indian Rosewood body, G.Mahogany neck, Ebony fingerboard, bridge. Sitka top and bracewood, and Indian Rosewood Bridgeplate , 25.4 scale Top radius 25' back 15' 

Factory Specs are XXXXX

If the back was thicker, it would do this

If the X brace stock was thinned out (it would be a D-35 sound) etc. 

 If the lower basebar was thinned it would have this effect

If the transvers bars were thinned to XX it would have this effect

If the neck was rosewood it would have a slight tone this way

This is basicly the basics of 'Luthier Secrets' people say dont exist, but obvioulsy they do.

There is a good DVD for rent on 'Advance Voicing ' by John Mayer. I didnt think much of his first one, but I hear good stuff about his second, it goes over the basics of the top, but you dont actually hear the results.

 

 

Parser wrote:
"I'm positive that it is possible to tune a back to a top, but I'm not sure this is practical as it would probably only be in tune for a particular set of frequencies (I'm at pretty much the same conclusion as Mr. Carruth).?"

Impractical? I do it all the time! So do a lot of other makers, whether they know it or not.

It seems to me that a strong response in the 'bass reflex' range of the guitar can 'put a floor' under the tone of the instrument all the way up. In this respect the guitar has a lot in common with the violin. In the best pschycoacoustic experiment I know of on violins, Dunnewald found, from measurements and audience response of about 700 instruments, that the a strong low range response was absolutely necessary. If it lacked that you didn't have a good fiddle, no matter what else it did.

Most of the actual power output of the guitar comes from the bass reflex couple. Tuning the back to work with the top enhances that. It seems that the exact pitches of the modes are less important than how well they are working together. In other words, it seems to be the shape and height of the spectral envelope that counts, rather than the exact pitches of the peaks and dips, at least in terms of qulaity of tone. The pitch values will help determine how useful the guitar will be for a particular style: Dreads have lower pitched mode tunings than OMs, and archtop jazz guitars are much higher pitched than most. but a good Jazz box and a good Dread will have spectra that are remarkably similar in form when you take the pitch displacement into account.

BTW, a good example of a 'perfect' reflector back would be an Ovation. I've never been able to scare up much in the way of resonant modes on one of those, and I'm sure that's part of the characteristic tone. It's not that the shape 'focusses' the sound so much as that it's simply dead.