Official Luthiers Forum!

(This was started in a tangent to the thread on "Boat Anchor Acoustics." Rather than further hijacking that thread's topic, I felt it was worth starting a new thread. I'll quote the posts, to this point. Though this looks like a dialog between me and Alan Carruth, I am hoping to get feedback from anyone and everyone who is interested in the topic.)

So far...

Alan Carruth : 25 July 2005 at 3:14pm

On acoustics it seems that a light _top_, particularly in the lower bout, is a good idea. There's only a limited amount of horsepower in the strings, and that's the part that makes most of the sound. If it's too heavy the strings won't be able to move it much.

From what I can tell now it might be a good idea to have a fairly heavy back. The back tends to 'steal' energy from the more effective radiator of the top, so making it hard for the back to move (for the most part) seems like a good idea.

Some poeple swear by heavy liners. I dunno.

There is a resonance of the whole body moving like a xylophone bar at low frequencies that can work with the air moving in and out of the soundhole to produce sound in the bass range: arounf low G or so. Normally this 'neck mode' is too low in pitch to help out, but sometimes you can get it. Since the headstock and neck are moving a lot it helps to make them as light and stiff as you can.

You can hear this 'neck mode' if you hold the guitar up so that it hangs freely by pinching the neck right up around the nut between your thumb and finger. Tap on the back of the headstock up near it's top end, and listen for the low pitched sound as the headstock moves. The air resonance can be found by singing into the soundhole if you're a bass or baritone. You can also pinch the low E string as tightly as possible between your thumb and finger up near the nut, and pluck it while sliding your hand up and down to change the pinch point. The string will not ring out clearly, but will be noticably louder at the pitch of the 'main air' resonance that you are looking for.    

I tend to keep the neck light if I can, but I have found that ample can be stiff enough to work ut well, particularly if you make a deep V-shaped profile.

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Alan Carruth

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Dennis Leahy : 26 July 2005 at 1:22pm

[Quote=Alan Carruth]...From what I can tell now it might be a good idea to have a fairly heavy back. The back tends to 'steal' energy from the more effective radiator of the top, so making it hard for the back to move (for the most part) seems like a good idea. [/QUOTE]


This concept intrigues me. From what I have read of Kasha/Schneider theory, the neck should be as stiff as possible. From a number of top-notch classical builders, the sides should be as stiff as possible (thus the 2-ply rims, as well as solid linings on the rims.) Now, you're saying the back should be as stiff as possible.

I guess that leaves the soundboard, top bracing and the bridge as the only components that should be engineered to move.

Do your findings conflict with the builders that are using softwood for backs (and engineering the backs more like tops), or is that a whole different animal? Is the problem (of robbing energy from the top) with typical backs because the back's bracing, density, and MOE make it impossible for most back to resonate at the same frequency as the top?

Dennis

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Alan Carruth : 26 July 2005 at 1:35pm
Dennis brings up a good point.

Backs are far more variable than tops, and this is not surprising. The top accounts for most of the sound, and poeple have pretty well homed in on the 'best' ways to make them. The back presents you with the opportunity to employ different strategies, depending on the wood and tone you want to get from it. Most makers go with the 'heavy and stiff' strategy, for the reasons I posted.

Another workable stategy is to use a light back that can move a fair amount of air itself. Mahogany and Spanish Cypress backs don't weigh much, if any, more than the tops do, and can move comperable amounts of air. Most of this, of course, is pumped through the soundhole, so the main beneficiaries of it are the lower note around the 'main air' resonance. It also adds a lot to the 'attack' part of the sound, and (usually) cuts down on sustain, and this seems to translate into a 'bright' tone even if there is not a lot of energy in the high frequencies.

As far as making the soundboard and bridge as the only 'moving' parts, that's the Smallman strategy in a nutshell. It's also the way resophonics work, and I'll nte that the one Smallman I got to spend any time with sounded to me like a wooden Dobro. I gues I'm just not the player that Williams is....

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Alan Carruth

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Dennis Leahy : 27 July 2005 at 9:27pm
Alan, Thanks for the clarification, and the info.

I have a hunch that the lightweight backs that are engineered to move like tops would work best if they are engineered to flex in tandem (or more precisely, in sync) with the top. Otherwise, I envision something akin to "noise cancellation" (except, this is hopefully not noise!)

This also makes me think that a stiff back would not be advantageous on a guitar without a soundhole.

Question: Can the sonic advantage of a stiff back be demonstrated by simply holding or removing a hand on the back of the instrument when strumming, or does too much dampening come into play to hear the advantage?

Dennis
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Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

Then again, I guess I could leave the back off, like a banjo.

DennisDennisLeahy38562.8404282407

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Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

Another option is wooly mammoth skin for the back, stretched tightly and tied with sinews.

Dennis

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Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

As to what part the back actually plays in these theories, I don't know. I always went with the philosophy that the top is the most important in absorbing the vibration of the strings. Therefore my focus in ways to improve the top performance was to let it be as flexible as possible without sacrificing the strength it needs to support the pull of the strings over the long haul. I experimented with many ideas and the best sounding guitars I made (and this was always one trick I swore to secrecy !!) I used a walnut bridge. My theory behind this was...it is a very light wood. Perhaps not being as strengent and heavy as Ebony or Brazilian Rosewood and being lighter, this allowed the top the freedoom of vibrating more. And I only share this because everyone here has shared so many of their ideas and tips that I felt compelled to share it. Maybe someone else can try it and see if they have the same results.

Yes, I think there is 100% consensus about the top being the major player. I think most would then list the top's bracing and bridge and saddle, in that order, next. (Though it can easily be argued that the entire top/braces/bridge is a single structure.) Brazilwood (hey, what is your Real name anyway?), I would recommend that you put your Walnut bridge suggestion in with a bridge thread as well, so that when someone searches the archives for "bridge", they may find your interesting suggestion.

My questions have to do with the role of the back, and how stiff a back should be, anywhere from rigid to loose, and why. I suspect that a lot of builders have followed "on the shoulders of giants", and just copied the back thickness and bracing from an existing model or from a plan. And, some may think the role of the back is so minor, it's not worth the discussion. But, I do hope that we can kick some ideas around. (Thanks to Alan Carruth for his thoughts.)

Dennis

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Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

What about drums? head on top, rigid sides and head on the bottom. Can't argue with the projection. That is the model I follow.

I don't for a moment suggest I have any scientific reason for it. It just makes sense to me. I (again intuitively) don't think wave cancelation would be a problem. It might even help eliminate any possible standing waves. Different reasonant frequencies and different decay rates.

   I can see wanting the back to move in sync with the top, but I think we need to throw another factor into the mix. What about the player himself damping the action of the back with a big belly, or hugging up to it when playing? Wouldn`t this hamper, if not defeat the intention?

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[QUOTE=DennisLeahy]

My questions have to do with the role of the back, and how stiff a back should be, anywhere from rigid to loose, and why. I suspect that a lot of builders have followed "on the shoulders of giants", and just copied the back thickness and bracing from an existing model or from a plan. And, some may think the role of the back is so minor, it's not worth the discussion. But, I do hope that we can kick some ideas around. (Thanks to Alan Carruth for his thoughts.)

Dennis

[/QUOTE]

Dennis I think the role of the back in a guitar is marginal. We are all agreed here that the top/brace/bridge system is way dominant, probably up to 90% or so in the sound of the guitar. I use very stiff sides, usually double skinned with a layer of mahogany inside whichever B&S wood I'm using, the sides are only to hold the back away from the front right?. For the back I generally now use parabolic X brace on the lower bout so I guess I'm in the stiff in the middle flexible at the margins school (same as for the top).

By the way your "on the shoulders of giants" quote. When I did my PhD at Cambridge, I lived and worked in Isaac Newton's old rooms at Trinity College for three years! Couldn't say he helped me much with my maths though.

Colin

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I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

Agreement seems to be that the effect of the B/S is nominal. Then why do people salavate over BR and African Blackwood?

[QUOTE=bob J] Agreement seems to be that the effect of the B/S is nominal. Then why do people salavate over BR and African Blackwood?[/QUOTE]

Because, as Mario is so fond of saying, it's all in the details. They may not be the main source of the sound, but they certainly colour it, and have at least some effect (particularly the back).

Besides, often more importantly (rightly or wrongly so), zooty wood pretty!

One of the loudest and best sounding guitars I have built to date has back and sides that are .060" thick. The back does play an active role in my designs.

There seems to be two schools of thought on build structure:
1) Thick, stiff, heavy, reflective & passive.
2) Thin, flexible, light and active

I have more experience with the latter. I am not saying one is better than another. If you study the older Martins and Gibsons from the pre-WWII era they were built extremely light. Even into the 1950's they were still lightly built.

Put a guitar in a case and strum the strings. Now lift the neck so the back isn't dampened by the case and strum the strings. What you hear is the back taking an active role.

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Walnut bridges: yup. You can make it fairly large and stiff without adding too much weight. Tends to enhance the highs. I like them on cedar tops as they allow you to get a nice big footprint to keep te max stress level along the back of the bridge below the peeling strength of the cedar.

We don't usually talk about the top 'absorbing' the string vibration: that word implies some sort of loss, and we sure don't want to lose any of the little bit of power we've got. The top is a transducer: it converts the mechanical forces of the string into sound. As such it is the main player in the game, since all of the other parts of the guitar get whatever sound power they have to work with from the top more or less directly.

Guitars do seem to fall naturally into 'light back' and 'heavy back' groups. Whichever way you do it I've found it to be advantageous to 'tune' the back to work effectively with the 'main top' resonance. For one ting, it can give you more power in the bass and midrange without the need to make the top thinner, which, in turn, helps in keeping the treble response up. Otherwise the two ways of making the back end up sounding a bit different, but I'd hesetate to say that one is intrinsically 'better' than the other. It all depends on what you're trying to do.

Thanks for the input, from all of you. My original notion of creating an active, flexible back will still be my intent for the first one, and at some point I may try a stiff back on another instrument, to try to hear the difference.

Dennis

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Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

I accidently sanded a Quilted Maple back to thin a while back (the new General 25" sander sure takes it down fast. I asked advice on what people thought and the consensus was that it was to thin, especially since it was being built for a very high profile artist. Mario sugested I get a new back for the ordered guitar and save this back and build another guitar and compare. I did just that, braced the thin back with BC Fir to stifen it up a bit. as I compare the two guitars the thin back seems to have had very little if any affect on tone. The only problem is that it seems to have developed a bit of a ripple effect where each back brace is.