Acoustical effect of back woods

Construction and repair of Classical Guitar and related instruments
chiral3
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Re: Acoustical effect of back woods

Post by chiral3 » Mon Apr 08, 2019 8:08 pm

Thanks for the link and the summary.

What I don’t like about these studies, no matter how rigorous, is that they are usually univariate in the sense that they focus on one dimension of perception.

As others have stated the result is no surprise. There are other studies that I think are relevant from a multidisciplinary perspective. I can’t recall citations right now, but I’ll paraphrase one to give a sense: there was a study using wine where people were randomly given various wines ranging from bargain juice to Napa cult and also told their prices in some random assignment. fMRI suggested that awareness of the higher price point correlated with more brain involvement in a number of important regions which was further correlated with higher self-reported enjoyment. If this translates to instruments it’s arguable that people could legitimately perceive and enjoy music more from an instrument made from exotic woods over their non-exotic analogue even though they sound the same.
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Julian Ward
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Re: Acoustical effect of back woods

Post by Julian Ward » Mon Apr 08, 2019 8:13 pm

Chris Sobel wrote:
Mon Apr 08, 2019 7:39 pm
I would agree with Trevor in the sense that how the back is built can make a huge difference in sound. We recently re-backed a guitar that had laminate Brazilian for the back with a heavier solid Brazilian back... the sound difference was night and day.

I personally have never been able to make maple or cypress sound like excellent Brazilian... and to boot; the aesthetics of rosewood are second to none IMO.
Exactly, demonstrating that a solid back (and sides) makes a huge difference vs laminate. There are many people who seem to believe this is not so.
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Eduardo Bossa
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Re: Acoustical effect of back woods

Post by Eduardo Bossa » Mon Apr 08, 2019 11:36 pm

Trevor Gore wrote:
Mon Apr 08, 2019 12:22 pm
One can get very picky about the way these sorts of experiments are performed, but I'll try to stay away from that. However, trying to identify differences in sound due to the back woods when the T(1,1)1's and T(1,1)2's range over a semi-tone on non-live back instruments (no T(1,1)3's apparent, Fig 1A) doesn't give you much of a chance.

How the back and sides are treated can make a huge difference to the way a guitar sounds. Live vs. non-live backs, mass loaded sides etc. are ample proof of that. But what wood you use, much less so, UNLESS you systematically rely on the average species properties rather than build technique to effect the differences.

What you hear when a guitar is played are the modal resonances, defined by their center frequencies, amplitudes and bandwidth. If you can measure the mechanical properties of the wood (density, Young's modulus, damping) and use the right, well documented techniques, you can match the important modal resonances of guitars made of different woods pretty closely. That's what builders are doing (whether they realize it or not) when "The guitar maker, by treating each back in the way that his experience suggested was best, has to a very large extent compensated for any physical differences between the types of wood". So a wide range of woods can be made to sound alike, which means, of course, that players can't pick the difference, and guitar builders have a much greater range of woods at their disposal. I know that I can't pick wood species on my (modally tuned) guitars. All we need to do now is stop the dealers from perpetuating the species myths and get them to promote some of the more sustainable alternatives. :D
Great info there, Trevor. Thanks!

I tend to agree with your experience. One thing I have observed is that certain wood species (i.e. indian rosewood) tend to exhibit less variation in mechanical properties from piece to piece than other woods (i.e. maple). In my experience, these woods that exhibit less variation make it easier to hit the sweet spot I'm looking for in the completed instrument. Even if I do make significant variations in thickness and/or bracing, they seem to want to behave as I would expect them to. On the other hand, there are other woods that allow me to hit that sweet spot if I carefully match the mechanical properties of another known back, but if I so much as end slightly off my target, the result can be less than ideal. In other words, very sensitive to minute changes in construction. There is yet a third group of woods I have experimented with, and no matter how precisely I match the mechanical properties to a known back, I can never get them to fully satisfy me. They make fine guitars, but I don't get that specific tone quality I do get with others.
Do you agree with this according to your observations? I guess my backs are what you would call active backs, as their contribution in tone is quite noticeable.
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guitarrista
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Re: Acoustical effect of back woods

Post by guitarrista » Tue Apr 09, 2019 12:17 am

chiral3 wrote:
Mon Apr 08, 2019 8:08 pm
As others have stated the result is no surprise. There are other studies that I think are relevant from a multidisciplinary perspective. I can’t recall citations right now, but I’ll paraphrase one to give a sense: there was a study using wine where people were randomly given various wines ranging from bargain juice to Napa cult and also told their prices in some random assignment. fMRI suggested that awareness of the higher price point correlated with more brain involvement in a number of important regions which was further correlated with higher self-reported enjoyment. If this translates to instruments it’s arguable that people could legitimately perceive and enjoy music more from an instrument made from exotic woods over their non-exotic analogue even though they sound the same.
I remember that wine study as well. Great point transferring the results to this field.

BTW, how does the study discussed in the thread here relate to the Leonardo Guitar research project and the blind listening tests? If I recall correctly, that blind study found almost complete inability to differentiate between the sound of guitars with tropical vs non-tropical woods (traditional vs. non-traditional woods) as well as between different guitar makers.
Last edited by guitarrista on Tue Apr 09, 2019 12:37 am, edited 1 time in total.
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Re: Acoustical effect of back woods

Post by Alan Carruth » Tue Apr 09, 2019 12:32 am

My experience suggests that using a three-piece (or more) back makes no particular difference in the sound, so long as it is carefully done.

It's well known that there is a strong 'placebo effect' in the perception of guitar sounds; people 'listen with their eyes'. Just as in medicine, pushing a placebo can lead to undesirable outcomes. In medicine you get people paying lots of money for useless 'drugs', and refusing treatments that are known to work, to the detriment of their health. In the guitar world we are increasing the pressure on woods that are becoming rare and endangered when substitutes are available that are common and often under utilized. Osage orange is practically a drop-in replacement for Brazilian rosewood in it's mechanical and acoustic properties. Given the extent the B&S affect the tone the substitution should be imperceptible. The primary use for Osage orange wood these days is for fence posts. That's a worthy use, of course, especially since the posts are reputed to outlast the holes they're put in, but guitar backs would, IMO, be a higher use.

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James Lister
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Re: Acoustical effect of back woods

Post by James Lister » Tue Apr 09, 2019 8:37 am

Alan Carruth wrote:
Tue Apr 09, 2019 12:32 am
My experience suggests that using a three-piece (or more) back makes no particular difference in the sound, so long as it is carefully done.
That has been my experience as well.

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gjo
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Re: Acoustical effect of back woods

Post by gjo » Tue Apr 09, 2019 8:44 am

Now I am only waiting for the blind test police to join ...

Or, as Charlie Brown would say, „Good grief!“

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Re: Acoustical effect of back woods

Post by James Lister » Tue Apr 09, 2019 9:04 am

guitarrista wrote:
Tue Apr 09, 2019 12:17 am
BTW, how does the study discussed in the thread here relate to the Leonardo Guitar research project and the blind listening tests? If I recall correctly, that blind study found almost complete inability to differentiate between the sound of guitars with tropical vs non-tropical woods (traditional vs. non-traditional woods) as well as between different guitar makers.
To quote from the LGRP conclusions:
This test shows that the distinctive sound qualities and the supposed nature of T’s [tropical] and NT’s [non-tropical] were not distinguishable one from the other.
I was involved in the blind listening/playing tests conducted at Newark College on the classical guitars made for the project by the students. One of the issues with the project was the fact that the guitars were all made by students, so that there was bound to be more variation from guitar to guitar (regardless of woods used) than if the guitars had been made by professional luthiers. The first pair of guitars made by the students were quite different, and the tropical guitar was perceived to be "better" than the non-tropical by the majority of listeners and players (although certainly not by all). My own opinion was that the differences were largely due to small differences in the soundboard/strutting, and to the significantly lower mass of the bridge of the non-tropical guitar.

When the second pair of guitars were made, we took much greater care in matching the soundboards (I did the final tuning myself), and used better matched woods for the bridges (laburnum and Indian rosewood). As far as I can recall, no-one was able to reliably identify which guitar was which.

Just to pick out a couple of significant points from the results of the online listening test (conducted by the LGRP), out of 226 listeners who responded, less than 5% claimed to be able to distinguish the tropical from the non-tropical guitars, and of those who claimed they could, their success rate in identifying the nature of the woods used was less than 10%.

None of the above proves that there is no difference between guitars made from tropical woods and those made from non-tropical woods. What both studies do show clearly is that it is VERY difficult to hear those differences when the guitars are otherwise well-matched.

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Martin Woodhouse
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Re: Acoustical effect of back woods

Post by Martin Woodhouse » Tue Apr 09, 2019 11:15 am

Tropical vs non-tropical seems like an odd choice of question: tropicalness isn’t a physical characteristic of the wood, it just tells you where the tree grew/originated... Is the question really 'relatively high density woods which tend to grow in tropical climates vs relatively medium/low density woods which are more common in temperate climates?' Or maybe wood from warmer countries gives a warmer sound? :-)

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Re: Acoustical effect of back woods

Post by robert e » Tue Apr 09, 2019 2:06 pm

Martin Woodhouse wrote:
Tue Apr 09, 2019 11:15 am
Tropical vs non-tropical seems like an odd choice of question: tropicalness isn’t a physical characteristic of the wood, it just tells you where the tree grew/originated... Is the question really 'relatively high density woods which tend to grow in tropical climates vs relatively medium/low density woods which are more common in temperate climates?' Or maybe wood from warmer countries gives a warmer sound? :-)
This is an odd way of agreeing with the the study's conclusion, which is that guitar buyers' preference for tropical woods is irrational. :) The choice of question is odd, yes, because it was precisely on point.

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Re: Acoustical effect of back woods

Post by James Lister » Tue Apr 09, 2019 2:23 pm

Martin Woodhouse wrote:
Tue Apr 09, 2019 11:15 am
Tropical vs non-tropical seems like an odd choice of question: tropicalness isn’t a physical characteristic of the wood, it just tells you where the tree grew/originated.
Fair point, but in the case of the Leonardo project, the object was specifically to investigate the use of alternative, non-tropical species to reduce the reliance on rare/expensive tropical hardwoods, many of which have trade restrictions imposed (by CITES and the Lacey Act), or are being illegally harvested.

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Re: Acoustical effect of back woods

Post by Alan Carruth » Tue Apr 09, 2019 4:23 pm

The point of the study of B&S woods in question was that makers can compensate to a large extent for variations in wood properties by varying the way they make the guitars. It has nothing to do with where the woods originate.

When you think of it, though, there are a lot of tropical woods that share the high density and Young's moduli and low damping of the canonical rosewoods. What seems rarer in tropical woods is lower density, although cedro comes to mind. Perhaps it has to do with hot climates and high pest loads.

There are non-tropical woods that share these properties, but they tend not to be commercial species. High density is usually not a desirable property in construction wood, although it's useful in cabinet woods to some extent. Most of the really dense 'local' woods tend to be light in color, and often plain in figure. Most cabinet woods are either dark or have some 'fancy' grain, or both. For most purposes low damping is not a factor, and it may be less of one in guitars than many people think.

My experience suggests that when trying to compensate for varying wood properties in the back, density is the hardest to 'fake', with damping being far less of an issue. Obviously, the closer the properties of the substitute are to those of the original, the easier it should be to make the swap.

This brings to mind the case of walnut. All the tests I've made of wood samples suggest that walnut (particularly American black walnut) tends to be a very close match for soft maple, either American Red maple or European maple. The received wisdom is that maple and rosewood guitars sound very different, and yet many people accept walnut as a reasonable substitute for rosewood! I suspect this is another example of listening with the eyes.

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Re: Acoustical effect of back woods

Post by robert e » Wed Apr 10, 2019 9:28 pm

Very interesting and appreciated, Alan, as always.

I assume the question regarding tropical species comes up because of the Leonardo project. It's in the project's stated mission, after all. This study on the other hand speaks of "sustainable" vs "non-sustainable" woods, but these terms seem to refer to non- tropical and tropical woods, respectively. AFAIK, the fact that these woods have market prestige, at least in part due to misconceptions about all the factors that determine the audible characteristics of a guitar, is a primary rationale for both that project and this study. (Leaving aside exactly why this matters to various interested parties.)

And it is an odd question to ask, if we ignore all that socio-economic-ecologic context, but in reality it's the reality that inspired the question that's wacky.

As you and other luthiers have said in this thread and elsewhere, most guitar makers know better but that's less than half the way there. Most dealers and buyers, for various reasons, act according to traditional misconceptions or at least practices.

In the US there's at least one major producer of acoustic guitars that appears to be highly motivated to change these perceptions and has the market footprint and prestige to possibly succeed, which gives me hope, as does every study like this that comes along and these discussions that follow.

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Re: Acoustical effect of back woods

Post by attila57 » Wed Apr 10, 2019 10:16 pm

Julian Ward wrote:
Fri Apr 05, 2019 7:21 pm
This is simply not true at all. I have demonstrated this in my local guitar shop when the assistant said the same thing to me. I said he was mad, and the back and sides make a big difference. I 'played' the guitars to prove it. Players that cannot tell are those that have a soft technique or rather a 'weak/quiet' one. In this case they are simply not able to drive the top enough to get that sound fully moving through the body. I can absolutely tell if a guitar is solid wood or laminated plywood - on nylon strung and on steel strung guitars. I can also hear a lot of difference between the mahogany bodied vs rosewood guitars of the same guitar in the steel string category. And a big difference in the sustain of rosewood vs cypress. But you really have to 'dig in' and "play' the guitar to hear it easily.
I fully agree.
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Re: Acoustical effect of back woods

Post by Martin Woodhouse » Sun Apr 14, 2019 7:52 am

I meant to say that 'tropical vs non-tropical' seems like an odd question from the perspective of this study, which is more specific, and more about trying to relate differences in sound to differences in physical characteristics of the wood. The Leonardo project is a very good idea in terms of public education, demonstrating what can be done with "non traditional" woods.

Sustainablity-wise, I think it’s more important to consider the provenance of the particular piece of wood that you’re buying/using, rather than lumping all tropical hardwoods together and assuming the worst: there are sustainably managed tropical hardwood plantations, and unsustainably managed non-tropical forests. Though I agree that one key to any kind of sustainable future for guitar making is to diversify and use a wider range of species, so anything which helps to dispel the myth that there are a few magical “tonewood" species is worthwhile.

Mainly, we seem to be talking about rosewood vs non-rosewood though: I don’t remember ever having a customer ask for 'tropical hardwood', but people ask for rosewood back and sides, because it’s what they know and expect. Most people don’t really seem to mind what the neck, fingerboard and bridge are made from, as long as they work and look good, so the main issue really is with the rosewood back and sides. When I tell people that I’m not using rosewood any more, most of them don’t mind choosing something else, some are happy about it, and some I never hear back from... I suspect that dealers are more "traditional" than the average guitarist though.

Part of the problem is that the reality is more complicated (in various ways), and much harder to explain/sell than a simple species-based or climate-based division.

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