Acoustical effect of back woods

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Trevor Gore
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Acoustical effect of back woods

Post by Trevor Gore » Thu Apr 04, 2019 8:53 pm

According to the Journal of the Acoustical Society of America (of which esteemed body I am a member) one of the most downloaded articles from the last year published in The Journal of the Acoustical Society of America is this one:

“Effect of back wood choice on the perceived quality of steel-string acoustic guitars” (Samuele Carcagno, RogerBucknall, JimWoodhouse, Claudia Fritz and Christopher J.Plack). Some fairly famous names there.

https://asa.scitation.org/doi/pdf/10.11 ... ?class=pdf

This article has been made free to read, download, and share for a limited time (apparently).

After 16 pages of intense analysis their conclusion is:

“The results of our study indicate that steel-string acoustic guitars with backs and sides built using traditionally prized, expensive, and rare woods are not rated substantially higher by guitarists than guitars with backs and sides built using cheaper and more readily available woods. The poor ability of guitarists to discriminate under blinded conditions between guitars with backs and sides made of different woods suggest that back wood has only a marginal impact on the sound of an acoustic guitar”.
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guitarrista
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Re: Acoustical effect of back woods

Post by guitarrista » Thu Apr 04, 2019 9:02 pm

Thank you, Trevor!
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souldier
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Re: Acoustical effect of back woods

Post by souldier » Thu Apr 04, 2019 9:19 pm

The conclusion is not surprising at all and I expect it to be the same for nylon stringed instruments. I also wonder if it really matters whether the back and sides are all solid or laminated.
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Echi
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Re: Acoustical effect of back woods

Post by Echi » Fri Apr 05, 2019 1:34 pm

My conclusion is that the material used for the sides ( or used to laminate the sides) makes more difference.
I don’t mean you can spot the difference of sides made with woods of the same family but in the case of flamenco guitars (otherwise similarly built and with the same back) you will notice if the sides are made with cypress or rosewood: faster and clearer attack of the note with the cypress.
I suppose it’s not just a matter of mass.

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Julian Ward
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Re: Acoustical effect of back woods

Post by Julian Ward » 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.
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Re: Acoustical effect of back woods

Post by printer2 » Sat Apr 06, 2019 3:28 am

Trevor could you take a look at page 3536 of the paper and comment on the chart with the admittance curves of the test guitars. I am wondering if there should be an extra impedance peak, it looks like there is no back peak. I would have just chalked it up to the instrumentation but the Yamaha guitar seems to have an extra dip and peak in the right neighborhood.
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Re: Acoustical effect of back woods

Post by zupfgeiger » Sat Apr 06, 2019 4:18 am

Echi wrote:
Fri Apr 05, 2019 1:34 pm
My conclusion is that the material used for the sides ( or used to laminate the sides) makes more difference.
I don’t mean you can spot the difference of sides made with woods of the same family but in the case of flamenco guitars (otherwise similarly built and with the same back) you will notice if the sides are made with cypress or rosewood: faster and clearer attack of the note with the cypress.
I suppose it’s not just a matter of mass.
Exactly. Ask Daniel Friederich.
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James Lister
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Re: Acoustical effect of back woods

Post by James Lister » Sat Apr 06, 2019 9:16 am

No surprise to me.
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Stephen Kenyon
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Re: Acoustical effect of back woods

Post by Stephen Kenyon » Sat Apr 06, 2019 9:51 am

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 actually agree and think they do make a difference, but would point out that its possible builders match their best materials and efforts in the matter of the soundboard, with the "perceived-widely-to-be-best" woods on the back.
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Martin Woodhouse
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Re: Acoustical effect of back woods

Post by Martin Woodhouse » Sat Apr 06, 2019 10:40 am

A lot of the disagreement about this (I think) comes from the fact that there are two different questions which often get conflated:

1. If you use different woods for the back/sides, with a standardised design, without any adjustment to the thickness and bracing to compensate for the different properties of the wood, will there be a difference in sound between the different woods? Answer: probably yes (at least to some extent, on average)

2.If you use different woods for the back/sides, can you adjust the thickness and bracing to compensate for the different properties of those woods? Answer: yes (or at least mostly, and probably to a point where nobody can really hear a difference)

The stiffness and weight of the back/sides do affect the sound, and different woods do have different density and stiffness, so if you build exactly the same design with different woods you will get a different sound. If you adjust the design to suit the wood you’re using, then you can get (virtually) the same sound, as this and other studies have shown.

If you're comparing factory made guitars, which are made to a standard design, then there may be a difference in sound between the different back/woods. If you’re commissioning a hand made guitar, then the luthier should be able to get the sound that you/they want with any reasonably sensible wood.

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Re: Acoustical effect of back woods

Post by Eduardo Bossa » Sun Apr 07, 2019 1:32 am

Very interesting discussion!

Martin, I believe you are right when you state that we are answering different questions. Perhaps another question is:

If you do hear a difference in sound, can you accurately identify the wood species every time?

Sometimes we do hear tonal differences, even significative ones, but the problem doesn't seem to be determining if the back has a contribution on the guitar's overall tone. The problem seems to be adscribing specific tonal characteristics to a specific wood species. It is apparently very elusive and sensitive, varying tremendously with minute changes in guitar construction, the particular piece of wood used, etc.
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Trevor Gore
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Re: Acoustical effect of back woods

Post by Trevor Gore » 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
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Alan Carruth
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Re: Acoustical effect of back woods

Post by Alan Carruth » Mon Apr 08, 2019 4:12 pm

On reading the article I think it would have been better to title it something like "Makers can largely compensate acoustically for the different properties of back woods". Manufacturers can't, so, as with tops, to the extent that the average properties of different species differ, you're more likely to hear that in a factory instrument than one from a luthier.

Most of the makers I know are on board with the idea of using 'alternative' woods, and would do so far more if we could only sell them. I have had people stand and look at an oak guitar for a half hour and refuse to even try it.

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Chris Sobel
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Re: Acoustical effect of back woods

Post by Chris Sobel » 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.
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Re: Acoustical effect of back woods

Post by regow » Mon Apr 08, 2019 7:56 pm

Aside the woods, what effects can be expected of a three-part back, with a wedge like part in the middle?

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