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Hi, I was just doing some bracing on a Small Concert guitar I am currently building, and I noticed that the plans APG-01 show the soundboard face brace opening to the treble side. Whereas the Orchestra model APG-14 shows the soundboard face brace opening to the bass side. Or maybe I just read the plan wrong.

Does it really matter which way the soundboard face braces open, treble or bass?

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Michael Lloyd

“I was born to ignorance, yes, and lesser poverties ...
I was born to privilege that I did not see ... I didn’t know it, but my way was paved” – John Gorka

I doubt there's a ceteris paribus difference, but an opinionated customer might think otherwise.

But I think you're reading the plans wrong.

I think the OM plan shows from the inside, while the concert shows from the outside. So it is probably supposed to be open on the bass side too.

That being said, it would only be a fairly subtle difference. Take any guitar you have and reverse the e strings and they won't sound much different...

No telling, I think, without making two identical guitars with mirrored bracing. And we all know how likely that is.

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Peter Havriluk

With the Scott Antes plan, Small Concert - AGP-01, it is drawn as though you are looking through the top. Most plans are drawn looking from the bracing side but that one is not.

I agree with comments that the plans are being read wrong some plans look through the top some plans are view on the brace side of the top. I do not think it makes a noticeable or predictably difference. My understanding is that energy is transferred to the top by the bridge rocking, not vibrations of the string being transferred directly and locally to the top under a particular string. Mirroring the brace pattern looks the same to the bridge.

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http://www.Harvestmoonguitars.com

Thanks for your responses. I have to agree, I had read the APG-01 Small Concert plans wrong.

With the second Small Concert on the go I'll wait and see if there's a noticeable difference. However, as you know there are many things in a build which will influence the sound between the same model.

Thanks again!

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Michael Lloyd

“I was born to ignorance, yes, and lesser poverties ...
I was born to privilege that I did not see ... I didn’t know it, but my way was paved” – John Gorka

Peter Havriluk wrote:

"No telling, I think, without making two identical guitars with mirrored bracing. And we all know how likely that is."

Actually, having tried without success to make identical guitars that sound the same, I'd say it's probably impossible, as least as long as you're using wood.

John Parchem wrote:
" My understanding is that energy is transferred to the top by the bridge rocking, not vibrations of the string being transferred directly and locally to the top under a particular string."

If you're talking about a forward and back rocking of the bridge, due to the tension change, that's a common misunderstanding. I've done several experiments on this, measuring the string signal forces and doing playing and listening tests with controlled mechanical plucks, as well as other ways of driving the strings and 'listening' to the sound, so I'm pretty confident about this.

"Mirroring the brace pattern looks the same to the bridge."

I can get behind that, for the most part.

I built my first Antes plan size 2 with the brace reversed. I have built a lot more from the same basic plan with the lower face brace correctly installed. Not a discernible difference to my ear.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

That dang plan. This post comes once or twice a year it seems. Are there others backwards like that? It’s always the Antes parlor plan.


Sent from my iPhone using Tapatalk Pro

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I think this deserves showing again.

Intonation might be a bit off....
(posted for all the non-luthier "civilians" who want to make their guitar a "lefty")

I was talking about side to side rocking of the bridge

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http://www.Harvestmoonguitars.com

John Parchem wrote:
"I was talking about side to side rocking of the bridge"

OK; thanks for the clarification. The rocking-under-tension-change idea has gotten pretty well embedded, and having spent the time on the experiment that disproves it, I tend to jump on that reflexively every time I see it. Sorry.

Side to side rocking happens, as, indeed, does even the fore-and-aft rocking from tension change, but side to side rocking is sort of a by product. Most of the actual sound is produced by the vibration of the strings perpendicular to the top, which pulls the bridge up and down, and drive the top as a unit in a loudspeaker like motion. There is in most tops a 'cross dipole' resonant mode where one end of the bridge goes 'down' as the other comes 'up', but even if you drive the bridge at the location of, say, the 5th string, using a 'pure' sine wave signal, and right on the cross dipole pitch (on a steel string, usually somewhere near 300 Hz), the loudspeaker like 'monopole' motion still predominates and produced more of the sound. Resonances have peaks at certain pitches, but they can be driven off peak fairly easily for the most part, so what you actually see for motion in the top is a sum of all of the modes as driven from the particular point. It's a lot more complicated than it 'should' be, for sure.

If the bridge doesn’t rock back and forth, what causes the long dipole mode?

Hahahaha... no disrespect, but I remember pondering the same thing while looking at those plans after building 10 Jumbos in a row, with a very clear expectation of what angle the braces should run. for some reason I decided to order the LMI plexiglass template for the parlor (I made my own for the jumbo and dread models) and it showed the "correct" placement of that tone bar.

I haven't tried it both ways, but I also feel like there can't be too much of a difference. Unless we start to factor in the amount of arm contact a given player will have with the instrument.

meddlingfool asked:
"If the bridge doesn’t rock back and forth, what causes the long dipole mode?"

The mode itself is a product of the distribution of mass and stiffness within the top. If you drive the top at that pitch it's likely to move that way.Most tops do have along dipole mode, interestingly enough, often around 350 Hz, both on classical and steel string guitars, despite the differences in the bracing.

John Parchem wrote:
" My understanding is that energy is transferred to the top by the bridge rocking, not vibrations of the string being transferred directly and locally to the top under a particular string."

and I replied:

"If you're talking about a forward and back rocking of the bridge, due to the tension change, that's a common misunderstanding."

As I said in my clarification to him, I spent rather a lot of time looking into this, and have spent even longer responding on line. In this case I was trying to cut my usual lengthy answer down, which may have been confusing. I was not trying to say that it doesn't happen, but rather to rebut the assertion that it's the main way in which the strings drive the top, and produces most of the sound. Now comes the lengthy answer.

If you measure the forces the string exerts at the top of the saddle as it vibrates, as I have, you'll see that there are actually three 'signals' being produced: a 'transverse' signal, based on the frequency the string is tuned to, with it's nearly harmonic partials, a 'tension' signal, based on a pitch an octave higher than the 'transverse', with the upper partials also doubled, and the longitudinal 'zip tone', a compression wave within the string material, with a pitch that is determined by the string length and material properties, and no necessary connection to the pitch the string is tuned to. Although it has some interesting effects on the sound sometimes, the 'zip tone' can be ignored for now.

The 'transverse' signal pulls the bridge along with it. If the string is vibrating purely 'vertically' with respect to the plane of the soundboard, it pulls the bridge and top 'up and down', driving the top as a loudspeaker, generally in the 'monopole' mode. If the string is moving 'horizontally', parallel to the soundboard plane, the bridge will be pulled sideways, which could drive the 'cross dipole' mode. It's a lot harder to get the top moving in that way (try it!), so at a given amplitude 'vertical' string motion makes a lot more sound than 'horizontal' at most frequencies. In practice, a normal pluck has some of both kinds of motion.

The 'tension' signal does indeed pull the top of the bridge toward the nut twice per cycle of the 'transverse' motion the string is tuned to, and this does rock the top fore and aft in the dipole motion. This is not, however, an effective or efficient way to drive the top to produce sound, for three reasons:
1) the 'transverse' force is usually much greater,
2) bridge rocking pulls part of the top 'up' while it pushes another part 'down', so the signal tends to cancel out, and
3) we make tops to resist this sort of torque deformation, while making it fairly easy for the top to move in and out in the monopole mode.

I ran a couple of different tests of this, and got largely similar results. In one I used an alternating current down the string and a horseshoe magnet to produce either purely horizontal or vertical motion of the string at whatever pitch,and it the other I used a mechanical plucker to start the string moving in either plane. A dB meter out in front of the guitar gave a reading of the sound produced. 'Vertical' string motion, perpendicular to the plane of the top, produced about 20 dB more sound than 'horizontal' in both cases. That's 100 times as much power. Horizontal string motion can only really drive the top through the tension change signal, which is limited, as pointed out. Vertical string motion drives the much more effective and easily moved 'monopole' motion of the top.

A later experiment, looking as the effects of string break angle and height off the top, found that the 'tension change' and 'zip' signals can contribute to the sound, not by adding power, but by altering the balance between the partials of the sound. Raising the strings off the top gives more leverage for the torque wise bridge motion that they drive, so there is a little more energy in the output at the second and, sometimes, the fourth partial from the tension signal, plus some more output at the 'zip' pitch when the strings are higher off the top. However, overall, looking at the rise and fall times (attack and decay) of the sound, and the peak amplitude, there did not seem to be any more actual power in the sound when the strings were raised. There's only so much energy in a plucked string, and if it comes out more at one pitch there has to be less elsewhere. People could hear a difference when the string height off the top was changed, but it didn't make the guitar more or less powerful.

Does that clarify things?

Yes, thank you.