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I was rewatching the old Bourgeois video on top voicing and noticed Dana braced the top in question with one lower x brace scalloped and the other not.
Oddly (if you are holding the top in front of you and looking at the inside bracing) the left side he left unscalloped with the intent of decreasing boominess... while scalloping the right side.

I realize there are a LOT of ways to skin a cat... and I’m NOT someone to question this guy... I’m just curious
1: are there actually bass and treble sides
2: why would the left side be the bass response side if it is under the treble strings?
3: inversely if one wanted to make more bass response and less treble... would you do it the other way around ?
Here’s a pic.




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Look at it again snowman. The left side under the bass strings is indeed the scalloped side.

Having said that it's my understanding that has been more or less debunked. There is really no bass/treble side so to speak. The monopole mode doesn't care what side the strings are on. Along those lines I have converted a few guitars to left handed and there is no way you could tell the difference because the so called tone bars are set up the 'normal' right handed way.

That's not to say that Dana's scalloping of only one side of the X doesn't do anything, it does.

Thanks for the response... I guess I was thinking that by scalloping the bass side it would make it MORE responsive... instead of less boomy... perhaps there is a lingo issue at play:)

He left the side closest to your leg strait... which I see would be the treble side ... (if there was a side)
Maybe I’m just confused and need more coffee.
This is what I get for dreaming about building while I’m at my day job:)


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To me they're just labels like left side and right side, but help keep track of what you're doing regardless of orientation or whether it's a left or right handed guitar.

Bracing designs with a "loose" and "tight" side can help to reduce cross dipole cancellation. Violin family instruments use a soundpost to essentially transform the cross dipole into a monopole mode. On a guitar where you don't have continuous energy supply from a bow, a soundpost would kill it. But stiffer bracing on one side can create a similar effect without too much detriment to other modes.

Most likely it would sound about the same regardless of which side is loose and which is stiff (or which side has a brace and which side has a soundpost on violin family instruments), but it seems pretty universal that the bass side is made loose and treble side stiff. Superstition and tradition more than anything, I think. Much of the way people hear instruments seems to be psychological than acoustical. If you know it's "backward" and believe that to be a problem, you'll hear a problem

If it's a design that also includes bridge mass asymmetry and you're building a left handed guitar, then it's probably worthwhile to reverse the bracing since since mixing and matching loose/stiff with light/heavy might have some audible effects.

Another possible exception is guitars with armrest bevel, where the bass side of the soundboard has its area reduced. Might be better to make bass the stiff one in that case, since it will already be naturally stiffer due to smaller span. Fan fret bridges also create asymmetry, and seems like bass would be the naturally stiffer one due to the reduced bridge-to-tail span.

I attended the top tuning session lead by Dana Bourgeois at the last Fretboard Summit. He discussed his intention to get a lot of different resonances when the top is held and tapped at different locations. The more different frequencies you can get out of a top, the more overtones are available and the more interesting the sound (his words).

This is like a Ramirez classical bracing which has a cutoff brace on the treble side. It is designed to introduce asymmetry so that the top can resonate at many different frequencies. There is no true treble and bass side of the top as it will resonate as an entire system. It's more of a naming convention for which side is treated differently to introduce asymmetry.

I have always done Dana's asymmetric bracing on most of the dreds I have built. I think they are less boomy and have better definition of the highs as you go up the neck. I think you are basically just tightening up the top a bit.

I bought a used 1975 D28 in 1977. The time when they had a massive Rosewood bridge plate and of course heavy non scalloped bracing. It took a while but it opened up into a great singer/songwriter guitar with balanced highs up and down the neck. No truss rod but the neck has been dead stable all these years and other than a refret it has not needed any work and still has a perfect neck set. ( Of course the bridge is in the wrong place, I've just lived with that )

Building a little tighter and letting the guitar play itself in is something I have come to believe in more and more. Hans Brentrup reinforced my feelings there and felt quite strongly about it.

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

I attended one of Dana's tap tuning workshops a couple of years ago. I use a 'tech' version of tap tuning; using Chladni patterns to find and visualize the resonances, and I took along a double-X braced Dread top to work on. I also (with Dana's permission) brought my signal generator and other toys along to check out some of the other tops, including his, which used his asymmetric pattern. What I found was interesting.

I had always had problems getting 'good' patterns on asymmetric scalloped braced tops on student guitars, and I was hoping that the workshop could help me sort out some of the problems, and establish some rough limits. Dana was pleased with the tap tones of my 'roughed' top, and made a couple of useful suggestions. The guitar ended up being quite nice: for those of you who got to Woodstock in '17 it was the Dread with the oak B&S that I had.

On checking Dana's top I found that the mode shapes were much less asymmetric than they are on fully scalloped tops. A little thought and some hand flexing showed why.

We often flex along and across the grain of the top, of course, and it seems to help to get things more or less 'even' both ways. There are other ways to check them, though. In particular; if you flex a fully scalloped top on the diagonals you'll find that it is much 'softer' along one diagonal than the other. That's because the tone bars a parallel (more or less, depending), so that when you flex on one diagonal you have to bend them, while on the other diagonal you don't. When you flex the top holding the at the 'X' crossing and bending the bass side leg of the X the tone bars resist the bending, while if you bend the treble side of the X they don't, so it's much easier to flex the top. That asymmetry is what throws off the shapes of the modes. Leaving the treble side of the X brace un-scalloped restores much of the stiffness along that diagonal, and the modes are more symmetric.

In terms of tap tones, more symmetric modes tend to be better defined in pitch and more active. Technically they usually have higher Q values, and lower losses. This seems to help the tone of the guitar, particularly in the higher frequency ranges: the sound tends to be 'clearer' and possibly more 'responsive'. As my voice teacher said to me many years ago:"If you want the basses to sound good, work on the trebles".

Bass comes from the whole lower bout moving like a loudspeaker. Scalloping the braces gets the center moving more, and pumps more air. At higher frequencies the top breaks up into smaller vibrating areas, and what you hear is a sort of sum of all of them as they reinforce or cancel each other out. High frequency sound comes from everywhere, and no place in particular, so you really need to get everything working well to make the highs come out.

Now that makes a lot of sense even intuitively.

Alan, that’s super interesting ... and a little mind bending for me as a new builder....but I think simply put. Thanks for your input


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