Why doesn’t Martin scallop the upper arms of the x-brace?

[Pitar]

Originally Posted by jpricewood

I noticed Eastman scallops the upper arms of the x-brace. I wonder why Martin doesn’t.

Because the sound box is basically the lower bout making upper bout brace scalloping unnecessary. Eastman probably does it just because.

[Scotso]

Originally Posted by jpricewood

But doesn’t it make the top lighter, which allows it to vibrate more? I’ve never heard of an Eastman imploding.

Give it time.....

[Mr. Jelly]

Back up a second. The first rule of building a guitar is to built it so that it doesn't fail. That was Martins first rule. The business is to sell guitars not repair them. Building great sounding guitars that last days is not a good business model.

[John Arnold]

......did you ever personally get the sense that they were experimenting with bracing thickness/shape and top plate thickness in some effort to find a "sweet spot"...or...did you ever see evidence to suggest that they seemed to adjust the bracing size shape thickness and scallop length and depth guitar to guitar in a given years production run, like the various craftsmen were trying to use some simple form of voicing process on the Golden Era guitars?

I believe guitar-to-guitar variations were entirely due to the process, which included hand shaping and sanding of braces, and some hand work when preparing the bodies for finish. In other words, I see no evidence of voicing of individual guitars, and there is ample evidence supporting this. That is not to say that there was no evolution in the bracing over the years, even in the golden era (1930-1944). But this was a gradual fine-tuning involving thousands of guitars. In general, guitars built within a few months of each other (except for the step changes in bracing) tend to be very close dimensionally, considering the methods used.

[ljguitar]

Originally Posted by jpricewood

These are cool images!

Makes me wonder why people say 12 frets produce more bass than 14 frets.

Hi jpw…

Not everyone says/thinks that.

As to why people say things about guitars, I'm still trying to figure out their source of expertise.

[zoopeda]

Originally Posted by jpricewood

Makes me wonder why people say 12 frets produce more bass than 14 frets.

Because it's a bigger body cavity, for one (which creates more bass because more air has to woof through the sound hole). For two, on many guitars, the bridge is slid further into the lower bout to compensate, which also makes more bass. Everyone hears what they want to hear, but most builders will confirm a degree of objective truth here that--all other things being "equal"--a 12-fret guitar has more capacity for bass response than a similar 14-fretter.

[Alan Carruth]

If the center of the lower bout was the 'best' place to put a bridge, why aren't banjos made that way? Or violins?

In point of fact, the geometric center of the top may not be the same thing as the 'acoustic center', which depends on how you brace the top, among other things. The bridge goes in the acoustic center, and the brace layout and profile are chosen to get that in the right place.

As John says, scalloping the braces above the X crossing risks strength more than it helps sound overall. Eastman may have designed their bracing to allow for that.

[Mr. Jelly]

Originally Posted by Alan Carruth

If the center of the lower bout was the 'best' place to put a bridge, why aren't banjos made that way? Or violins?

First off best is subjective. Your mileage may vary. Secondly the comparison is classic apples and oranges. But they are are all fun to play. Even the ones that don't sound so good.

[dneal]

Originally Posted by ljguitar

As to why people say things about guitars, I'm still trying to figure out their source of expertise.
[/size]

For me, it's either the internet or a Holiday Inn Express. When I really need some expertise, I use the internet while staying at a Holiday Inn Express.

[colins]

Originally Posted by Alan Carruth

If the center of the lower bout was the 'best' place to put a bridge, why aren't banjos made that way? Or violins?

In point of fact, the geometric center of the top may not be the same thing as the 'acoustic center', which depends on how you brace the top, among other things. The bridge goes in the acoustic center, and the brace layout and profile are chosen to get that in the right place.

As John says, scalloping the braces above the X crossing risks strength more than it helps sound overall. Eastman may have designed their bracing to allow for that.

Thanks Alan, always nice when a guy who builds guitars contributes to this sort of thread.

[Alan Carruth]

Mr. Jelly wrote:
"First off best is subjective. Your mileage may vary."

Usually the argument goes that 'the best place to drive a soundboard is in the middle.' I'm just making the point that the acoustic center may not be the same thing as the geometric center. In fact, it seldom is.

"Secondly the comparison is classic apples and oranges."

There certainly are lots of way in which violins and banjos differ from guitars, but I've made enough different sorts of stringed instruments in the past 4+ decades to know that the acoustic center of the soundboard is the 'right' place to drive it. Where that is depends on how the soundboard is built. It's generally someplace close to the geometric center, but almost never right on.

[mlr1122]

How does the popsicle brace or lack of one factor into this? I know some people like to have it removed, but according to the visual, it looks like it may not have an effect.

[pbla4024]

Originally Posted by Alan Carruth

Mr. Jelly wrote:

I'm just making the point that the acoustic center may not be the same thing as the geometric center.

What do you call acoustic center then? That would be something like the spot with highest amplitude for first stationary solution of corresponding hyperbolic PDE.

[John5]

Originally Posted by Scotso

Give it time.....

So you're of the opinion that Eastman guitars will implode in time?

[Alan Carruth]

pbla4024 asked:
"What do you call acoustic center then? "

At the risk of seeming to state a tautology, it's the best place to drive the top.

On a guitar it will generally be the spot on the top that gives the strongest 'tap tone'. Usually this will be the place where the 'main top' monopole resonance shows the highest amplitude when driven. Since that's the mode that probably produces most of the actual output of the instrument that makes some sense.

It's something of a self-fulfilling prophecy, though. On a completed instrument the bridge is there; a large mass concentration that's either going to be on or near a node line, or at the antinode of a mode. Even without the bridge in place the acoustic center is usually reasonably well defined, since the bracing is designed to provide the necessary stiffness at that point.

I'll note that it's not anything like a mathematical point: it's more like a low-Q resonant peak that's spread out a bit. It does seem to be more tightly defined on 'better' instruments.

I have weak math chops, which is one reason I tend to rely on experiments and verbal descriptions. This is also a complex system that is not well understood or defined in many respects, so that there are probably limits as to the utility of a mathematical description.