Loss of bass lowering the saddle?

[Clay Schaeffer]

What type of strings are you using? Bronze wound tend to sound bassier than nickel wrapped. Finding the right gauge can also make a difference - strings that don't over tension the top, but also have enough tension to drive it.
From the pictures the bridge looks O.K. and the top doesn't look over braced.
Some Factory guitars fall on the lower end of the bell curve and don't sound that great. It may have a dense spruce top that was sanded to the same specification as the run of the mill tops and left too thick. Trimming the braces might not help.
I would try swapping strings and if that didn't work pass it along in original condition to the next buyer. Someone will like the sound of that guitar.

[jorge rodriguez]

Thanks Clay. I'm using phosphor bronze medium strings.

[jorge rodriguez]

Here's another picture of the inside... I don't know if this looks overbraced or not... but doesn't everything look very square? I would expect the cross section of the braces to be more parabolic (except on the places where it's scalloped), at least that's what I've seen in most guitars. Also the tone bars look very thick where they meet the x brace. Is that normal?
I don't want to mess with shaving the top bracing... but I just wondered.

[Alan Carruth]

Those braces do look heavy.

I'd feel a lot better about that top if the X-brace crossing were reinforced. The don'w facing open side of the half lap joint is in tension, and because of the sudden change in stiffness there's a tendency for the brace to split at the level of the lap. Anything that helps hold the open side together is a big help; Martin has used a cloth patch there forever,and a lot of modern makers put on a wood patch. If you opt for a wood one keep in mind that it doesn't have to be thick to work; what's more important is the glue area. I commonly use one that's no more than 1/16" thick, and about 2" long, and as wide as the brace top.

Trevor Gore points out in his book that you can use top deflection as a guide to whether the braces are too stiff. When you put on the strings the bridge rotates toward the neck, and he maintains that two degrees of rotation is a good number to shoot for. Much less than that means the top is over built, and not capable of producing the sound it could, and much less than 2 degrees is probably too light.

A lot of folks drop the brace height out toward the edges to increase the bass response. Lowering them in the middle (a 'scalloped' profile) is more effective, but also riskier. It looks to me as though you could reduce the tone bars some, at least, but as always with the top, take it easy. The 'rule of 'half' is good: take off about half of what you think you want to, and see what happens over a period of time. It's easier to take wood off than put it back on....

[jorge rodriguez]

Those braces do look heavy.

I'd feel a lot better about that top if the X-brace crossing were reinforced. The don'w facing open side of the half lap joint is in tension, and because of the sudden change in stiffness there's a tendency for the brace to split at the level of the lap. Anything that helps hold the open side together is a big help; Martin has used a cloth patch there forever,and a lot of modern makers put on a wood patch. If you opt for a wood one keep in mind that it doesn't have to be thick to work; what's more important is the glue area. I commonly use one that's no more than 1/16" thick, and about 2" long, and as wide as the brace top.

Trevor Gore points out in his book that you can use top deflection as a guide to whether the braces are too stiff. When you put on the strings the bridge rotates toward the neck, and he maintains that two degrees of rotation is a good number to shoot for. Much less than that means the top is over built, and not capable of producing the sound it could, and much less than 2 degrees is probably too light.

A lot of folks drop the brace height out toward the edges to increase the bass response. Lowering them in the middle (a 'scalloped' profile) is more effective, but also riskier. It looks to me as though you could reduce the tone bars some, at least, but as always with the top, take it easy. The 'rule of 'half' is good: take off about half of what you think you want to, and see what happens over a period of time. It's easier to take wood off than put it back on....

Thanks Alan.

I will definitely follow your advice and take care of the x-brace joint.

Someone told me the way to measure top deflection with a bubble level app... I might do some tests. But I would definitely think it over a few times if I decided to shave the top braces and go very slow.

[Steven Smith]

Square topped braces are not an issue. I know of one top luthier that leaves his braces square. If the top is voiced properly it just doesn't matter.

[Alan Carruth]

Yes; the cross section of the braces probably hardly matters. All of the bracing put together is something like 25%-30% of the overall weight of the top. The most efficient section probably would save you no more than about 10% of the bracing weight, so, say, 3% of the total top weight. You can do better than that simply by choosing a lower density piece of wood for the top and actually figuring out how thick it needs to be based on a measurement, rather than the usual guess work based on species. I round them off out of habit, and because that's what people expect, but I don't go to great lengths to 'triangulate' them or anything.

[Barry Daniels]

You really can't tell how large those braces are in the photo as they could be magnified from the close-up photo. A measurement of the height of the X-braces at the joint would be helpful, as well as the width of the braces.

[jorge rodriguez]

You really can't tell how large those braces are in the photo as they could be magnified from the close-up photo. A measurement of the height of the X-braces at the joint would be helpful, as well as the width of the braces.

I measured that today... About 5/8'' tall and 5/16'' wide, which is pretty standard if I'm not mistaken.

[Marshall Dixon]

I measured that today... About 5/8'' tall and 5/16'' wide, which is pretty standard if I'm not mistaken.

That sounds about right, and standard for this model certainly. Do let us know how it sounds when you're done please. I'm really curious.

[jorge rodriguez]

Thanks Marshall. I will, though I still need to think carefully what I try doing.

Finished gluing the back bracing today. Finally found 4 loose ends... so it took a few days. The rattle is gone, I will string it up and see how it sounds later.

This is a picture of the inside, it has two 5/8'' tall x 5/16'' wide braces on the upper bout, and the two at the lower bout are about 1/2'' tall and 1/2'' wide. Does it make sense to shave the back braces to try to enhance the bass like Alan suggested?

[Marshall Dixon]

The following is from Jansson's work on guitar and violin acoustics (edited).

“2.3. THE MECHANICAL AND ACOUSTICAL MEASURES OF THE RESONATOR
One very interesting question is: How much are the acoustical measures changed if one of the mechanical measures is changed? The changes we shall study correspond to doubling of the stiffness, mass and friction, c.f., Fig. 2.7.

If the mass is doubled the resonance frequency decreases half an octave (from 500 to 350 Hz, i.e. 1/1.4 times). If the mass is halved the resonance frequency is increased half an octave (from 500 to 700 Hz, i.e. 1.4 times). The peak heights (the levels) are approximately the same and the levels of these curves are the same at low frequencies (100 Hz for instance). At high frequencies the level (the vibration sensitivity) is larger for the smaller mass.

If only the stiffness of our resonator is changed, then the following happens... For doubled stiffness the resonance frequency increases half an octave and for half the stiffness the resonance frequency decreases half an octave. For high frequencies the level (the vibration sensitivity) is the same but for low frequencies it is higher for the smaller stiffness. The peak heights are little influenced by the stiffness changes.

Thus we have found that the effect on the resonance frequency by a doubling of the mass is equivalent to halving the stiffness and vice versa. There is, however, one large difference. The doubling of the mass decreases the vibration sensitivity at high frequencies only, while halving of the stiffness increases the vibration sensitivity at low frequencies only.”
__________________________

I have read Alan's previous posts on this subject, and have experimented with it only briefly. It has now occurred to me to try this more objectively to the guitars I make. Especially since it requires no modification of the structure to try out. So far I have concentrated on the top plate, braces and bridge. As I mentioned before; putting them on heavy and hearing what happens when taking them down.

I'll give you the gist of what I've read in Alan's previous post, hoping he'll give a clearer version later.

What Jansson says about stiffness and mass can be applied to the back braces; What you can do is take a small weight and with double sided tape stick it onto the back over the middle of the long brace(s) of the lower bout. You could use fishing sinkers flattened out or coins and add more weight with the double sided tape. Experiment with that over various braces. The addition of weight reflects the loss of stiffness in the brace, giving you an idea of where to shave the brace. By how much is up to you.

I get an idea of how much by taking a similar piece of wood to the brace and giving 10 swipes with a sanding stick, trying to simulate the pressure applied awkwardly inside the box. Measure before and after. That gives me some kind of idea how far I've gone. One of my guiding principles is that if 10% is removed and I don't hear any difference, then stop. Also I find that my hearing perception to subtle changes dulls after 15 minutes or so. Only to be restored by caffeine.

Remember that a 10% reduction in height reduces stiffness by over 30%.

[jorge rodriguez]

Thanks Marshall, that's very interesting info. I will make some tests.

Yesterday I made some tests on the bridge rotation with a mobile, a bubble level app and some blu tack. The results seem to be that rotation is about 0.8 degrees. So I guess I could try shaving the top braces a bit, but I want to be very careful and go slow if I do. Any suggestions about where to shave (tone bars, x bars, scalloped parts, high sections...) and where not to shave are welcome.

[Barry Daniels]

I would remove material evenly over the entire brace system.

[Clay Schaeffer]

If you want more rotation I would lower the X and the "legs" in the forward portion around the soundhole. Aside from being easier to get to, they are the parts of the brace system designed to prevent bridge rotation.

[Barry Daniels]

Clay makes a good point. I amend my recommendations to reflect his.

[jorge rodriguez]

Thank you both.
Well what I'm after ultimately is more bass response. Is bridge rotation itself a way to achieve that... or only a way to measure top deformation?

[Alan Carruth]

Bridge rotation is just an indicator of whether you're in a safe stiffness range with the top. Reducing the stiffness will usually increase the bass response, but may not do as much as you'd expect depending on what the condition of the top is and where you take material off. It could also be far more effective an you might think, again, depending.

As has been said, adding mass is a non-destructive way to test the response of the top to removing material from a brace. You should always try to find the most effective spot for the top in question before you do anything that can't be easily reversed.

In general, taking material off the braces in the center of the lower bout should give the greatest change in the bass response. It's also the most risky structurally. I'd try to do most of my brace shaving behind the bridge, and stay clear of the X crossing.

[Randy Roberts]

Kind of an old thread but this might help picturing the reason for all the advice of take any thinning of the braces slowly, as you get close to where you want to be, very small changes can have very big effects. Also demonstrates pretty clearly why to bridge across the junction of the X braces.

https://www.mimf.com/library/Randy_Robe ... -2010.html

[Marshall Dixon]

When you consider bass response I think it is important to consider that the middle of the range in “standard” tuning Of a 21 fret instrument is around D(293.7 Hz). The 3rd fret of the B string. Some people refer to a “bass side” and “treble side” of the sound board as if separated down the middle, but the whole top is in play with any note.

Another basic concept is restorative force. When a string pulls the top down in front of the bridge, the top pushes back. When the top is pulled up behind the bridge it pulls back. Ideally you want those forces to be equal. You want the top to respond to all the partial harmonics that the string produces. Those harmonics can effect each other depending on how they interact, either by augmenting or nulling frequencies. And this creates a feedback loop to the strings. Probably a million variables, even the temperature and humidity are a factor.

Here are several graphic representations of how the body of a guitar vibrates. The first is Jansson's again. Note the interferometry of how the plate subdivides in response to different frequencies.

The second is from Pennsylvania State University professor Dan Russell. Many cool graphics related to Jansson's work. Follow Russell's link back to the home page for much more. Great website!

Look at what Jansson says about tuning marimba bars. I took short pieces of 2x4 about 2 feet long and listened while shaping them trying to interpolate what's going on with a brace.

http://www.speech.kth.se/music/acviguit4/part6.pdf

https://www.acs.psu.edu/drussell/guitar ... gbird.html