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Old 05-09-2021, 05:15 PM
Dan Pack Dan Pack is offline
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Default 7th guitar build, OM base response?

Hello everyone. I'm starting my 7th guitar which is going to be my variation of an OM, Sitka spruce top, IRW back and sides. I've built this model 3 time so far, but I'm trying some different things on this one with the hope to get a little more base response. The top is braced and the braces have been rough shaped, but I haven't done the final shaping and voicing yet, I probably should have started this thread a little earlier before I braced the top (I'm new to the forum).
So far the changes I've made from the original are: forward shifted x-brace, widened angle of the x-brace, lattice braced lower bout, slightly different geometry of bridge plate, thinner top (.100 in the middle, .095 at the edges)
My question is primarily in regards to the x-brace. not sure whether to scallop the x , or do a parabolic taper. my goal with all the changes is to encourage more of the monopole response. last year I built an MD (Modified Dreadnought) with lattice brace and parabolic taper on the x-brace, and was really pleased with the sound, but this one is a much smaller instrument. I would love to hear all of your thoughts.

Here's Some pictures of the top so far.

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Old 05-09-2021, 05:24 PM
Dan Pack Dan Pack is offline
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Here's a picture of the rosette, just for fun
Manzanita burl with Pau abalone
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Old 05-10-2021, 09:50 AM
Alan Carruth Alan Carruth is offline
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Scalloping is usually the best way to increase the bass response. It helps the top to 'pump' more air in the low 'bass reflex' range. It tends to produce a sharper 'attack' with less sustain, all else equal. You have to be careful not to get too carried away; it's possible to weaken the structure in such a way that the problem doesn't show up for a while. Many of the people that do after market scalloping tend to do it in stages, with some settling time in between, to avoid this.

Another way to increase the bass response is to shave the back braces. If you can get the 'main back' tap tone to be fairly close in pitch to the 'main top' resonance the two will work together in a way that also helps move air in the low range. Effectively, the back becomes a sort of extension of the top. In theory having the two tap pitches match exactly gives the strongest bass response, but it also invites some fairly nasty 'wolf' notes. I find I get good results if the top tap tone is about 1/2 semitone lower than the back before I glue the bridge to the top. The bridge generally adds enough mass to drop the top tap tone about another 1/2 semitone, and the semitone relationship is close enough to give strong coupling without much risk. Try to avoid having the back tap tone lower than the top; sometimes the top will 'play down' over time, and you could end up with them too close. One advantage of this is that you're less likely to get into structural problems than you would from shaving top braces.

You might also try making the sound hole a bit smaller. If you have a wide margin inside the rosette you can always enlarge the hole.
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Old 05-10-2021, 10:16 AM
charles Tauber charles Tauber is offline
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Regarding the top bracing you have, I'd do four things. First, I'd cut the height in half of the finger braces coming off the X. That will reduce the stiffness in that area. Second, I would leave more mass on the lower-bout portions of the X brace by not shaping their cross section into a triangle. The increase in mass and decrease in stiffness can help bass response. (I scallop the X braces and contour the lower portion, but not to sharp, triangular peaks.)

Third, your bridge plate looks a little thick. I'd probably make it about .10" thick. Doing so reduces stiffness, again, favouring bass response. Fourth, I'd also make it smaller, reducing stiffness of the overall area.

These are things that I've found work well for me, but need to be taken in the context of the entirety of the guitar.

Your work looks very crisp and clean.
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Old 05-10-2021, 12:21 PM
Howard Klepper Howard Klepper is offline
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Dan, your work looks very well executed. I get what you are trying to achieve, and suspect your are influenced by Ervin Somogyi's work.

To address your questions: I pretty much agree with everything that Alan and Charles have said. It is too late to thin or reduce the bridge plate by much, although you could bevel its lower edge more between the braces. Do extend the slopes on the finger braces so that they reach a peak (and I agree with what I see you tending to: that the peak should be toward the X side of them rather than in their center). You could also extend the scallop at the lower end of the X arms a little further up, and scallop the lower ends of the lattice pieces so they are concave and get down to nothing at their ends. There is almost no need for bracing right next to the rim in the lower bout, and that is where the monopole needs compliance. I leave about 1/32" on the bottom of the X arms, and everything else below the bridge goes down to zero at the rim.

I tried an X with lattice on a couple of guitars, but I found that the guitars had some uneven response among their registers.

Regarding Ervin and the emphasis on the monopole and two dipoles, I'd say that this will get you bass, but that is just one thing that you may find desirable. A pleasing character to the sound, and liveliness in all registers do not come from focusing on those first few modes. I see that kind of design focus as akin to the old joke about the man looking for his keys under the streetlamp. He didn't drop them there, but the light is so much better! We know more about how to make those low modes stronger than we do about the higher frequencies that give the instrument pleasing and balanced timbre, so we look where we can see better. We don't have much theory to help us with the higher modes; that is where experience, tradition, and trial and error count for more. I suggest learning to live and work with that.
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Old 05-10-2021, 12:25 PM
Dan Pack Dan Pack is offline
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Thanks guys. Great advice, I'm going to wait until I have the back braced to do the rest of the voicing, then I'll voice them together. the voicing process has always been a little tough for me, I just don't know what I'm listening for, and I don't do it often enough to have a sufficient sound library memory. I'll do my best to tune the back and top as Allen said.

For the finger braces, I've always been a little confused as what to do on those. For now I'll shave them down like Charles said. Its hard for me to hear any difference in the tap tone when shaping them so I just don't know what I'm changing. I usually end up just shaping them small and kind of like some pictures I've seen. but I've noticed some builders leave them tall all the way to the edge (Doerr, Somogyi, Greenfield), while others taper them down pretty small (Ryan Gerber, martin, Taylor). Maybe you guys can tell me what their function is as far as what vibrations they are controlling. maybe the cross dypole?

As for the bridge plate, the picture is deceptive on the thickness due to the chamfer on the edge. It actually measures between .085 and .090. I radiused the whole X assembly in a 35' dish after the bridgeplate was notched into the x-brace, hence the variation in thickness. I may try making a smaller one on a guitar in the future (i chickened out this time). I started making them big like that only because I was copying some of Ervin Somogyi's designs. However, I've noticed many builders (both modern and traditional) that use a narrower bridgeplate. I know Tom Doerr uses a smaller plate in combination with the lattice braced lower bout, I've played a few of his guitars and really liked the way they sound. so all that to say, I just have to get the courage to try it on one.

Thanks for the help and for the compliment, Cheers
I'll post more progress when I get time to work on it
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Old 05-10-2021, 12:32 PM
Dan Pack Dan Pack is offline
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Thanks Howard, also great advise. I'll do my best to take all of this into account when I do the voicing.

Thanks so much
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Old 05-10-2021, 01:19 PM
Talldad Talldad is offline
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Great work Dan
I too think a lot about voicing a soundboard. It is a physical fact that the response you get from a free board will bears no resemblance to the response you get from a board bound by the sides and that closed into a box. But I really want to think that there is a secret sauce.

I try to go for a musical tone with some variation in frequency and hope for the best.

Somogyi's book is excellent, but there is a book that is equal but completely opposite written by the Australian Trevor Gore, his book is a masterpiece of physics and equations and recordable, repeatable experiments and analysis.

Interestingly Trevor Gore's work doesn't place a great deal of importance on fine tuning X bracing at all. He puts a lot more emphasis on getting the soundboard the correct thickness for the sound you want to produce. As a last resort he will adjust the bracing once the instrument is completed over time if required.

I have spent time bracing guitars, shaving them down to virtually nothing and recording the results, I find that big shavings off bracing don't equal big changes to tone, but they do make big changes to deflection tests. My heart wants to believe in the dark art of voicing, my heads thinks that Gore might just have a point!
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Old 05-10-2021, 04:55 PM
charles Tauber charles Tauber is offline
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Quote:
Originally Posted by Dan Pack View Post
Maybe you guys can tell me what their function is as far as what vibrations they are controlling. maybe the cross dypole?
I can't, no.

As the antithesis of modern scientific work on guitar response, consider a very simplistic approach. In this approach, braces are there, in sizes, quantities and locations, sufficient to ensure longevity of the instrument. That is, to ensure that the instrument is structurally sound for a sufficient period of time. Ideally, the braces exist - in size, quantity and location - to be just enough to provide that structural integrity - not more, not less.

History has shown us that there are many different bracing sizes, shapes, locations and materials that will accomplish that.

From this simplistic approach, the finger braces should be as small as possible to assist in accomplishing the above. In general, we know that the guitar top is diaphragm-like and "rigid" around its edges. That is, very stiff at its edges and not very stiff towards its centre, which also happens to be where the forces are applied. Thus, we need minimal, if any, added structure towards the rigid edges and "lots" towards the centre.

Given that the finger braces are short and near the edges, they don't need to add much stiffness. Some small-bodied, early 1900's Martin guitars, for example, have shockingly light finger braces, but nearly 100 years later are still structurally sound.

We also know that the closer to the edge of the guitar we get, the less added stiffness is required. Given that stiffness of a beam of rectangular cross section is proportional to the cube of the height of the beam, small changes in brace height produces large changes in stiffness. Putting the two together, braces, generally, should be taller towards the centre of the guitar, and less tall towards the edges. That is true of the finger braces as well: taller towards the X brace/center and shallower towards the guitar's edge.

The shaping of the finger braces in your photo seem to show that the finger braces maintain a constant height for about an inch or so before tapering in height towards the guitar's edge. You'll get less stiffness if you bring the height to a maximum and then immediately taper them, rather than maintain a constant height. As previously mentioned, you'll also get less stiffness simply by reducing the maximum height.

Changes in cross sectional area increase localized stresses (i.e. stress concentrations). To reduce localized stresses where one brace abuts another, what is often done is to reduce the height of one of the braces adjacent to the abutment. In the example of the finger braces, most makers reduce the height of the finger braces adjacent to the X brace. Doing so reduces the local stiffness of that brace and reduces the likelihood of the brace end peeling off from the top at that end.

Taking all of the above together, one gets the often seen shape of the finger braces: shallower next to the X brace, rising to a peak not far from the X brace, then tapering to little or nothing in height towards the edge of the guitar. The cross-sectional profile of the finger braces can be left relatively rectangular or peaked towards a triangular shape, depending upon how one want to balance the mass vs. stiffness of the brace.

Upon assembly of the instrument, one can press on various parts of the top to observe the amount and nature of the deflection that results. The "nature" of the deflection refers to how much of the top deflects, versus a localized depression. A method that Jose Romanillos taught for classical guitars was to press with one's thumbs on the wings of the bridge to see how the entire top moved in response to the applied force. It is a subjective deflection test. I do that on steel string guitars as well, though the response to pressing on the bridge differs between classical and steel string guitars.
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Old 05-10-2021, 06:09 PM
Howard Klepper Howard Klepper is offline
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Quote:
Originally Posted by Dan Pack View Post
Maybe you guys can tell me what their function is as far as what vibrations they are controlling. maybe the cross dypole?
Dan, this is a place where it really shows that you have been reading too much Somogyi, and that you have done too little repair of old guitars. The finger braces are mainly there to keep the top from warping from the stress that the bridge applies to the areas that they support. Think less about dipoles and more about stress and strain, and how you get the structure to be strong where strength is needed without overdoing it. Example: I do not taper the upper finger brace to nothing at the liners; I tuck it into them. My reason has nothing to do with vibration modes and everything to do with seeing old guitars' tops that are sunken at that location.
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Last edited by Howard Klepper; 05-10-2021 at 06:22 PM.
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Old 05-10-2021, 09:07 PM
Taylor Ham Taylor Ham is offline
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Default 7th guitar build, OM base response?

If I may inject my humble opinion (and maybe my own questions) I agree with shaving the back braces and finger braces to get some more bass response. The finger braces look like they can keep the same profile but squashed down to a 'bit more than the height of the lattice' and still be fine.

I recently completed a guitar that is 10-8.25-14.25 wide x 3.375 deep, and I put on parabolic back braces that were not very tall- probably about 9/16 +0 / - 1/32 for the tallest 2 across the lower bout, and about 7/16" at the waist and upper bout. the guitar is very loud, projects well, and has more bass than you'd expect for a relatively small body. I wonder if bracing the back lighter can be explored further.

Last edited by Taylor Ham; 05-10-2021 at 09:19 PM.
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Old 05-11-2021, 07:25 AM
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warfrat73 warfrat73 is offline
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Quote:
Originally Posted by Alan Carruth View Post
Another way to increase the bass response is to shave the back braces. If you can get the 'main back' tap tone to be fairly close in pitch to the 'main top' resonance the two will work together in a way that also helps move air in the low range.
Curious about how that works, and what would be the "main" tone. One of the things that Dana Bourgeois talks about in voicing is getting multiple tap tones.

Which would you consider the main? The loudest? The most fundamental? The lowest frequency? Is there a particular nodal point that you'd be holding it?
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Bourgeois DR-A / Bowerman "Working Man's" OM / Martin Custom D-18 (adi & flame) / Martin OM-21 / Northwood M70 MJ / 1970s Sigma DR-7 / Eastman E6D / Flatiron Signature A5 / Silverangel Econo A
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Old 05-11-2021, 07:29 AM
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warfrat73 warfrat73 is offline
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Or is this a Chladni pattern thing?
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Bourgeois DR-A / Bowerman "Working Man's" OM / Martin Custom D-18 (adi & flame) / Martin OM-21 / Northwood M70 MJ / 1970s Sigma DR-7 / Eastman E6D / Flatiron Signature A5 / Silverangel Econo A
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Old 05-11-2021, 09:47 AM
Dan Pack Dan Pack is offline
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Quote:
Originally Posted by warfrat73 View Post
Curious about how that works, and what would be the "main" tone. One of the things that Dana Bourgeois talks about in voicing is getting multiple tap tones.

Which would you consider the main? The loudest? The most fundamental? The lowest frequency? Is there a particular nodal point that you'd be holding it?
My understanding is that the multiple semitones that Dana Bourgeois talks about is in reference to free plate tuning. The main top or back resonance is what is referred to when the they are coupled to the sides, or when the are held in a rigid temporary form.
Is that correct?
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Old 05-11-2021, 09:52 AM
Dan Pack Dan Pack is offline
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Quote:
Originally Posted by charles Tauber View Post
I can't, no.

As the antithesis of modern scientific work on guitar response, consider a very simplistic approach. In this approach, braces are there, in sizes, quantities and locations, sufficient to ensure longevity of the instrument. That is, to ensure that the instrument is structurally sound for a sufficient period of time. Ideally, the braces exist - in size, quantity and location - to be just enough to provide that structural integrity - not more, not less.

History has shown us that there are many different bracing sizes, shapes, locations and materials that will accomplish that.

From this simplistic approach, the finger braces should be as small as possible to assist in accomplishing the above. In general, we know that the guitar top is diaphragm-like and "rigid" around its edges. That is, very stiff at its edges and not very stiff towards its centre, which also happens to be where the forces are applied. Thus, we need minimal, if any, added structure towards the rigid edges and "lots" towards the centre.

Given that the finger braces are short and near the edges, they don't need to add much stiffness. Some small-bodied, early 1900's Martin guitars, for example, have shockingly light finger braces, but nearly 100 years later are still structurally sound.

We also know that the closer to the edge of the guitar we get, the less added stiffness is required. Given that stiffness of a beam of rectangular cross section is proportional to the cube of the height of the beam, small changes in brace height produces large changes in stiffness. Putting the two together, braces, generally, should be taller towards the centre of the guitar, and less tall towards the edges. That is true of the finger braces as well: taller towards the X brace/center and shallower towards the guitar's edge.

The shaping of the finger braces in your photo seem to show that the finger braces maintain a constant height for about an inch or so before tapering in height towards the guitar's edge. You'll get less stiffness if you bring the height to a maximum and then immediately taper them, rather than maintain a constant height. As previously mentioned, you'll also get less stiffness simply by reducing the maximum height.

Changes in cross sectional area increase localized stresses (i.e. stress concentrations). To reduce localized stresses where one brace abuts another, what is often done is to reduce the height of one of the braces adjacent to the abutment. In the example of the finger braces, most makers reduce the height of the finger braces adjacent to the X brace. Doing so reduces the local stiffness of that brace and reduces the likelihood of the brace end peeling off from the top at that end.

Taking all of the above together, one gets the often seen shape of the finger braces: shallower next to the X brace, rising to a peak not far from the X brace, then tapering to little or nothing in height towards the edge of the guitar. The cross-sectional profile of the finger braces can be left relatively rectangular or peaked towards a triangular shape, depending upon how one want to balance the mass vs. stiffness of the brace.

Upon assembly of the instrument, one can press on various parts of the top to observe the amount and nature of the deflection that results. The "nature" of the deflection refers to how much of the top deflects, versus a localized depression. A method that Jose Romanillos taught for classical guitars was to press with one's thumbs on the wings of the bridge to see how the entire top moved in response to the applied force. It is a subjective deflection test. I do that on steel string guitars as well, though the response to pressing on the bridge differs between classical and steel string guitars.
Thanks again, makes sense
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