Chladni and VA tests ???

[Trevor Gore]

I am leaning towards the lighter 20g walnut bridge and side mass to adjust.

That's the way I'd go.

[blackalex1952]

Current report. I have glued the 20g walnut bridge to the guitar. The resonances I have now are 103Hz and 200Hz...too high.
When I carved the soundboard, I experimented with a scalloped edge around the inside perimeter of the top for two reasons.
Firstly, I had this hair brained idea that scalloping the edge would minimise the amount of edge thinning post assembly, and that the scalloped edge would give a slightly larger area of the soundboard at full thickness. Taylor guitars do a similar thing with a routed groove.
The downside of this is that there is little control over the amount of edge thinning post assembly, for top tuning purposed. With the edge tapering method, the amount of spruce removed can really only be measured by the frequency drop, the flexibility change, and by measuring the thickness of the shavings as they come off the plane, but the thickness of the scalloped edge where it glues to the sides can be measured before the top is attached.
However the experiment is a total disaster because I failed to scallop far enough in from the liners, and the glue squeeze out has filled the void, thus rendering the scalloping useless as well as preventing me from any dramatic edge thinning now the guitar is assembled.
The only thing I can think to do is either remove the top, which I don't want to do as it has been lacquered and the neck attached, or try and reach inside with some sort of tool and carve away the excess glue.
Moral: 1)stick with the old tried and true edge tapering method.
2)don't apply finish until all resonance tests are done with
the bridge glued in place, allowing for edge thinning pre lacquer.
Well, that's what I will probably do in future...
That 200Hz resonance is a bit high for me, and from what I can work out it will require a lot of side mass to drop it by 20Hz...edge thinning would most likely help but the horse has bolted...
The up side is that the guitar sounds pretty damn good already, in spite of everything and is loud and resonant with a nice bass!

[jeffhigh]

Since you have regular X bracing, it probably will not take much scalloping, or general height reduction, to bring the top down to 190 and that will push the air down towards 100.

[blackalex1952]

Since you have regular X bracing, it probably will not take much scalloping, or general height reduction, to bring the top down to 190 and that will push the air down towards 100.

Thanks, Jeff. It is not "regular X bracing" It is graduated and not scalloped and is SO LIGHT that I am scared to take anything more off the braces.
Also, the Chladni tests indicate that the nodal line for those two modes is not all that close to the edge of the soundboard, so I am making a mini scraper to try developing the soundboard edge scallop further. A mini toothed Ibex violin makers plane would be best if there is such a small one...I have left the bracing above the bridge at normal height, and tapered the main X braces down to nothing on the lower bout and they are not let in to the sides, so it's feasible to loosen the soundboard edge, as I can get my hand in the soundhole and reach all the way to the tail block. I thought of taking the neck off and then the soundboard, but I have A frame bracing into the heel block and that would be difficult to remove without risking the top.
Interestingly, I proved one of Trevor's assertions in the book for myself. Attaching side mass with tape does very little. I also found that DS taping the bridge on for tap testing gave a different result than the taps when I glued the same bridge on.

Here is a question: I have ordered some camel bone bridge pins. Can any body tell me if they are much heavier than plain old bone bridge pins?

[jeffhigh]

How high are your braces at the X joint?
I seriously doubt you will drop 10Hz by working the edge of the soundboard.
Camel is not too different to regular cow bone in density,

[blackalex1952]

The X brace height is 15mm
I'm just waiting till I get some brass flat for the side mass and I'll see how far I get with it.
The other thought I have is to experiment with a Tornavoz
Right now, however, disaster has struck...sudden freak storm two days ago has wreched my access road and flooded the machine section of my shed...tools rusting timber warping and more rain on the way I think, so trying to deal with the humidity in there, move stuff out, I don't know where, and clean up all the wet sawdust! AAARRRGGGHH!

[jeffhigh]

Bugger, hope you get it sorted.
A Tornavoz will lower the main air without much effect on the main top frequency.
I still think working your braces is the way to go, that 200hz frequency is telling you that you are plenty stiff and 15mm is not real low.
Maybe check the bridge rotation under string tension?

[blackalex1952]

Thanks Jeff! Bridge rotation is the next step...I forgot to mention it...I took the box to the Luthiers meet in Melbourne and had a few luthiers check it out, as it's the first fixed bridge guitar I have made, have been mucking around with Selmacs, also lots of repairs over the years.
The general consensus was that it would be fine, albeit light. One guy felt that the top was too thin. Another issue with that box is the top join
ended up a fraction thinner due to misalignment but it still sounded good to me. Many makers think that the centre of the top needs to thicker, so that the plate is encouraged to vibrate as a single diaphragm.
The top is quite domed-my Selmer influence going on there....
Not much info around regarding ladder braced design, the top being divided into rectangular sections...interesting! I have seen old Gibson ladder braced guitars with one of the ladder braces at an angle-tighter on the treble side than the bass. Love a good pliage!

[jeffhigh]

Love a good pliage!

What's your take How to produce the pliage?
I have the Collins book but really did not see the merit in bending uniformly right across then gluing to a flat rim.
I ended up tapering the bend from max in the middle to nothing towards the rim then shooting and jointing the two halves after. Seemed to produce a good solid lift that way.

[blackalex1952]

My pliage method. I have a shooting board which is angled in two planes. See pix
I made a bending pipe jig with elements in it which allows me to repeatedly bend to a set angle. It would be possible to make it work in two planes as well, so that the edges of the assembled board are not forced straight again but I don't think this is necessary as the original Selmers weren't bent that way. See pix. The pipe is a piece of stainless steel filled with aluminium, with 10mm holes drilled either end and Chinese bending iron ceramic elements(cheap). There is also a cheap Chinese pid control and sensor, all off eBay...The bolt adjusts to get whatever bending angle is required, in this case 6 deg as in the FCharle plans. I spritz the board and place it over the pipe then put the top piece with the hinge in place and hold one side down whilst pressing on the handle to form the bend

The joining of the halves can be done the Collins way, or with a jig like the one in the Selmer book, or with an angled jig a bit like the Collins one and a go bar deck and tape.

[jeffhigh]

Interesting, thanks.

[blackalex1952]

I have a theory about the Busato "bombe". Busato made his soundboards from multiple pieces of spruce. I think he probable bent each one to a different angle, shot them and joined, the outside ones not as bent. That way he minimised the stress on the top and was, perhaps, able to recycle timbers from old piano soundboards and smaller spruce billets, given that times were tough both during the depression and during the war when he was active.Also his guitars were bigger, so it was the logical way to go. Just my theory...

[56nortondomy]

Couldn't the top plates be joined and then bent? Is there anything wrong doing it like that? Seems to me that would ne easier, but I've never made one, so I'm just thinking out loud.
Wayne

[jeffhigh]

That's the Collins book approach.
I'm sceptical, try it with cardboard, and then press the edge down flat, will the bridge area stay up? just simple geometry suggests otherwise.

[blackalex1952]

My method works, I've done it. Bend two halves to 6deg, shoot at a slight angle crosswise,and straight lengthwise (see shooting board) so the join has the least amount of stress and candles well then when the braces are glued on the top is clamped at the edges and it is fine fine fine Eccles!In fact as soon as the braces are glued I have found that the dome is formed in the top and the edges seem pretty flat even before they are glued to the liners. If the top plate is joined then bent, the stress can open up the centre seam in the soundboard, or the plate buckles weirdly. Best to bend then join. Jeff do you have Collins vids? Different from the book in some ways. I learned the method I use from another maker and by researching on the net and looking at the photos in the F Charle book. The other issue with the Charle plan is the neck angle. Completely wrong. The no1 brace or fingerboard support brace needs to be a different curve. I initially lined up the braces on the sides before gluing to the soundboard, then worked out where the bridge would sit above a line between the sides at the bridge position (pliage included), then used a jack plane and a sanding paddle to adjust the brace and the heel block so the angle was correct. Now I just make the brace from a template and set the heel block angle that way, which allows me to build with the top going on first. That way the braces can be messed around with by removing replacing the back bla bla..

[blackalex1952]

So...back to the X brace guitar I am learning about, you know, the guitar everyone makes, except for the intrepid falcate fellows. Jeff, my bridge rotation seems to be only 1 degree, even though the bracing south of the bridge is rather light. So I'll take the plunge and dive into the scary bit! At least I erred on the good side of disaster when I put the box together.
BTW this is good:http://www.rapidtables.com/calc/math/Sin_Calculator.htm

[blackalex1952]

Of course, where should I be carving the braces-where the X intersects, between the bridge and the soundhole on the X braces, the finger braces or the neck side of the X intersection. Finger braces pretty light already...?

[Craig Bumgarner]

Sorry to detract from the OP thread, but in response to Wayne, for a while, I tried bending the pliage after joining the two halves of the top and like blackalex says, got an opening of the joint in the vicinity of the heat, shrinkage actually, about 2" along the seam. Probably not a major structural problem if the joint is reinforced with strips on the inside and can be somewhat hidden by the bridge, but is cosmetically offensive, poor craftsmanship, not acceptable on a high end guitar, etc. Went back to bending the halves independently. The same shrinkage occurs after the bend of just the half so the final joint has to be shot and fit after the bend.

FWIIW, I use a heat blanket at about 275 degrees F in the bend area and some simple blocks, wedges and clamps to make the bend. Maybe less heat would save the glue joint, but I'd be surprised if the wood will take the set with much less than 275F. Wood starts to char at 300F, but if you are careful, this makes a better set.

[blackalex1952]

Love yr work Craig! Thanks for chiming in.

[Craig Bumgarner]

Thanks! Here's a little sketch of my pliage setup. I may have posted it before, but any thing worth saying is worth saying often

Pliage.jpg (7.08 KiB) Viewed 32872 times

The only changes since this is the block at the bend has a 1/2" radius. The block forcing up the angle is now a wedge instead of a rounded block. I heat the top before clamping, now using a a heat blanket, but early on, I laid the pieces on my wood stove until they got nice and toasty.

[56nortondomy]

Thanks Rosco and Craig, I have a Selmer to do for a guy so this information is very helpful, i'll probably be starting it late 2016.
Wayne

[blackalex1952]

but early on, I laid the pieces on my wood stove until they got nice and toasty.

We Aussies put vegemite on things that are toasted!

[blackalex1952]

I have added over 100g side mass for only a couple of Hz drop in top frequency. The sides are walnut with solid beechwood top liners and kerfed maple back liners.Is the reason for such small changes,7Hz drop,due to the fact that the sides are pretty solid already and the mass loading is not so effective? what is the upper limit of the amount of side mass that can be reasonably added? I understand from the Gore/Gilet books that added side mass vs frequency change is linear.

[blackalex1952]

What is the consensus regarding the topic of soundboard brace tuning? There seem to be many schools of thought, for instance, the idea that, apart from the structural issues to support string load, the soundboard should function as a single diaphragm in a similar way to the cone of a loudspeaker. Loudspeaker manufacturers "dope" their cones with all sorts of materials, one of the best being hemp fibre in order to stiffen the cone and prevent unwanted intermodulation. In other words, one main cone resonance. That resonance is theoretically below the operating frequency range of the speaker and when installed in an enclosure, the coupled resonance ends up also below the speakers intended range. On a guitar, for practical reasons, the resonances are set between scale tones to prevent wolf notes due to too high an admittance. I may be wrong...but that's what I have read.
So, how to tune braces on a soundboard? Should they all work as individual frequencies which couple to form a main coupled response within the soundboard? Or should they be treated as totally separate "tone controls" for different parts of the instument's voice, like a parametric EQ?
The reason I ask is that I have an assembled guitar, and am considering further brace shaping to lower the top main...ahhh, which braces to hack into first? Never done X bracing before...bass bars sound different frequency wise, X either side are also different. BTW the guitar sounds FANTASTIC already, tight solid punchy bass, silky poppy mids, not clangy, sustain to die for (CF reo neck with dual action truss rod) singing highs and LOUD...drowned out all the guitars it has played with. The issue is that it's coming out too strong in G and wolfing a little there. Main top 193Hz, coupled Helmholtz 100Hz.Before side mass, 220Hz, 103Hz... much more side mass and it will weigh in like a lignum vitae precision bass!
https://youtu.be/bNchJIfT2O8
In the attached video, Dana is talking different distinct frequencies...so should they theoretically be between scale tones? Are there notes that should not be paired, in that same way that back and top should not be an octave apart? I wonder...

[jeffhigh]

7Hz reduction with 100 grams is great.
I'd get it to 190 with very minor X brace shaving or extra mass your call
Nothing wrong with a 100/190 result
I have about 350grams on my last which is heavier than I would like
I don't really go with the speaker analogy , there is a lot more happening with a guitar soundboard especially with a fixed bridge as opposed to a floating bridge/tailpiece