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These are two dreadnought soundboards I recently made, using a modified Cumpiano bracing method. The tops are completely flat, all the braces are quartersawn spruce from an online supplier, and the tops are 0.140 thick. The differences are the padauk has a large diameter single ring rosette made from a strip of herringbone with a strip of b/w/b on both sides, and the mahogany has a double strip of b/w/b very close to the soundhole. Another difference is the lower face brace positioning, and obviously the fact that one is made of mahogany and the other padauk.

Here's the weird part now that they are both complete soundboxes with binding. The padauk guitar has a nice low resonant tone to it when tapped near the bridge location. It sustains for about 3/4 second before fading off. The mahogany guitar...well...sounds like tapping my front door. It's instantly dead. I glued them to the sides/back in a similar manner (I'm not ruling out a mess up in that area, or the eventual problem coming from that part of the assembly, but just for clarification) and made them in the same shop all during the same few days. Any thoughts as to why they sound so different? I can provide more pics if necessary. Thanks.

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Mohagany is a lot more porous than paduk. Air litteraly runs through it. This alone is enough reason to kill the tap tone. Do the same test once a few coats of varnish will be applied to it and you will hear world of difference.

Also, paduk and mahogany don't have the same weight and thus the back will react differently from one guitar to another when tapping on the top.

I'm guessing the mahogany one will be lowder on the tap tone once varnish will be applied.

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

Damp the top and tap the back, damp the back and tap the top. On both of them. Most likely, the pitch top and back on the padauk one will be very close, and on the mahogany, not so much. The echo happens when they resonate with eachother... and is not a good thing when string are on it, because it causes a dead/wolf note at that pitch. But gluing on the bridge on drops the top some, so matching them before the bridge is a good way to get a set interval between top and back in the final guitar.

Once you figure out whether the top or back is higher pitched on the mahogany, you can go about shaving the braces and tapping until it drops to match... but shaving one will probably affect the other as well, to a lesser extent, due to the interactions with the air volume.

At least that's how I understand it, with my one completed guitar. I ended up shaving quite a bit off the top bracing to get the echo. Turned out nice

Brian: I assume that you are using Cumpiano's book as a guide. It is an excellant guide on how to build a guitar and lots of folks here have it and have used it ,myself included.But for me his suggestions tend to end up with too tight a top.The X brace 3/4" depth at the cross over is way too heavy and top thickness more then I would use. Alaine has pointed out one of the big problems with trying to get a handle on contol and that is that each piece of wood is different even within any particular specises.Therefore they all have to be treated a bit differently in regard to size,shape,location etc.The results of doing this type of construction are hard to convey over the forum with out having the materials on hand. Also there are so many variables within any one guitar so that if someone did handle the material and give suggestions that the guitar may or may not come out as desired. For me this is where the repeated building comes in,the result of changes that one tries in successive guitars slowly gets transfer into a more intuitive base.As you may have guessed some of the folk who answered your posts have built a fair number of guitars and are still learing and trying to improve,me included.Someone suggested that you get Somogyi's "The Responsive Guitar".If you want to understand a bit deeper how a guitar works,this is the book for you.By the way your woorkmanship looks quite good.Just one question for you. Wonder what you have used for bridge plate for the Mahogany dread.A bit of rambling but hope it conveys the struggles most of us have or have had trying to understand these machines. Good luck.

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A person who has never made a mistake has never made anything!!!

It's hard to see what the brace shaping looks like on the Mahogany, but from what I
can see, the braces on that one look more like scalloped bracing a la old Martins, while
the Padauk one looks more like what's called "parabolic". So that could certainly cause
some difference in how they respond. Also, without any stiffness measurements on the
tops, it's impossible to know how different they may be. I tend to build a good bit lighter
than you do- thinner and lower braces, but I mostly build OM's with a smaller body than
the Dreads, so I'm not qualified to say your bracing is too stiff. Also, I'd recommend that
you cap the X-brace join area- that makes a big difference in how similar the two legs
of the X are in stiffness.

As Dennis said, find out the pitch of the top and back of both guitars. It's generally accepted,
from what I've read, that the top should end up about one semi-tone lower than the back.
If you get them very close at the stage you are at with the closed box, then when the bridge
mass is added, the top tone will drop some. You can shave braces on top and/or back to
help get them where you want. I'd recommend you search for Al Carruth's comments and
threads on voicing tops. His DVD is also very good. There's a tremendous amount of information
on voicing in the archives. Do some searching on voicing and bracing, and you'll find a lot
of information.

David Hurd http://www.ukuleles.com/ also has a lot of information on measuring
stiffness and working with it. Look under the "Technology" section of his website.

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Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

First a thank you, and second I hope that when I get a good coat of finish on the mahogany to seal it all up that the tone returns to the mahogany guitar. Also, now looking into the bracing I can start to see some of the differences that you all have pointed out. It seems that even though I literally built these two tops side by side, there were some areas where I didn't do the same thing on both, which is a contributing factor to the poor sound quality at this stage of the game.

Also, the Cumpiano book is where I got the basic pattern and sizing of my braces from, which I follow pseudo closely, and am having personal problems with just like some of you already mentioned. I am starting to see that there is way too much wood on the tops as reccommended by the sizes detailed in the Cumpiano book. The braces are left too thick, they aren't really shaped that much, and the tops are left too thick as well. It's odd that a poster mentioned that they build OM guitars because that's the style of top I have been working all this new information into from the forum posts and articles and it's way thinner. The top is 0.110" and the total mass of the braces is probably 2/3 of the dread bracing or less. My x-brace is still 3/4 high, I put a cap over the lap joint that I recessed into the leg by 1/8", there is no upper transverse graft (popsicle), my main braces (X and UFB) are 5/16 wide, and my others (LFB's, FB's, SHB's) are 1/4" wide. I also am in the process of shaping the braces to a tall skinny point as suggested by Somoygi. I have undergone a revolution in how bracing and structure work in the last week. I was always scared to try something out of the norm according to my reading material, and that was a mistake.

Also, I believe those were Goncalo Alves, quarter sawn, for the bridge patches. I'll also post a pic of the new top after this post.

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Here is the OM-15 cutaway top that I am working on right now with some of the new gems that I have been learning from everyone here. I think I want to drop the x-brace height a bit, which means I'll have to put in a new cap, but that's fine. I am also contemplating ramping the x-braces down faster to lower the weight even more. Everything still needs a final sanding. Let me know what you think.

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To my eye, this is much better. You've cut the brace mass down a lot, and I like
the tapers you've got on the tone and finger braces. In my OM's, I go for 5/8"
height on the X-braces. IMO you don't want to inlet the cap on the X-brace,
by doing that, you weaken the uncapped brace by cutting it's height down by
that 1/8". Just flatten the top of the X-brace junction, and put the cap over the
top of the brace that's cut on that surface. I think of it like this, the other brace
is "capped" by the top that it is glued to. You're just connecting the cut fibers of
this second brace like the first is capped by the top. So both braces have similar
height and structure at that point. If you decide to cut down the X-braces faster
in the lower bout, leave them up high at least until they clear the bridge. You want
that strength at least to that point to resist the string forces through the bridge and
down into the top and X-brace.

I'm on my 9th guitar, and I still am trying to figure out if I like the "parabolic" style brace
shaping you've got here, or the "Scalloped" old-Martin style shapes. I've done good
sounding guitars both ways.

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Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

Thanks Gozeirt, I actually made some adjustments since posting that picture. I did lower the height of the x-brace to just a shade over 5/8" at the lap joint. I have about 1/16" of cap left, which I hope holds up. I can see the point about how adding wood to one brace by weakening another is like rotating four bald tires...not really getting anything done there. I will try a 1/8" piece of spruce on top of the joint next time. It was the OCD luthier in me that saw a stub of wood glued to the x-brace and decided it would look far better if I recessed it into the brace. That being said, I would rather have function than beauty, especially in a part of the guitar not many people ever see. I also left the braces at a slow taper until they cleared the bridge area then ramped them down a bit faster. I'm still a little leary about removing too much stock. I want it to work after I make it, not rip the soundboard off when its brought up to pitch. The last step is to buy a scale and weigh it. I know that weight is not the only indicator of structural integrity, but on boards of similar flexibility and thickness with similar bracing patterns I think the information would be valuable from a uniformity stance. Also, as a side note, the board has a deep resonant tap tone that holds on longer than any other top I have ever made. I think it was all the good advice about not over-bracing and allowing the top to move a little more.

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I'm still a newbie, but it seems to me you could remove a lot of wood. .14 sounds too thick, and that popsicle brace looks quite massive. But what do I know?.

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"Preoccupation with an effect gives it power and enhances the error"
from "Your Owner's Manual" by Burt Hotchkiss.

Brian,
Sounds good. I'd go ahead and build with the top now. If it comes out stiffer
than you like, you can shave braces using a finger plane, or sand/scrape the
top thickness down a bit. For my OM's, I usually go to about .100 or a little
lower in thickness, depending on the stiffness of the particular top. But I
build very light, and normally use light strings.

When you do the cap over the top of the braces, you'll find that it looks fine
if you taper the ends down to meet the brace, then sand it smooth of
chisel marks and round the edges a little.

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Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

Sounds good. I just picked up some Bubinga from my wood supplier out here, and I was able to get enough wood for three back/side sets, three head/tail block sets, and some 2"X20" pieces that I dont know exactly what I'll do with yet, all for $75. Not a bad deal at $25 per guitar, and its all well quartered, not too much figure, but good color. I like working with Bubinga because it lets me get a harder wood on the rear end of the guitar without having to spend the $100+ for a rosewood set. Plus, it's not plain old Mahogany, which I'm getting bored with.

I think I'm going to do the side braces too on this one to help transfer sound from plate to plate, but I'm still debating. I've never done it before, and I don't want to fall back into the trap of adding more wood just for the sake of it being there.

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