Box tops & Sound

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Permalink Reply by Paul Doug on May 26, 2009 at 9:59am

Great question and I don't know the answer because I have not built enough. I know the best sound I have gotten is without the neck running through the box (attached to the end of the box instead). Coarse if you are using electronics, it's different. I prefer the sound of acoustical guitars, so that is what I am trying for.

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Permalink Reply by Mark Werner on May 26, 2009 at 11:06am

Number of things to consider. One, what is the top made of? Most boxes are plywood; some are pressed cardboard, and some are solid wood. My Cohiba mandola is solid cedar.

Second, how big is the box? Sound is dependent on how much air you move, and how the structure resonates. Generally, a smaller box will produce a "higher" range of sound.

How free is the top to vibrate? On several of my recent guitars, I've use a neck construction more like a real guitar, with the neck slotted and glued or bolted-on. This leaves a lot of air-space inside for sound to bounce around in.

I fear it's as much art as science...

I would like to take one of the nice Cohiba boxes and thickness-sand the top down to about the same as a regular instrument, then apply bracing. Have to find someone with a thickness-sander I can borrow...

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Permalink Reply by Dan Sleep on May 26, 2009 at 1:27pm

All my CBGs to date have had the neck run all the way through the box, and out the other end. The lid is then glued to the neck.

If I don't have the neck go all the way through, is the lid strong enough to support the tension of the strings, or do we need to glue some type of support inside so the box can hold up to the string tension?

Maybe someone can post a pic of the inside of one that doesn't have a through the body neck?

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Permalink Reply by Doc Oakroot on May 26, 2009 at 2:07pm

It depends on how stiff the box top is. A relatively flimsy top may require some support - either contact with the neck carry-through or extra bracing.

Contact with the carry-through will reduce volume. You can minimize this by rounding the carry-through (like a dowel) so that the contact is just on a line down the middle. Then use a bridge with two feet so the bridge vibrations push down on one side of the line, while lifting up on the other. This is how banjo ancestors like the akonting work.

In Shane's basic design, the carry-through touches the box at each end and right under the bridge. (This is more-or-less how Africa half-spike lutes, like the ngiri and guimbri work.)

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Permalink Reply by Mark Werner on May 26, 2009 at 2:50pm

I sure wouldn't want to try having a cardboard-type lid support any great string tension....That being said, my pressed-cardboard "aroma de Cuba" box (neck-through construction) has a really nice, loud acoustic sound.

The Cohiba box has a lid that's nearly 1/4" thick, solid wood. Even so, I used an X brace as it was going to be supporting the tension of 8 strings. So far, it's worked just fine; no sign of deflection.

There's a lot of room for experimentation with all this; cigar boxes are pretty cheap!

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Permalink Reply by Paul Doug on May 26, 2009 at 8:54pm

I want to add, the best sounding one I have I used the back as the front and the lid is the back of the guitar. I like that because it gives more solid face and you don't destroy the pretty graphcs on the box. I do brace the inside of the box and where I attach the neck.

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Permalink Reply by BT Cohosh on May 29, 2009 at 8:53pm

I've mostly done neck-through builds with the the neck "relieved" below the box lid. That said, through an engineering oversight I had an interesting result with my last build but one--

I had built three stringers on small Romeo y Julietta boxes for my two little boys for Christmas. Thought that b/c the boxes were so small they wouldn't produce decent sound. I was wrong--when they were done I was (predictably) jealous and built one for myself, but added a piezo pickup. I was bummed that when I got it strung up and played it that the sound, while tonally "good" was weak--not a lot of volume compared to the boys'. The trade-off was that when I played it through a *big* (500 watt) amp, I had to really work to get it to feed-back. It, frankly, sounded *great* amplified. I ripped it apart and found that the lid was pressing on some of the wiring below. I fixed that problem and the acoustic volume went way up, but the thing started feeding back when I played it amplified, even on my whisper quiet little 3/4 watt Ruby.

So, basically, the more play you've got on the lid, the more volume you're gonna get unplugged, but also more feedback tendency. Pick your poison.

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Permalink Reply by Brian Lemin on May 29, 2009 at 9:42pm

I can only speak through experience of making a few Lutes and Vehualas along with other Early Music Stringed instruments.

We take inordinate amount of care regarding the the soundboard. Its thickness, sometimes different in various parts of the soundboard. We then add braces and bars. Braces particularly prevent warping, bars add stiffness, in different places to allow for various tones to be emphasized or deadened. It is one hell of a science and one I never got to grips with, I just copied those bar patterns that had good reports or I knew would bring out the sound f the instrument i was making.
Soundboard thicknesses of 1.5mm are quite common, they vibrate better, but they need bars strategically placed. Particularly around the sound holes as they are near the bridge and have a lot of pressure placed on them.

Please, do not get the impression that I am flogging a superior line with these comments. I want a cigar box guitar not a classical guitar. Having said that, I am personally not going to have the fingerboards run under the soundboard. I will use the traditional guitar set up and leave the box open except for braces which are needed if you use this system. String pressure in an unbraced CBG or guitar come to that will result in coming down one morning seeing your instrument nicely bent! Believe me i have done it!

Bridge position, sound hole position etc are all important.

But here is the crunch. If you plan to to electrify your guitar then most of the above is very theoretical. If you want to improve your sound then get a better pick up and a better amplifier.

Having said that, I still like the idea of a freely vibrating soundboard.

To sum up. For a purely acoustical CBG then take care of the soundboard details. (Excluding reso which is another issue altogether) Thinnish, nice vibrations, support if required, bars if you think they will help (The CBG has a very small soundboard and as it is new to me I can not add much advice, except I think the effect of bars would be minimal.)

For amplified CBGs then I am not sure you really want to go to this trouble.

Finally. I want a CBG, twangy, noisy, bluesy. Bugger the theory! Maybe, one day I will make a pretty sounding CBG but not to start with. :)

PS see my other discussion on "what is inside the box?"... I am so curious as to what you very creative guys do that is out of site.

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Permalink Reply by Wes "I'm Baaaaack" Yates on May 30, 2009 at 8:36am

Now this right here has convinced me to allow the top to more-or-less float free and not have the neck attached.

One thing I have seen with neck reinforcement is to cut a channel down the neck about 1/16wx1/2d* and put in an iron/steel bar. This was done only on the neck portion. I wonder if bringing the bar all the way to the tail would be a better idea structurally.

BT, Did you add any bracing where the bridge is?

-Wes

BT Cohosh said:

I've mostly done neck-through builds with the the neck "relieved" below the box lid. That said, through an engineering oversight I had an interesting result with my last build but one--
...
So, basically, the more play you've got on the lid, the more volume you're gonna get unplugged, but also more feedback tendency. Pick your poison.

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Permalink Reply by BT Cohosh on May 30, 2009 at 9:57am

No bracing. Most of the boxes I use tend to be pretty chunky things. The Romeo y Julietta box had a 1/4 inch lid made of some kind of *very* dense pressboard. Definitely does not have a warm, woody tone--more of a nasal twang (which I actually really like in a CBG) but it does sustain like crazy (which I also like).

For a simple 3 stringer you probably don't need much in the way of reenforcement. I haven't had structural problems yet on any of my 3 string boxes, including my Don Tomas, which is made of a much less robust, thinner plywood.

The great thing about CBG building is that you can afford to be pretty impulsive about how you build. Unless you're into sophisticated lutherie, none of the materials you use is going to be terribly expensive--if something doesn't work out, salvage what parts you can and then move on.

Wes Yates said:

Now this right here has convinced me to allow the top to more-or-less float free and not have the neck attached.

One thing I have seen with neck reinforcement is to cut a channel down the neck about 1/16wx1/2d* and put in an iron/steel bar. This was done only on the neck portion. I wonder if bringing the bar all the way to the tail would be a better idea structurally.

BT, Did you add any bracing where the bridge is?

-Wes

BT Cohosh said:

I've mostly done neck-through builds with the the neck "relieved" below the box lid. That said, through an engineering oversight I had an interesting result with my last build but one--
...
So, basically, the more play you've got on the lid, the more volume you're gonna get unplugged, but also more feedback tendency. Pick your poison.

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