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I'm in the process of researching concert classicals as I prepare for my first classical build. I've found a pretty wide variations of body depths. From as shallow as less than 3.5" to 4.25"+ at the tail block. For those of you who build classicals all the time what are you using? Also how much of a taper do you use between the tail and neck joint?

Thanks

Josh

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Josh House

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The air volume is likely related to the air resonance, and bass amount and quality, but I think it is rather subtle, unless you go to extremes (like those very narrow electro-classicals, or some ultra deep early 1800s Spanish guitars, or a very large shape (rarely classicals match an OM in surface)) I built a cutaway classical and it didn't seem to do any wrong.

I think the plate weight and stiffness, overall stiffness of the box (all surfaces plus linings influence) and soundhole size have a greater impact.

I guess most builders make them with more practical reasons in mind to start with. Material width, and guitar size. A big guitar with shallow sides looks and feels a bit ill proportioned, and same does a small one with deep sides.

My guitars varied from 103-94mm (end-neck) to 97-85. Perhaps the most important consideration is: what sort of build to you want to achieve. A light, very traditional guitar, or a more modern, heavily bodied instrument. When I use deep sides, I also add a number of perpendicular struts meant to increase stiffness and weight, and also heavier stiffer linings. A big guitar which lacks efficient stiffness in the body can easily sound too boomy and unfocused in the bass, at least for my taste.

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Build log

My builds vary from 91mm to 98mm at the neck and 93mm to 98mm at the tail. The waist usually comes in at about 98 to 103mm using a 15 ft radius arch across the span of the back. The lesser values are for std. classicals and the higher/deeper boxes are for those with a soundport.

I shoot for 98 to 102hz. (~ G-G#) for body resonance and have learned that stiff back and sides will help with attenuation of string energy much like Alexandu mentions. For flamencos I've been getting about 92hz. main air and the additional bass is welcome in those boxes.

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Peter

Josh,
My classicals are about 95mm at heel to 100mm at end block. Don't profile the back and rim assembly in a symmetrical dish, it won't look right. As you can see there is not a big taper in the sides.

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You could start with a makers demensions that
has a plan available from one of the
resorces above.
The G.A.L. has lots of them.

I usually think inside body depth when planning a guitar.
That usually makes the outside come out 5 mm+/-
different.
My tops are built in a solera with the area under the bridge 2+mm
scalloped out.
The # 1 brace under the f.b. is almost flat-just
a bit of a radius.
The # 2 below the s.h. matches the bridge area radius.
All braces either fan or what ever are glued on in a go-bar
deck with a dish that matches the solera radii.

"Flammers"inside for me-Heel 84-6mm-butt 90/92mm
classicals heel-inside-90mm heel to 95mm butt.
These depths give me the tone/volume that I'm looking for.
My biggest classical is only 36cm. wide
NOT a big guitar.

Mike

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Mike Collins

I mostly build with rosewood for my classicals and they are usually 86mm @ neck and 90mm @ tail +/- a mm. However, I am finishing up a maple guitar and have a myrtle guitar started (among several others) and for these two I decided to go 90/94mm.

Thanks for asking this Josh. I'm currently in the same situation. I've been looking at both Bogdanovitch and Courtnall's books. They are narrow fella's. No dreadnaught that's for sure.

Interesting Bogdanovitch's plan is 86.5mm tail and neck end. No taper! That seems to be on the narrow end.

The tough question for me will be, nitro or..... sigh...... French polish.

Cheers,
Danny

Thanks everyone for the replies. It is very helpful information and gives me lots to go on as I continue to draw up plans for my guitar.

Tom, why won't it look right to profile the back in a symmetrical dish? What am I missing?

I've got some other classical related questions, but I will save those for other threads.

Josh

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Josh House

Canadian Luthier Supply
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House Guitars - Custom Built Acoustic Instruments.
http://www.houseguitars.com

Josh H asked:
"Tom, why won't it look right to profile the back in a symmetrical dish? What am I missing?"

I'm not Tom, but the answer is that Torres didn't do it that way...

Eugene Clark gave a talk about the geometry of the classical guitar at last summer's GAL. According to him, the edges of the rim on both top and bottom are supposed to be flat.

The modern Spanish classical guitar is typically built top down on a solera. The lower bout area of the solera is dished and used as a form when gluing in the fan braces. Since the surface starts out flat, and the dish is not carried all the way to the edge, the rim ends up flat.

Once the sides are plugged into the neck slots and the tentellones done the back edge is dressed down flat, with a slight taper: according to Clark, about 6mm, the same as the thickness of the fingerboard. The back liner (often solid, and bent, which reinforces the back edge while you work on it) is put in, and dressed off to a slight angle around the edge to accommodate the doming of the back. The back braces can be glued on to the back, or inletted into the liner and glued to the sides. Each back brace will have a different curvature, so that the center line arch is smooth (although possibly not a circular arc) and the sides don't dip at the waist (this is tricky). If the braces were glued to the sides, you apply HHG to them, and then glue the edges down, using something like string to hold the back in place. Once it's secured you warm up the back at the brace locations to re-activate the glue. Torres apparently brushed on alcohol, and set it on fire; he'd have loved a heat gun, if he'd had a place to plug it in.

Clark as insistent that the wavy edge produced by fitting the rim to a spherically domed back looked strange, and hurt the sound. He could not say how or why the sound suffered, though. It would be nice to have something other than 'tradition' to back this stuff up, but I'm not holding my breath.

Totally flat results in a dip at the waist, it kind of looks like a violin plate carving. This is stressful on the joint to the lining and can make thin spots when scraping the purflings. It can also make smoothing the back and finish difficult. I try to leave that spot about 1mm higher (maybe a bit more) than the surrounding areas.

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Build log

[quote="Josh H"
Tom, why won't it look right to profile the back in a symmetrical dish? What am I missing?
Josh[/quote]

Alan's answer about Torres is probably correct about the genesis of the shape of the back. I follow Clark's method regarding the top rim being planar, but differ slightly from him on the back rim. I radius all three braces to a 20' radius and drop the sides about 5-6mm at the widest part of the lower bout. The back linings are sanded to a 20' radius with a board that has a 20' radius cut in only one direction, i.e. not spherical. The flat block with radius cut then trues up the surface from the heel to the waist by sanding in an x-pattern, which approximates a sphere. The net result is the back is about spherical in the upper bout, cylindrical from the waist to the brace across the lower bout and then the combination of the shape of the lower bout plus the radius of the back braces means the back can be brought to the rims with very little deflection, and it completes the longitudinal arc of the back. It gives the look I am after and the fact that the back is not forced into a tight spherical radius probably lowers the primary resonance of the back.

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--Reynolds Large