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This might seem like a silly question, but why a back arch? Does it improve the sound? Does everyone have a formula for this arch? If the tailblock is X" then the heel should be Y"? In other words, why does the sides get cut so the back has an arch from front to back? I'm sure there is an old tradition for this design, and I'm wondering how far off the traditional arch I can cut. Clintoncrich38766.8578472222

I think the great the arch on the back improves projection, but at an
expense of the bass response after a certain point...

edit.. my response made sense before the orginal post was changed....John Mayes38767.0496180556

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John Mayes
http://www.mayesluthier.com

    Depends on who's theory you want to belive. I think if you want to look at it from a practicle point , the radius allows the wood to shrink without spliting. If you build backs or tops flat you will see the wood pull as it shrinks and this will cause them to sink. If wood shrinks enough it will pull itself apart and crack. This is why Martin changed the back bracing from the lowe 2 being flat and shallow to the modern higher peaked back brace.
       While the back will influence the tone and sound it does so more from the reflective acoustic properties than vibration. ALso the arch is stronger than a flat.
    If you are talking of the back "wedge" this is to aid reflective acoustics as I understand it.
    john hall

Yes, I meant the wedge,I think? Maybe I should of named this post differently. But the arch I'm talking about it where your neck block is shorter than your end block. Is there a way to re name this post? Clinton

you can try the edit button

I think I'm understanding you.
I would have called it an arch Too.
I have built an instrument without a back arch and one with an exadurated arch. The one without tends to get flat and almost concave durring dry season but has great bass responce. The other one has a consistant responce year round and does not move with humidity changes, but a little light on the bass responce compared to the first one.
I think a little arch is good.
Wade

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Wade
Nashua, NH
http://www.wadefx.com

[QUOTE=crich] This might seem like a silly question, but why a back arch? Does it improve the sound? Does everyone have a formula for this arch? If the tailblock is X" then the heel should be Y"? In other words, why does the sides get cut so the back has an arch from front to back? I'm sure there is an old tradition for this design, and I'm wondering how far off the traditional arch I can cut. Clinton[/QUOTE]

I've never heard a really good explanation for this. My understanding is that that perfectly parallel top and back plates could lead to some weirdness in body resonance - in the modern classical guitar, this leads to often no more than a 4 or 5 mm narrowing of the body towards the neck block. In the steel string, the geometry is much more exaggerated. A good reason???

JK

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Jim Kirby
kirby@udel.edu

Yeah, I know you don't want parallel walls in a recording studio because of frequency cancellation. Same thing in a guitar? Clinton

Crich,

You can have a front to back arch in the back with the heel and neck blocks at the same height. This puts the largest body depth somewhere near the waist. This is what I did on the "Slim Jim" guitar I posted recently here and the design came about by accident as much as design. I really liked the sound.

I like the look of an arched back, suspect that it gives better structural stability with changes in humidity and think it adds to the projection of the guitar - but this all depends on your overall guitar design. You can deal with the bass responsiveness by the bracing pattern and profiling of the back braces.

I also saw on Sylvan Wells website that he made a guitar with the reverse "wedge" - higher neckblock than tailblock - that he liked the sound of.

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

[QUOTE=Dave White]
I also saw on Sylvan Wells website that he made a guitar with the reverse "wedge" - higher neckblock than tailblock - that he liked the sound of.[/QUOTE]

Yeah, it looks all kinds of weird, but it sounds wonderful, and is incredibly comfortable to hold (got a chance to play it very briefly when I was over in the US last year).

Dave, I went to your site but you don't have the Slim Jim listed in your instrument log. I love the looks of your shop! You have some very nice instruments, I'll just look and keep dreaming, maybe someday,naybe someday. Clinton

Crich,

My website is due a big update but I'm waiting for the hosters to install a new version of their software. Here's the OLF post on Slim Jim with some pics:

Slim Jim

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

John Hall hit it on the nose. the arch allows the back to expand and contract with out cracking or splitting it also is a stronger structure than a flat back. just like a dome roof can bare agrater loading than a flat roof. the same is true with the arched or domed back.

Now part of your questins was Why deeper at the Lower bout than the upper. I am no acoustic engineer but the lower bout of the top is the main diaphram you need a volume of air to displace. the upper bout is the monitor. you want to restrict this area to create the pressure drop that send out and takes in air displaced and drawn in but the diaphram.

I wasn't speaking the "Dome" or radius but the slant of the back that tapers the sides. The displacement of air idea hasn't sunk into my thick head ,but I will work on it. If the depth of the guitar( end pin) is 4 1/4", is there a formula or consenus of how much to taper the sides? Clinton

I had a little chat with Blanchard about this a couple of years ago. He said that he had tried different tapers and noted sound changes. Iirc, he said that more taper made the tone more 'even'. It makes some sense.

If you don't taper the depth along the length of the guitar the dome will tend to give you an odd looking variation in the side height. They will tend to be deepest just below the waist. Classical guitars often taper about one centimeter along the length, which is just enough to keep the side height nearly uniform from the tail block up to about the waist. I also think that a tapered box is a little more comfortable to hold, particularly if it's a big, deep guitar. Dreads and Jumbos tend to use more taper than smaller guitars.

Just for reference, my Santa Cruz F model has a 1" slant from tail block to head block. It looks like too much of an angle for me, so my last guitar I went 3/4 of an inch, with most of the angle starting around the waist. I also believe the angle puts another stress on the back causing some more stiffness. But that is just my opinion and has never been proven. Hope this helps!
Tracy

Awe come on John, are ya sure it made sence?

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Canada

According to Cumpiano, you start the taper 2 inches behind the waist. The tail block is 4" tall and his headblock is 3 5/8" tall. I don't have 4" tail block(4-1/8")So I was wondering .........? Allan, during your chat with Mr. Blanchard, sounds like you shouldn't put too much of a taper or you risk a bland sounding guitar? I guess like with everything else, moderation! Clinton

I put 1 1/8" difference between tail and headblocks !

It sounds magic , is very comfortable ,and looks groovy.

KiwiCraig

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CRAIG LAWRENCE of AUSTRALIA
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