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I build steel strings rather than classical style. I have some mahogany that I could use for back braces, but from a sound perspective should I stick with spruce?

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Steve Sollod (pronounced sorta like "Solid")
www.swiftcreekguitars.com

Can't tell much, if any difference. If you want to alter the tap tone of the back, change the height of the braces.

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John

Spruce would be stiffer for the weight, but for back braces sometimes you need weight more than you need stiffness. I usually use spruce, but more of them and low wide ones in the lower bout.

I've used walnut, mahogany, cherry and even oak. I tend to prefer spruce now just because it's light and strong but like JA said you can adjust the stiffness accordingly by reducing and or adding height.

What Clay said.

Cherry is a pretty good substitute for mahogany, as is walnut.

Okay - What I'm hearing is that mahogany, cherry, or walnut can be used for back braces. Spruce is lighter, but that may not matter much. "...you need weight more than you need stiffness". I'm not sure I get this... Does anyone want to discuss this some more? Clay? Alan?

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Steve Sollod (pronounced sorta like "Solid")
www.swiftcreekguitars.com

don't rule out Poplar and Catalpa

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

There are two ways to lower the pitch of the "plate". One is to reduce stiffness and the other is to increase mass. Mahogany is less stiff and heavier than an equally dimensioned piece of spruce, so it should do both. The shape and orientation of the braces can also have a great effect on the outcome, just as it does for the soundboard braces.
Some people tune the back to fall within a range of tones to the top, so sometimes a less stiff heavier brace will help (low ,wide mahogany) or a stiffer lighter brace might be called for (tall, narrow spruce) This is more important when building an "active" back.
As I mentioned I tend to use a combination of tall narrow and low wide spruce back braces and more of them (5 on a "parlor" size guitar). There is more than one way to skin a cat (non-live back) and spruce is what I usually have on hand.

Another way to think of it is that the top is the part that is directly driven by the strings. Everything else gets the energy to vibrate from the top. In the very low 'bass reflex' range the back can actually improve the output of the guitar if it can work together with the top; that is, if it's not too stiff, so that it has a resonance that is not too far above the 'main top' resonance. Above that range (which is near the open G pitch on most guitars) the top will be far more effective at turning string energy into sound, so making the back or sides too loose will cost power. It's not all bad: this is where a lot of the 'tone color' of the guitar seems to come from. So what you want to do, IMO, is to get the back to be 'active' in the bass reflex range, to improve the low end power, and then have it be a 'reflector' for the most part above that. You can't avoid having resonances in the back that cost power, but by keeping the activity level down and the band widths of those resonances small (it's only costing energy when it's moving) you limit the cost. You keep the activity down by using a heavy, stiff back, and the band widths depend mostly on the loss in the system. That's why a dense, stiff, low loss wood like Brazilian rosewood works so well for backs. Light weight in the top is helpful, as it means more sound for a given amount of power, but the back has a different job.

how thick do you leave your backs?
Mike

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Mike Collins

I generally make backs about 2.5mm thick (.1"), with some variation depending on how dense and stiff the wood is, and the size of the box.

At least I have one thing in common with Mr Carruth.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.