Woods for bracing

[Guitar Hack]

I'm curious as to why we use spruce as brace wood on the back of a guitar or even on the top. Seems to me the spruce would tend to absorbs sound so a hard wood would tend to reflect it better. Since most the tone comes from the top why wouldn't we use hard woods on the back braces?

[Darryl Young]

I'm curious as to why we use spruce as brace wood on the back of a guitar or even on the top. Seems to me the spruce would tend to absorbs sound so a hard wood would tend to reflect it better. Since most the tone comes from the top why wouldn't we use hard woods on the back braces?

Some folks do use softer hardwoods like mahogany for back braces. But mostly spruce is used there for the same reason it's used as brace wood on the top.......it has a very high strength to weight ratio. There is a limit to the energy the strings can put into the top to make it vibrate producing sound. It takes more energy to move more mass so in general, lighter is better. Said another way, more mass would typically mean more dampening or less volume or less sustain. Even though lighter is generally better, you still must have sufficient stiffness to resist the pull of the strings.

Taking these two factors into consideration, woods with high strength to weight ratios work best.......and few, if any, work better than spruce. Since the back isn't as critical to sound as the top (doesn't vibrate quite as much) you can get by with mahogany or similar for back braces.......just not the optimum (at least in my mind). It looks nice though.

[tippie53]

I like Kens description as you can make it the way you want but don't expect it to sound like what we think a guitar should sound like . I think you may have some wood properties confused. You may be thinking about dampening as sound absorbing. The top must be able to move , yet be strong enough for the load that is placed upon in..
This is why spruce is used . It is also used in houses for load carring beams. You ever see an Oak 2 by 4 ? Spruce is an efficient load carrier. To make an oak stick to carry the same load would be heavier than what the spruce would be. I have heard a few guitars with Mahogany as top bracing. While it didn't sound bad , there was no volume at all.
I am a firm believer in natural selection when it comes to engineered products. A guitar has been around a long time and it has evolved to what we see and hear today . I agree most real discoveries come from non status quo thinking . So with that . do some homework and look at the material you want to use , and here is another thing to think about . The shape also has much to do with the way the brace will behave.
I cannot speak for Ken or anyone else here but early in my career I played with the shape and size of the braces. I now use a deflection test to tell me how to voice the bracing for the tops. We all have to find that way to control the process so we have repeatability in the product. Have fun and enjoy , we are all here to help .
If making a top very flexible was the key , we would have figured out how to brace a piece of paper by now .

[tippie53]

I was asked how I measure deflection . There are a number of ways to do this. I have a 5 lb weight. When I make a top I will take it to .125 and measure the drop from the lead and then take the top to .115 . This lets me know how much the top may move.
When I do the bracing I will then use the same weight and I want to get the top to be stable with the 5lb weight when placed at the bridge location. If I see .005 I don't get worried. This gives me an idea how the top will take the load , it is at this point just data I collect to see a correlation . I don't trust tapping other than to heat what the top note tone happens to be/

[tippie53]

I want to see that the top is ready to deflect at 5lb. I look to see that the top is just starting to move , remember this is not what the guitar will see in actual force and I do the final adjustments on the guitar . By haveing the top ready to move , I know the top will . I look for about a .050 move once strung up.
This isn't an exact science but more of an art. This just gives me information that the top is going to deflect with the braces and the final thickness to a point I want to see. I like to see that the top is just at the point of moving. We all have to find what works for us . It beats just tapping on it.

[Tom West]

Weight in the top is a tone killer. So why not use Balsa wood for braces...? Not stiff enough...!! As a couple of folks have pointed out,Spruce has the best stiffness to weight ratio. A lot of folks measure deflection of the raw plate but I do not see much about measuring the braced plate as John H. is describing. Doing a combined before and after braces deflection test certainly makes a great deal of sense as you have more firm data to guide you. Think I may try the second deflection test myself but will also continue to do my tapping. Thanks John.
Tom

[Guitar Hack]

I think you guys are missing the point here. I was talking about the back of the guitar which generally reflects sound. It would seem to me that a soft wood like spruce might tend to absorb sound where a harder wood would more effectively reflect it. Otherwise we would use spruce as a back wood as well.

[Darryl Young]

If the back was just a sound deflector, then making the back very thick (and thus stiff) would make it sound better. It's more than a reflecter. The back vibrates and at some frequencies the vibration couples with the vibration of the top which can affect the power and tone of the guitar.

Also, what do you mean by "absorb sound"? Are you referring to dampening?

[Guitar Hack]

Is there a difference between the velocity of sound traveling within the wood and the woods reflective properties. Spruce is a soft wood. Soft things tend to absorb sound. Harder substances also tend to reflect sound better. If I want to sound proof a room styrofoam is great but it is really soft. Spruce makes a good top because it is light and vibrates well. This allows the top to move more freely and project sound out the sound hole and off the top. In doing this function it's sound absorptive capabilities don't matter much because you're are just trying to get the air to vibrate. Common sense tells me that a soft wood will tend to absorb sound when it hits it and harder woods will tend to reflect it.

I could be all wet but wasn't able to find any data on the web about the reflective properties of various woods.

[tippie53]

When you say spruce is a soft wood , that is just a term , some conifers are harder than deciduous trees. Too hard a wood will make a very tinny instrument. Balance is the key .