So... Why is it best/preferrable to use a soft wood for a acoustic sound board?

[William Shafer III]

Hello everyone I'm new to forum and have had a nagging question I feel you all would be best to tackle for me. Each book I've read and most material I've discovered online pretty much assumes that you will use sitka spruce or at least a English spruce or cedar. Why exactly is that?

I'm in the process of finishing my first instrument (a gourd mandolin) and am using bolivian rosewood for the top. I knew that it was against common knowledge but it looks great and will off set the lightness and softness of the gourd. Also, since I'm a glutton for punishment and like exotic woods I did the neck out of bubinga with a hormigo negro fret board. I'm assembling it now and guess I'll find out how it sounds.

What are all of your experiences with sound boards and how do you feel about using hard African wood in acoustic instruments?

[Ryan Mazzocco]

I'm sure the real experts will chime in soon, but for now I'll go ahead and jump in.
I wouldn't say it's a rule that you have to use soft woods. I've seen plenty of guitars that have hardwood tops such as Mahogany, Koa, Cherry, Maple, Alder, etc etc....
But here's the simple answer; the way I best understand it. While some hard woods might make decent sounding tops they simply don't have the strength to weight ratio as a softer wood like the spruces and cedars. This means that in order to have a hardwood top with the same strength as a softwood top it usually has to be heavier, thus less vibration, less tone etc.
That's not to say that it can't be done. It's just that USUALLY you get a better top with one of the "accepted" softwoods.
I think I hit all the high points. Now we'll let the pros fill in the rest and make clarification where I messed up.

[G.S. Monroe]

I would consider myself to be far from "expert" in the subject of tonewoods, but from what I have learned from building various folk instruments, I would say Tradition plays a huge roll. Tonewoods can be found amongst both softwoods and hardwoods, and every variety of tonewood brings with it particular characteristics. I would say the common use of spruce and cedars can be explained in that it is a known quantity. We know that spruce produces a pleasant tone, so we use spruce so that we can get that tone.

I sometimes compare instrument building to cooking, Music is a matter of taste. If you want your chocolate cake to taste like chocolate cake you use cocoa. That is not to say that it is wrong to use other ingredients, because many famous (and infamous) chiefs will create "signature dishes" by not following the common recipe, and blend ingredients in their own fashion. That takes a lot of guess work, instinct, and a willingness to toss out the dish and start over if it happens to just not be palatable. It's more of a risk, but when it does work, it really becomes something special and unique.

Every culture on this orbital rock has "traditional" folk instruments that were fashioned from local materials. Each culture also has it's own style/form/variety of music. You can play Mexican mariachi with a wide range of guitars, from a solid pine body Strat, to a Koa Uke, but it really won't have the same feel you get from a Spanish Cypress Acoustic.

[Trevor Gore]

If you want a technical answer:

There's a property called the sound radiation coefficient, which combines a factor indicating how easy the wood is to bend (Young's modulus, E) with how easy it is to accelerate (density, rho) and the sound radiation coefficient which is SQRT(E/(rho)^3) is an index of the acoustic power that can be radiated from a driven vibrating plate before damping losses are taken into account. As it turns out, the usual top woods have a sound radiation coefficient about 2 to 3 times that of typical hard woods. So the softwoods always make a more efficient soundboard and I guess people figured that out long before there was a technical explanation for it. Because the softwoods performed better, they became the default choice and the default sound.

More info here.

[William Shafer III]

Thank you for all the information. I've skimmed the document Trevor posted and it already has given me a much better idea of the why. I'll have to read it thoroughly later! I gathered from what you'll said it directly has to do with volume and secondarily with tone. I'd say that it is safe to say that a beginner should stick with tried and true. The paper did get me thinking though... properly braced some tight grained, quarter sawn hardwoods taken very thin could be made to vibrate beautifally. Hmm, I'll condiser that for maybe #4 and #5.

[Alan Carruth]

To put it another way:
There is not much horsepower in a plucked string. If you want to make a car with a small engine that will have good top speed and acceleration, you need to keep the wight down. In a guitar, power is more or less equivalent to top speed in a car, and high frequency response to acceleration.

Softwoods have a different basic structure that hardwoods in some ways. As a result, softwoods tend to have much lower density for a given stiffness along the grain than hardwoods. It's long grain stiffness that does the most to keep the top from bellying up, so you get the needed stiffness from a lighter top if you use softwood. It's interesting that many samples of Indian rosewood I've tested have about the same Young's modulus along the grain as Sitka spruce, so you'd need to leave an IRW top about as thick as you'd normally make a Sitka one. IRW tends to run about twice as dense as Sitka, though, so the IRW top would weigh twice as much.

Hardwood tops can have a very nice tone, they just don't tend to make as much sound as softwoods. Sometimes you want that: acoustic/electrics that are made to be played primarily amplified can work very well with hardwood tops, and they will be less prone to feedback at high gain.

Alan Carruth / Luthier