Why do different woods produce different tones?

[Maud]

I could be wrong but, I don't think Alder is considered a "cheap" wood. Compared to something like Teak then yes it is inexpensive. But Teak makes for better furniture than guitars

Maybe cheap was the wrong word but it's certainly not an exotic wood I (maybe wrongly) associate with high end basses.
Hopefully you know what I mean .

 

[SGD Lutherie]

As others have said, the body and neck form a resonant system. The string's vibration excites the body/neck and some of that energy is absorbed and turned into acoustic energy. The lighter and more flexible the body/neck system, the louder the sound and less sustain the string has. An example of where this is undesirable is dead notes on some bolt on neck basses.

An extreme example is a banjo. Lots of volume, and very little sustain. All the energy is spent exciting the drum head. The opposite is a composite instrument which would have little sound output and lots of sustain.

Wood, being what it is, does not absorb the string's energy in an even fashion, so it sets up a notch filter type response. Hard woods like maple and ebony tend to be brighter than soft woods like swamp ash or basswood. Woods with a lot of oil, like rosewood, have a darker tone even though they are dense and hard. All this is a generalization, but when you work with enough of the woods you get an idea what kind of tonal qualities they have. And every piece of wood is a bit different.

We can also get into the body shape setting up standing waves inside the body, but that's a whole other can of worms.

 

[MostlyBass]

Disregarding all the interactions of components and thinking of just the wood - like a bow or violin - it often comes down to....density.

 

[Calaverasgrande]

there are a couple things;
sound travels at different speeds depending on the density of the material through whcih it travels. The denser the material the faster it moves.
There is a principle of acoustics that when sound crosses a transition between materials it changes direction. Think of water in a glass and how it refracts light. Sound proofing often takes advantage of this by using multiple material transitions in a small space to cause the most loss of sound. Often doing better than an equivalent amount of concrete!

 

[pilotjones]

there are a couple things;
sound travels at different speeds depending on the density of the material through whcih it travels.

Yes. And depending on the stiffness.

The denser the material the faster it moves.

No. Slower.

Speed of sound in a solid is proportional to the square root of the stiffness, and inversely proportional to the square root of the density.

There is a principle of acoustics that when sound crosses a transition between materials it changes direction. Think of water in a glass and how it refracts light. Sound proofing often takes advantage of this by using multiple material transitions in a small space to cause the most loss of sound. Often doing better than an equivalent amount of concrete!

Sound insulation takes advantage of the inefficiency of energy transfer between materials of widely varying mechanical impedances, somewhat was you said.

 

[Calaverasgrande]

Yes. And depending on the stiffness.No. Slower.

Speed of sound in a solid is proportional to the square root of the stiffness, and inversely proportional to the square root of the density.


Sound insulation takes advantage of the inefficiency of energy transfer between materials of widely varying mechanical impedances, somewhat was you said.

looked it up, you are right,
the wiki entry even points out that some textbooks mistakenly assert what I had said about density and speed of sound.
Thats what I get for being out of college for 25 years!
(stupid worthless degree...)