The harmonic character of the resonance is what matters. The stiffness and hardness are same as THD, meaningless. Some soft woods sound great (spruce) some sound bad (pressure treated fir.) Some hardwoods sound great (maple) most sound bad.
I built my first DIY speaker with "Finnish White Pine" (spruce) boards from Home Depot, against the DIY know it alls' advice. It was well behaved and intoxicating on all kinds of music. No bracing, no stuffing, no wood finish. Of course it resonated, but the box resonance sounded like music. Even room treatments diminished the goodness from these speakers. They wanted to ring and fill the whole room with love. The president of Feastrex heard them and said they sounded really good. That was a high praise because he was notoriously negative and closed minded about his drivers playing in any box other than the ones he sold. In the 8 years since then I have never seen any speaker made from spruce, except
Robert Altmann's.Then, heeding the know it alls' advice I built another set of similar design speakers from baltic birch with bracing. Same driver, same amp, same room. It had a much stronger personality that worked very well on some kinds of music, but very bad on others. The best it ever sounded was playing jazz drum solos, because drums shells are made from birch plywood. Absolutely real, Tony Williams in my house! Paper drivers and drum heads, in birch cabinets and drum shells. Dual horns per speaker made the real dynamics. Too bad it couldn't do much else that good!
If you want to play many different kinds of acoustic instruments, especially classical music where oboes and violins and bells and tubas all play together on the same song, then the material of choice is totally non-resonant, so as not to enhance the flavor of one at the expense of the others. You want tonal accuracy above all else. Wilson uses non resonant phenolic, Magico uses thick metal braced with internal metal frame, Vandersteen uses MDF with internal damping layers of stone and rubber. Many use bent thick laminated wood or MDF with dense bracing.
Interesting and entertaining articles about why wood and other carbon based materials sound good:
http://www.mother-of-tone.com/vibration.htmhttp://www.mother-of-tone.com/mother.htmMechanical resonance and musical materials is a dominant distortion type that we use to color the music we play to suit our personal taste. When given the choice, carbon based materials tend to sound better than metal and plastic, but certain woods are best of all, like spruce and maple. Carbon fiber is being used to make musical instruments now too, and speaker cones and boxes.
