a few years ago at sharper image, i saw speakers that were advertised as using a certain wood with tight rings, similar to what a luthier would use to create instruments. it gave me a huge laugh. that's a great gimmick that would trick most suckers who don't understand speaker design. the wood most speakers are made of is medium density fiberboard or particle board (i.e. compressed sawdust), because it a very non-resonant wood. also it has very good self damping properties. it is also the choice wood because it is very inexpensive. though i'm not sure, i'd imagine that the resonance with plastic varies, depending on the type of plastic. perhaps the reason why speakers aren't made of plastic might be because a non resonant plastic with good damping properties might be too expensive (probably much more expensive than particle board). on such a small product as headphones, it's probably more economically feasable. not to mention headphones endure a lot more stress than speakers (speakers sit in one place, while headphones get moved around a lot), and headphones must be made of a more durable material, such as plastic. a non resonant wood like particle board can flake and would be unsuitable for headphones.