Back in Copper #66, I wrote a Cynic column entitled "Nothing New Under the Sun?"---and judging from the comments on that article, my point was either unclear, elusive, or misunderstood. I'm about to comment on a similar topic, and will do my best to be clear---clear as a flawless diamond, a material which will be mentioned momentarily.
I'm a materials science nerd, and I make no bones about that. I worked in the world of racing engines, where there were frequent debates over the comparative properties of 4340 steel versus 5140 or 300M (materials used in forged or billet connecting rods and crankshafts), where titanium is preferred to steel (again, rods), tool steel versus metal matrix composite (pushrods), 4032 versus 2618 (aluminum alloys used in forged pistons), and on and on. I found it fascinating, and in recent decades, the number of newly-developed materials has exploded---metaphorically, one hopes.
In the audio world, the use of advanced or newly-developed material seems to go in waves. Think of what Stephen Jay Gould called "punctuated equilibrium": put briefly, Gould postulated that things stay the same until they don't. --Yes, that does sound a bit like a zen koan. To be more precise: rather than viewing evolution as a continuous, linear process, Gould indicated that new species appear in a sort of a blip---"punctuations"---and then continue in stasis.
Back at the beginnings of audio technology ---say, the end of the 19th century and the beginning of the 20th--- materials used were largely natural. Gramophone soundboxes used proprietary blends of materials for their diaphragms, in search of the magic combination of durability, low resonance, and extended frequency range. In some ways the requirements are conflicting, and that continued as cone drivers began to be developed: cones were generally molded or stamped pulp paper, with competing brands extolling the virtues of their particular mix. Cone surrounds were either an accordion continuation of the cone itself, or leather, or cloth. The materials used generally possessed a low Q---meaning that they were not highly resonant.
Not surprisingly, the concept of Q first appeared in work conducted by Western Electric, where much of the fundamental development of early audio and electronic products occurred (as discussed . The term was originally used to describe the tendency of an inductor to resonate, but the term's usage broadened to include mechanical resonance (or lack of damping), as well as the roll-off characteristics of a loudspeaker enclosure.
As speakers were developed with more-extended high frequency response compared to a standard cone driver, the conflicting requirements of stiffness/rigidity (to allow controlled movement) and lack of resonance (to enable flatter frequency response and greater listenability) came into play. Again, Western Electric was involved; the legendary 555 was developed in 1926, and is generally considered the first compression driver. The 555 used a carefully formed domed diaphragm of fairly soft aluminum, with an attached surround formed from the same piece. The diaphragm's "swirly" surround would be carried forward in almost all compression drivers, and can still be seen in modern units. It's easier to show that, rather than explain it:
It's been nearly a century since the 555 was developed, and those conflicting demands haven't vanished. In the last decade or so, the benefits of ever-stiffer cones, domes, and diaphragms have been trumpeted, using everything from beryllium, ceramics, even vapor-deposited diamond. Tweeters using such materials generally have response well above the range of audibility, but those very stiff materials can do very ugly things when they go into breakup mode. Paper cones or soft domes simply mush out, so to speak, when they break up, with fairly low-Q resonance modes, and just won't go any higher. The ultra-rigid driver materials, on the other hand, can generate very high-Q resonant spikes that are inharmonic and offensive to the ear. Careful crossover design becomes extremely important for such drivers, to ensure that they are never driven at their breakup frequencies.
Newly-formulated materials aren't always a panacea, and sometimes seem to be used more for marketing value than for their performance. Even the cost of certain materials---like diamond or to a lesser extent, carbon fiber---can have a certain status appeal completely separate from their performance. Indeed, if the "snobby" material is used improperly, performance may be worse than good ol' paper and plywood.
Exotic materials can be used in audio in almost any application, from cartridge cantilevers to speaker boxes. Whether speaker enclosures of aluminum, proprietary laminate, or carbon fiber perform better than the old standards of medium density fibreboard or plywood, has to be evaluated on a case-by-case basis. Our friend Peter Ledermann at Soundsmith makes a wonderful cartridge with a cantilever made from a treated cactus needle, for goodness' sake. Generalizations are just generalizations---not fact.
Audio is both fascinating and frustrating, simply because there are no easy answers. No one approach is IT.
For those dogmatists who extol the virtues of their ONE TRUE WAY on audio forums, I say: have fun! ...But you're wrong. ;->