Okay, my take on soft domes vs. metal domes; I've designed systems with both>
Soft domes came first. I first used them in the mid seventies in an attempt to find a substitute for the ubiquitous 2" phenolic-ring paper-cone tweeter. The phenolic ring was a great tweeter in that it cost about half that of the soft domes at the time (<$2.00 vs. about $4) for the same sensitivity.
The soft domes where harder to make light enough to get enough sensitivity out of them and you needed a really good Japanese or European silk dome with precisely sprayed, very light doping to give consistant results and frequency response out past 20KHz.
There were of course the Dynaudio's, Seas and Vifa's with fabulously flat frequency responses and free air resonances (FARs) below 1000Hz. But most commercially viable soft domes had FARS from 1400Hz to 1800Hz so you had to watch how far down you brought the little guy and you needed to use a second order filter at the minimum.
Listening-wise, I always loved the soft domes. By virtue of their cloth-with- sprayed-dope-domes they were inherently self damping and could therefore be forgiving or complementry to electronics which had a touch of transient crispness in the lower treble region. Truth be known, soft domes as seen by high speed photography can be seen to deform as they launch soundwaves so you might want to condemn them as romanticisizing the music. I wouldn't.
Well, what can you say? Mate a really well designed and excuted soft dome with decent electronics which are on the crisp yet accurate side, like a Yamaha Piano versus a Steinway, and what do you have. Music! Sweet, relaxing, accurate music. I love a good soft dome.....
Okay Hard domes. First off,all hard domes have two separate resonances. The free air resonance from ~1000Hz to 1800Hz and the so called "oil can resonance" whith is the resonance of the material itself. That's usually up around 25KHz and it's peak value can be fierce and wide, sometimes 10dB high and from 22Khz to 28KHz in width.
Hard domes started off on the wrong foot for several reasons. The materials, aluminum mostly, were too heavy and/or with incorrect annealing. Some were made in one piece with an all metal "suspension" which of course couldn't move very well and rang like mad. But they did measure well so the bad designs were produced for many years until gradually everyone figured out how to make a proper metal dome.
Take a soft material like non-annealed aluminum and anodize it. You've got a metal dome with internal damping, so internal resonance is minimized. No weird ringy sound riding on the music signal. Now give the dome a suspension made from rubber or doped fabric with enough throw to really excurt in it's low frequency range and you've got a very stiff, accurate piston radiator.
And that's where were at today. There's some nice metal domes which can go head-to-head with soft domes double blind and you'll be hard pressed to tell the difference. Listening in the Harman Multichannel Listening Lab I did become accustomed to hearing the accuracy difference of metal (CMMD) domes vs. some of the Infinity competition which uses soft domes. But there were usually other sonic qualities that that came into play also. So I won't say definitively that I could pick a super soft (expensive) dome design from a metal dome because there are usually too many other sonic factors that come into the picture when listening double blind.
