About getting harder. A popular belief has it that soft dome tweeters sound soft, hard dome tweeters hard. The problem with the latter isn't being hard. It's not being hard enough. Key are first breakup modes as a function of geometry and material. Those modes are proportionate to the material speed of sound. That is the square root of the Young's modulus divided by the mass of density or m4/kg/s. Some associated acoustic figures of merit—the higher the better—are 0.63 for steel, 1.13 for titanium, 1.86 for aluminium, 4.92 for diamond and 6.97 for beryllium. The mass of the dome as a function of size and thickness plus its geometry factor as well. A harder but heavier substance may not score quite as well as a somewhat softer but lighter one. Here we see that whilst the Young's modulus or hardness scale steadily increases from aluminium (71) to titanium (120) to beryllium (318) to diamond (1'000), density thus weight (Kgm) jumps from aluminium (2'700) to titanium (4'500), then drops well below aluminium to 1'850 for beryllium before increasing again to 3'500 for diamond. Watch this YouTube video with Dr. Gary Geaves, head of research for Bowers & Wilkins, explain the subject. Read up on some of the modern uses of synthetic diamond here.
Massimo Costa of Italy's Albedo Audio has successfully worked with hard ceramic Accuton unis for decades. "Regardless of cost, I'm starting to think that diamond is the only way to avoid even subtle treble hardness completely. It might seem contrary but the really annoying issue of hard membranes is their high breakup distortion. Meanwhile their distortion in the audible band is low; exactly what you want. Ceramic tweeters push their breakup beyond our hearing threshold, to ~30kHz depending on size. Even though that seems sufficient, it's not. Sometimes, a little harshness creeps in despite a low-pass filter. Diamond tweeters push their first breakup mode up by 10kHz or more*. Now breakup effects in the audible band are less. That's why diamond tweeters sound softer than ceramics." From this we take away that harder sounds softer; and that we can hear (the effects of) sounds we can't hear. Otherwise, how would we ever perceive domes break up at 30kHz in the first place? Probable cause is intermodulation whereby distortion beyond human hearing interacts with sounds we do hear.
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* According to Accuton, already their first prototype diamond tweeter had its breakup mode at 72kHz, a full octave beyond ceramics.
About dome geometries and their effects on the first breakup mode, we recall Laurence Dickie's unusual catenary dome profile (the natural form taken by a chain suspended at both ends) and the carbon-fiber damper rings around the edge of his driver membranes for Vivid Audio. They push up first breakup by an octave beyond where it usually occurs. That means 44kHz for an aluminium-alloy tweeter! From this we take away that it's not just harder and harder materials which push the first breakup mode further out into the ultrasonic range. It's optimized dome geometry as well. [Diamond membrane photo from here.]
Diamond as a tweeter material is obviously dear. It leads us to drive units just 1" across or smaller where a pair costs more than good three-way speakers from established brands; and where said pair operates across a very narrow meaningful bandwidth whilst the speakers cover (nearly) all of it. Cost-effective this is not. But getting ever harder comes at a price. With Franck's models, it's nearly six pair of magnesium alloy versus one pair of diamond. Which is about three pair of Amphions' very best monitor called Argon 3S. Today is thus squarely about polishing up what you already have. It's not about covering any basics. If all you had were a pair of super tweeters without main speakers, you'd hear very little indeed. Sizzle without the steak. Fizz without the champagne.
On the Avantages: "Both come in on a 1st-order 6dB/oct.slope. The magnesium unit's breakup occurs at 35kHz, that of the diamond at 75kHz. I auditioned a few available super tweeters. None convinced me because 'air' alone is insufficient. I also need correct tonality. By frequency doubling, my units open up the main speaker. Increasing treble dynamics and optimizing tone is what led me to an omni design. The difference is like that of listening to your speaker outdoors (without dispersion lens) or in-room (with the reflector). To confirm this, I built a directional super tweeter with the very same drive units. No doubt the omni was far superior.
"As to cost, magnesium is clearly far more attractive than diamond. But due to physics, magnesium can't capture the full harmonic tonality or extend as high as diamond. If I vapor-deposit a thin layer of diamond atop a magnesium substrate, I can push its first breakup to ~50kHz. This might become a future option for those who can't afford pure diamond but wish to go beyond magnesium. Of course performance depends on dome thickness too. The thinner and lighter it is, the higher its resonant frequency. Once we add a diamond layer atop our magnesium dome, we increase its thickness and weight to lower the breakup frequency again.
"By the way, I'll include a CD. It's a great recording which I recommend to all audiophiles to set up and adjust their systems with. Its recording engineer works with me on different projects. Now we're thinking of producing a demo disc. Joël Grare is a personal friend and an artist full of creativity. He visited me with Alban Sautour who records him. They really enjoyed their album's sound over César. The CD you already have is Paris, Istanbul, Shanghai which was recorded almost 10 years ago. Des Pas sous la Neige just came out. Alban progressed a lot in his work since the first Joël Grare recording. You'll be able to judge for yourself."
By now you might wonder how diamond tweeters are actually made; certainly not carved from solid which would take hideously costly rocks to begin with. Instead they're grown a bit like the 3D printer concept. Starting with a substrate to lock in the desired shape, powdered diamond is chemically vapor-deposited (CVD) atop it one layer at a time. If it's a diamond-coated dome, that layer can be very thin since it remains bonded to a substrate. If it's pure diamond, the layer must grow thicker so the now silicon substrate can be dissolved by acids afterwards. Hence the cost of a pure diamond tweeter isn't just its raw material. It's the laboratory hi-tech process to layer it up into a final dome of sufficient thickness to be mechanically fit for purpose.
Watch this YouTube video on how gem-quality synthetic diamonds are grown. This FAQ sheet on CVD diamonds has more answers. Finally in this now hifi-specific paper, we learn that diamond tweeters were first pursued in 1999 as a collaboration between the Fraunhofer IAF and Thiel & Partner GmbH aka Accuton. They also show that diamond tweeters reduce 2nd and 3rd-order harmonic distortion by a whopping 40% over already excellent sapphire membranes. Brands which exploit Accuton diamond tweeters include Albedo, Avalon, Ayon, Elixir, Estelon, Gauder, Kharma, lumenwhite, Mårten, Tidal and Vroemen. [$7'152/pr Seas T29-001 diamond tweeter at right.]
To conclude, at and beyond self resonance, speaker drive units stop behaving as pistons and start to deform or break up. Pushing this distortion further away from the bandwidth of human hearing has audible benefits. Harder thinner lighter tweeter dome materials do just that. But having compared conventionally directional prototypes to his omni finals with identical drivers, Franck Tchang heard that exploded treble dispersion not extension was the more dynamic, spacious and tonally correct. Whatever sonic benefits his super tweeters made would be primarily due to omnipolar not narrow beaming radiation like that add-on Tannoy. That's a bread crumb for those who insist on explanations before we proceed to subjective listening impressions. On how to use his super tweeters, Franck kept it simple: "At 90/91dB for the magnesium's and diamond's sensitivity respectively, they match the majority of speakers to market. I've tested many different kinds of speakers and they all worked great. Simply select 15kHz for older models, 18kHz for modern speakers. Also, experiment with the phase of the connection. The cable connection can be made from your amplifier or main speaker. Do use good cables. " C'est ca.
