Advantages of beryllium?

Completely ignore what johnk suggests about vintage speakers. The very few Be drivers made back 40 years aren't representative of the modern state of the art. What's more, modern speakers are built and perform vastly better than the 50's, 60's, and 70's products and the measurements leave no room for debate. 
As for the safety of Be drivers, you definitely don't want to break them. Just sitting there doing their job they're completely harmless. I don't think anybody makes more Be drivers than Focal. They're very nice drivers but they take a long time to break in. I personally wouldn't hesitate to buy them. 

@shadorne 
Huh... What are you talking about? Where's this ringing you're pointing out? Ringing, as you describe it, is called "break up", and there's no sign whatsoever of it in the Maestro data. Neither links contain data that suggests ringing. 

@shadorne 

Kinda like this?
https://www.stereophile.com/content/focal-aria-936-loudspeaker-measurements
That's what a pretty clean plot looks like with the exception of the excursion seen around 16kHz, which IS the resonance of the tweeter, which also reveals itself in the impedance/magnitude plot. That is NOT it's point of break up though. The internal damping of a Be tweeter is pretty much irrelevant because it's so easy to push it well beyond it's pass band. You seem to be confusing that with mechanical damping of the suspension. That's an entirely different matter. 
You seem to enjoy making broad, unequivocal statements about tweeter materials that don't reflect the reality. 

@erik_squires 
It's not so much as just a lack of necessity. The voice coils in the inverted domes are way smaller than anything you'll ever see in a convex dome even 3/4ths the size. You just don't need a magnet as large to concentrate the same energy in the smaller coil. 
And yes, the 936 uses an Al-Mg tweeter. I think I mentioned that. My point was to dispell the myths and dogma some like to generically attribute to metal drivers. 

@erik_squires 
I'm gonna ask you to prove that jazz. 

@shadorne 
@erik_squires 
https://www.stereophile.com/images/417wilsonal.Walexfig5.jpg
https://www.stereophile.com/images/1114FA936fig8.jpg
https://www.stereophile.com/images/817Rav2fig8.jpg
https://www.stereophile.com/images/217MagS52fig7.jpg

Which of you two smack talkers is going to explain to me exactly how ugly the decay plot of a Focal 936 is? 2 of these graphs are from $40,000 speakers, one is from a $109,000 speaker, and one is from a $4000 speaker. Help me, because I'm confused. I'm not exactly seeing the superiority in the silk dome and paper at least in terms of measurements. Looks to me like that Al-Mg micro-motor inverted dome is holding it's own a bit better than the silk and Be. In fact, it looks like you can blow WAY more money on speakers and get much less impressive measurements. Shadorne is up there whining about 5 small 1.5ms decays. That $109,000 Wilson is pretty much averaging 1.5ms decays all the way up to those TWO nasty spikes around 15kHz. 
So Erik, I did the homework you couldn't/wouldn't do. I just randomly picked some of the best reviewed speakers in the $40,000-$110,000 range and looked at their measurements. As far as I can tell, you're pretty much wrong. 
Though not exactly a meticulous examination of all the available tweeter designs out there, It appears that Be is a decent material capable of exemplary performance, Al-Mg holds it's own decently well, and silk absolutely blows. I mean, if ANYBODY is going to get silk right, wouldn't it be Wilson?

@shadorne 
You're quite right about the dispersion and waterfall. They say a lot. That Magico is a stunning achievement. As JA said in the measurements for the 936, it's measurements wouldn't be out of place in an expensive speaker. That it costs just $4000 makes it an excellent value. And as we can see from the raw data, even Focals better stuff doesn't measure a whole lot better. 

So......
@erik_squires 
You're more than welcome to claim I'm arguing about speaker prices, though that really is a canard as well as intellectually dishonest. And you can argue JA has some slanted opinion, which I find every reviewer does. It's my job to discern that slant and interpret what his description might translate into through my ears. Regardless, his opinion has very little influence on how a computer measures a speaker. He cannot will the microphone to detect what isn't there. Another canard tossed out there on your part. Like what you like. Try sticking to facts you can actually back up though. 

@erik_squires 
You're just making up things to claim I said which I never did. I'm pretty sure I said to shadorne that the Magico was an exemplary performer. You've got your warped, quirky opinions just like those you choose to ridicule and that's fine. You can't seem to back them up though. 

@johnk 
You just say silly things. You and your weird ideas about technology and it's evolution are laughable. I gave up respecting your opinions a while ago with that thread about how great ancient gear sounds. 

@erik_squires 
Yeah, and I've noticed that's often a low standard. All too often hard statements of fact are left uncontested and given the leeway of opinion. You know, like yours up above. I'm pretty sure you've got more technical knowledge than I do and it shouldn't be a matter of me just randomly selecting a few waterfall plots to prove you wrong, but that's what I did, didn't I? 
It's just arrogance on your part that you think you're word alone is the only proof I deserve. That seems a bit more trollish than me challenging you. Oh well. Just an opinion I suppose. Deal with it. 

@johnk 
Really? Unlike Erik I did prove my point.