Thanks that is a very interesting analysis...and a prime example of why this forum can be so great!
I see you measured some SEAS drivers different from the ones used in my particular Joseph Perspectives Graphene 2 speakers. Outside what JA provided in the stereophile review, I've never been able to confidently find measurements of the drivers used in my speakers. The spec mentioned for my speaker's drivers is "two 5.5" (140mm) magnesium-cone woofers." Early on I found that someone had measured the "new" SEAS graphene drivers, though the size is referenced as "SEAS Excel Graphene...5.25" Driver. So I'm not sure if it's the same driver. Anyway, the measurements are similar to what you got:
In this Test Bench, Vance Dickason characterizes the SEAS W16NX003 5.25” midbass driver, one of the new graphene-coated magnesium cone drivers from the brand's Excel Graphene range. This product line has been an ongoing project for more than two years, combining the already highly prized...
audioxpress.com
The writer seemed fairly impressed all things considered.
They are all of the same family.
No doubt Joseph uses modified versions of the Seas shelf-items. This is extremely common, all of these well-known manufacturers use drivers modified to suit their purposes. In some cases new surround or special tweeter like these very nice Jamo speakers that also use Seas Excel, the tweeter uses silver wire voice coil
:
Or even cheapen the driver by ditching the cast frame and using a stamped steel, or lesser magnet.
In your case, it does look like a non-standard surround with the foldy-ridges, which should help with some of the surround's non-linearities. Here are the Seas 18cm Excel models of Graphene, non-Graphene, and Nextel drivers:
The measurements above that I posted are of the middle one, the non-graphene version... All are part of Seas' ultra-stiff low mass approach. Perhaps the Nextel has a more damped peak, where the Graphene has less damped, but all of those drivers will make a similar screeching sound unless dealt with! The peak moves in frequency with the driver size:
The cone material can affect this resonant frequency, but to first order these light and stiff cones' resonant peak is determined by the size. For instance, the 18cm vs. 16cm versions of the same Graphene driver moves the resonance from ~5kHz to 6.5kHz.
As to break up modes, I never care much about that since I only care about how things sound in the end product. If the break up is dealt with well enough, it's no issue.
Joseph Audio was originally known for using Richard Modafferi's "infinite slope crossover," which I'm guessing you'd know more about than I do, but is a purportedly super steep crossover (purportedly dropping 120dB per octave for the first 42dB). Which if correct would seem to help realize the potential of many metal drivers. I understand the slope is somewhat controversial, and also Jeff Joseph has modified the crossover through time (I think he's crossing over the tweeter lower these days too). Further, I don't think the stereophile measurements showed anything particularly heroic was going on with the crossover?
It is very possible. I am not aware, and can't speculate! That may be a path that was taken. I like to use a series notch filter using an inductor, capacitor, and resistor as mentioned
above.
In any case, it seems to me Joseph knows what he is doing in terms of the final result, because despite the nasty break up stuff in these drivers, the end result is super smooth sound. That is the specific feature of these speakers that is remarked upon over and over by listeners at audio shows, reviews, and by owners. The smoothness and apparent lack of distortion is the very first impression they made on me, and continue to make on me.
The drivers are certainly extremely low distortion, the resonance is manageable. It's no wonder good speakers are made from them. Obviously Joseph is aware of the tradeoff here, and dealt it and was able to take advantage of the low distortion.