@deadeye – Thought you might enjoy this quote from Audioholics:
Editorial Note by Steve Feinstein on Crossover Frequency Selection
A basic, ages-old but still true, rule of thumb states that a designer is usually safe when he crosses a driver over at double its resonant frequency. If a tweeter has an Fs of 1500 Hz, use a 3000 Hz crossover, minimum. If a midrange is 300 Hz, use 600 Hz.
Another good rule of thumb says, “18 dB down at resonance.” If a tweeter’s resonance is 1500 Hz, the voltage curve of the crossover should show the tweeter section being down 18 dB from “0 dB.” That kind of conservatism all but assures no tweeter burn-out.
This was the “rule” at a major speaker company I used to work at, and the engineers all hated it, because it was so conservative and resulted in very high tweeter crossover points. But we almost never lost a tweeter and our warranty costs were vanishingly low. ‘Real world’ vs. ‘theory.’
So the doubling of the resonant frequency, which coincides with the peak impedance, which is the hottest and hardest for a tweeter to play, is to protect the tweeter from being rode too hard and dying prematurely.
I like the second rule, though, which is another way to check for it: 18dB down at resonance. Now, the reason I like this one is in the event you are using a third or fourth order crossover (rare and takes into account issues that can happen in phase if you go out that far), you have another measure as you may wind up being fairly close to the double Fs or just barely under, but you can look at the resonant frequency, make sure it is down 18dB, and see if the tweeter is being rode too hard.
As was put but the editor, the engineers believed that was way too conservative. But depending on your price range and the amount spent on your tweeters, what is being a little conservative for a little peace of mind on something that brings you this much joy.
Thought you might like that reference.
Edit: also, for ribbon tweeters (i.e. AMT and planar), do you calculate the distance from the center like normal speakers or, because of the origination coming from the entire area, can you use the lowest point of the opening. Basically, trying to figure out how low I can cross the Foster mentioned above.