Free air resonance?

Sorry i didn't get back to you on this via email Mitch. It's been one of those weekends for me....

A tweeter with a measurably lower Fs can typically use a shallower crossover slope and / or be crossed over lower without as much of a problem. In order to achieve a lower Fs, the suspension of the driver would have to be measurably different and would also probably have greater mass. This mass could come from a heavier diaphragm and / or a heavier winding in the voice coil. Either way, increased mass is what you have to move and is directly related to transient response.

Having said that, the added weight due to increased motor structure is what you would want, not a heavier diaphragm. This is especially true of a tweeter since it has to oscillate in fast, short strokes. If the diaphragm / cone was heavier, not only would it be harder to get moving, once it was moving, it would be harder to stop. That's what overshoot and ringing are derived from i.e. lack of motor strength and internal damping.

As such if everything else looks good, take a look at the moving mass of the drivers. If the driver with the higher Fs is a sealed back design and the driver with the lower Fs is vented, that would also explain at least part of the differences in resonant frequencies. Sean
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Does that mean that since I'm going from a tweeter with an fs of 1500 to an tweeter with a fs of 600 that it is a good thing? Do I have more versatility with the tweeter with the fs of 600? I ask because this is what I'm going to try.I'm going from the Hales Vifa (fs 1500) to the Seas Millenium(fs 600).

Considering the lower fs ALONE, you're better off: the lower fs tweet will be well within its linear range if the x-over frequency was chosen for a higher fs tweet. Generally speaking, the further away from fs you cross a tweet, the better it is (lots to be gained in avoiding distortion).