How Many Tweeters Does A Speaker Need??

A line-array like the Genesis is an altogether different beast than the 2-tweeter models. Line arrays do some cool things -- like losing only 3dB per doubling of distance (as opposed to 6dB), since there isn't significant vertical dispersion beyond the height of the array.

On some of the 2-tweeter models, 1 is a "normal" tweeter and the other is a supertweeter with a high pass of ~14kHz-20kHz that provides extension up to 35kHz - 60kHz (or beyond).

On speakers with 2 tweeters operating in the same range, there's no avoiding comb filtering effects.

And then there are those with a front firing and a rear firing tweeter as does my Dantax.

who needs tweeters. get a panel speaker and you'll get more realistic treble response.

This is a good question. I would like to hear a speaker designer's view. I thought doubling up usually to increase speed/response, in the mid range anyway. I always wondered how exactly those dual mid range cones could move in sync, and what issues that would lead to in terms of clarity.