Speaker Height / Tilt

I’m not understanding how a dome tweeter is directional... I have a d’appolito designed speaker.  How does that change things.  

But seriously, how does a dome tweeter radiate sound below its axis vs above its axis?  Am I misunderstanding you?  I would think that a dome tweeter radiates sound uniformly top to bottom, side to side...

And if its okay to sit below the tweeter and not above, can you fix that by turning the speaker upside down?  

Last, if its not good to sit above the tweeters axis, why are manufacturers making shorter floor standing speakers with tweeters below a listeners ear level?

I understand that tweeters sound / measure differently off axis, but I don’t understand how they radiate differently above vs below the axis. 

Also, changing tilt changes the tweeter axis, right?

So, here is the evidence tweeters are in fact directional. From the spec sheet of a typical tweeter:
https://www.madisoundspeakerstore.com/soft-dome-tweeters-vifa/peerless-dx25bg60-04-1-textile-dome-tw...
Notice the frequency response chart. Each line represents a different location off axis.


Next, please refer back to the measurements I pointed from Stereophile.

What happens is that the relative difference in distance to your ears causes constructive and destructive interference.  Speaker designers (like me) have to take the acoustic distance into account when designing crossovers.

D'Appolito is the exception in that above and below the tweeter axis is the same. As I tried to mention above, my comments about listening at or  below the tweeter axis applies only to traditional designs with the tweeter on top. With D'Appolitos it's the same amount of pain, and sometimes more so, below and above.

After he initially introduced his famous configuration, Dr. D'Appolito has suggested fixing this by increasing the steepness of the filters if you use his design. That is, if you'd normally do OK with a 2nd order (12 db/Octave) you should try a 4th order (24 db/Octave).

What erik says " I find a lot of speakers sound better if I listen below the tweeter axis, and sometimes with the tweeters not pointed directly at me"
That is exactly my experience with the Wilson Cubs. They sound best with the tweeters just above my ears & intersecting a couple feet behind my head. Same was true with my Tekton's, except those needed to intersect a few feet behind me (less toe in than the Wilson's).