[charmerci]

So I assume that the center dome is reflected back so that the high frequencies are dispersed over a wider field. There's even a tiny hole in the center to let the very high frequencies through.

Would like to hear more comments about it.

[srb]

I thought they were merely tweeter guards to protect against accidental indentation.
 
Steve
 

[HAL]

Looks like a phase plug on the Infinity tweeter.

It improves off axis response for the tweeter, but can create a cavity resonance. 

[charmerci]

What's a cavity resonance?

[Elizabeth]

The type with a circular area very near the surface of the tweet dome are 'phase plugs'. They usually are there to create a wider dispersion field for the tweeter, at the cost of clarity of the direct sound.
I had them on my Canton CD300/Cd360 speakers (for my 5.1 setup) and remove all of them fronts. center, and surround) taking them off MY Cantons helped the sound be clearer.
I would say they are a tradeoff put on 'lesser' speaker tweeters to accomplish something a different design would do better at.
For the rest of the designs for tweet covers that are just bars or a screen, they ARE just to protect the tweet dome from kiddy hands, and drunken guests. And those can also be removed to better effect with the hazard of squishing the dome (Which I have done on my B&W 805S in a drunken stupor, New Years Eve...)

[Letitroll98]

Yes Liz, one of the best tools in the old tweakers box is a pair of diagonal snips to get rid of those silly phase plugs.  Anyone that still has these affixed to their tweeters should carefully remove them.  Be advised this tweak is non-reversible, but don't know of anyone who wanted to replace theirs. 

[face]

Be advised this tweak is non-reversible

And detrimental to on axis response.

[rollo]

  When I owned Dalquist DQ 12 the Vifa tweeter had a thingie. Clipped it away, added wool felt around tweeter and the sound improved. Just took the edge off the sound.
 

charles