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All speakers have excursion.

Excursion - or more accurately, displacement (meaning, technically- how much air is moved as a piston of a certain radiating area moves in and out a certain amount of excursion), is how speakers make sound.

The relationship between "displacement" and "frequency" is in fact an interesting one.

Ever wonder why tweeters were only 1", and don't seem to excurt much... yet are capable of getting so loud as to drill a hole through your head?
...just as loud as your midrange speakers, that are maybe 6" in diameter, and can visibly be seen moving in and out?
Ever notice how far your mids move in and out when the bass hits?
Ever wonder why subwoofers have such large cones, and reach such tremendous amounts of excursion?
Ever play a subwoofer full range, or hit the "high pass" switch on the amp rather than the "low pass" - it plays sound, but hardly moves, right?

The relationship between displacement and frequency is that displacement naturally must more than double for every octave you move towards low bass frequencies.

Two ways that can happen, of course:

You can use a larger diameter speaker to pick up where that higher frequency speaker left off... tweeters are always smaller than midrange, right?

You COULD use a small diameter speaker also - but it would have to reach higher and higher excursion levels, the lower and lower you asked it to play.

Same goes for subwoofers...
Excursion really plays TWO roles:
1) more excursion = louder
2) more excursion = lower frequency capability.

It's one leg of Hoffman's Iron Law, in fact - which states the following three items are mutually exclusive:
1. small enclosure size
2. high efficiency
3. low frequency extension

Note that "efficiency" is "how much excursion you reach given a certain amount of power".
Note that "low frequency extension" involves reaching higher levels of excursion.

You - as the speaker cabinet designer - control whether you are going to ask your speaker to either be very loud at higher frequencies (look at SPL competitors, right? , or whether you'd rather have that speaker not be able to reach as high of an ultimate SPL level, but instead be able to play very low.

It's all in your hands - all of this for you to manipulate, to control as you decide what speakers are going to play what frequencies in your vehicle (and how you are going to make them play those frequencies - that's key, isnt' it? )...
...and it all ties to this relationship between frequency and excursion (strike that - "displacement" ).

Yet you never see it discussed... why do you think that is?

If the grill is touching the tweet, though, it will be a problem.

Tweeters do have excursion, but not much. Probably the highest excursion tweets I can think of are the Hiquphon home tweeters, they have a peak to peak linear excursion of 1.8mm-which is enormous for a tweeter. The Dynaudio Esotar is around .3mm peak to peak, SEAS Crescendo has a p-p of 1mm, can't remember what the Scan-Speaks are.

All of these are large format home audio tweeters with large motors. Compact domes like you see in car audio have much less excursion.

anything that makes sound will have some sort of movement...thats all sound is, moving air....however that being said i doubt that a tweeters movement is really very measurable at all....when you consider that at 50 hz a sub is only moving 50 times whereas at 16000 hz a tweet is moving 16000 times so its travel has to be extremely limited to do this per second

Quick question fellas and ladies:
Through some drunken act by one of my buddies the grill protecting my tweeter got pushed in at the very center...and it appears that the grille might even be slightly touching the silk tweeter dome. Now is this going to be a problem? If it is infact rubbing (its really hard to tell), is this going to be irritating and eventually wearing down the dome and making a hole in the tweeter?

Thanks for any input.