Do you say this because some tweeters can sometimes sound too "bright"?


In some cases, it's done on
purpose in a variety of different ways.


Can you elaborate?


"Muffling" a tweeter is generally done to tone down the brightness. An
important point here is that this can be very different from simply
attenuating the tweeter. Muffling a tweeter changes the tonal quality,
whereas attenuation simply...attenuates. For instance, take a look at
the off-axis frequency response of a tweeter vs. the on-axis response.
It's quite different.

Anyway, one way to "muffle" it is with mounting location. Another way
is to actually place an occluder in the path (as Eddie mentioned).