Christened with a tantalizing name, the Rumbler, it sends out low, bone-rattling vibrations, so it is not only heard, but also felt. One has been tested on the streets of New York, but the jury is out on whether it is effective, offensive or terrifying.
There are two types of floating cities: cities that float in water (see ocean colonization) and cities that float in the sky (such as Cloud City). Both are still theoretical; the latter is really theoretical.
Geoffrey Landis provides an interesting point though about building floating cities on Venus, whose surface is generally considered to be akin Hell:
However, viewed in a different way, the problem with Venus is merely that the ground level is too far below the one atmosphere level. At cloud-top level, Venus is the paradise planet.
(quote from Wikipedia)