Take a look at the humourous and effective noise flyer made by Rochelle Hartman (found via LibrarianInBlack.net.

The issue of noise in libraries has been around since the librarian at Alexandria told Archimedes to keep his water-pumping to a dull roar. LOL Recently however, there has been a realization that such strict admonitions to remain silent in the library has led to both a harmful stereotype of librarians and the discouraging of many potential users from using the facilities. There is a difficult balance to be struck between the concentration needs of the reading and researching users of the library and the interactive and casual (and very often youthful) users of the library.

This flyer is a great effort at serving both groups: to ensure a quiet environment without alienating and discouraging the users. It communicates the message, “keep your voice down” in a subtle and humourous way.

Here are some general noise related signs from Flickr.