Most alarm systems have some form of alerting equipment that sounds a warning, when the security at the premises has been violated, or if a potential fire has been detected.   These devices can be sirens, horns, bells, etc. and would be installed, preferably, inside of the building.  They are useful in scaring away an intruder, thus making the premises safer.

Local regulations need to be checked before any such equipment is installed on the exterior, because some municipalities ban this application.  Other communities require a maximum of fifteen minutes, automatic shut-off time on all external audible devices, so as not to create a neighborhood nuisance.