" There's a little witch in all of us" - Alice Hoffman (Practical Magic)
 
After 1000 AD, people made bells from iron. And, according to European lore, iron scares off or, at the very least, agitates faeries. So much so that folks kept iron horseshoes above the doorway to ward off the fae and evil spirits and employed iron bells as protection in various ways. In addition, church bells scared off trickster spirits.
And traditionally loud sounds, like those from bells or chimes, scare off ghosts and demonic entities. Is it the iron the bells are traditionally made of that has a cleansing, protective effect OR is it the purifying sound waves sent through the air that wards off the evil? Truly, I believe it is both of these things.