AD&D 2nd Edition

When an alarm spell is cast, the wizard causes a selected area to react to the presence of any creature larger than a normal rat-anything larger than about 1/2 cubic foot in volume or more than about three pounds in weight. The area of effect can be a portal, a section of floor, stairs, etc. as soon as any creature enters the warded area, touches it, or otherwise contacts it without speaking a password established by the caster, the alarm spell lets out a loud ringing that can be heard clearly within a 50 foot radius. (Reduce the radius by 10 feet for each interposing door and by 20 feet for each substantial interposing wall.) The sound lasts for one round and then ceases. Ethereal or astrally projected creatures do not trigger an alarm, but flying or levitating creatures, invisible creatures, or incorporeal or gaseous creatures do. The caster can dismiss the alarm with a single word. The material components of this spell are a tiny bell and a piece of very fine silver wire.