Audio Indicator EXL-450

(Saftey first! Wear ear protection when working with noise.)

These are similar to the Super Buzzer, but they're slightly smaller, weaker, and higher-pitched. They can produce 80dB at one meter at 18.9V and just SIX mA! That's about an 80-hour runtime on a pair of series-connected 9V batts. Heck, you'd be fine even with a stack of button cells! Neat.

As you can see, they're pretty compact. They're a buck or two each, so feel free to stock up. Try not to annoy your neighbors too much.