Update 7/2018:This is a good door alarm, but imo, it's not loud enough to scare an intruder away.

Will definitely scare off an intruder and wake you up at the same time.

Should be enough to deter an intruder.

It's very reasonably priced and the sound of each alarm is so unbearably loud and high pitched that I'm confident that it would scare away any intruder

Meant to scare off any intruder and alert anyone who is around

These are actually loud enough to wake someone up and startle an intruder.

Too easy for an intruder to stop the alarm.

Not quite as loud as I