Domestic internal alarms don't make a very loud noise. In our house we have a PIR-triggered external siren, 129db and difficult to get at. One of these in a hard-surfaced building will not only alert everyone nearby but more to the point it'll be pretty frustrating for the bad guys. It's quite difficult to think straight in high noise - the instinct is to get away from it. If the siren is fixed high up and behind wire mesh, with armoured power supply to it, it'd take a little while to disable (and not nice to stand in front of whilst doing that). 129db is pretty irritating but bearable for a short while if you don't mind hurting your ears. But two external sirens, at slightly different frequencies ... the dissonance would make a pretty shattering noise. Have some ear defenders outside in case you need to go in to turn things off.
