A number of years ago I wrote an article on alarm systems and we did a lot of testing - setting off alarms in various environments. What I have always found is that making a *lot* of noise is the best goal. You want to make enough noise that everyone around is well aware of what's going on. The ones breaking in won't want to be seen, and they won't want to be inside if they can't hear what's going on outside.
That means more than one siren/speaker, and a self-contained battery for the alarm system.
We also found a few units that not only emit a siren sound but also use a human voice. These were the most effective. One in particular that had a woman yelling "help me" even got mall security out of a shopping mall and into the parking lot when we tested it. A company called Bulldog was making them at the time, and I believe they are still in business.