First, you want a strong, loud siren.
If nobody can hear it, it’s not going to do its job. Some have multiple sirens that work on different frequencies to maximize the disruption. You can find sirens that go up to 120 dB (about as loud as a big clap of thunder, a chainsaw, or even a rowdy concert). So, not too shabby.
Be sure the alarm is easy to trigger, but not too easy. Look for one with a pull-out pin that prevents false alarms while you’re rummaging through your backpack. And, like pepper spray, be sure you test your personal alarm.
Finally, check the battery life. Most personal alarms last for months or even years. The one we tested uses batteries meant to last for 12 months.