Screecher Aerosol Personal Alarm
Mace Screecher Aerosol Personal Alarm emits an "ear-piercing" blast readily recognized as an emergency call for help.
I found this when I was looking for existing lo-fi solutions.
Mace Screecher Aerosol personal alarm has a high-pitched shriek which can be heard up to 1/4 mile away.
Its convenient size fits into your pocket or purse.
Safety alarm features a 10-gram unit with a push-top activator and contains approximately 20 short aerosol blasts.