Just a thought but I find the existing bike alarms/horns/bells inefficient, pitifully quiet, ineffective,wasteful, etc...
A while back at Canadian Tire, I found a "Variable, Reusable & Refillable Air Horn" that sells with the standard bike pump.
It lasts near indefinitely without use & about a minute full to empty (dozens of microbursts), weighs almost nothing, uses no power, near indestructible & very noticeable (even lowest setting!
It's also made from predominately recycled materials & costs about $25. Especially great as an alternative to the pointless, annoying horns on powerchairs...
I'll try to post a link to it...
Adam