DrIsotope

I don't think a white sound alarm is a very good option for us-- the sound is very directional, and dissipates very quickly in air. Also, there's the whole brutally expensive thing.

I also don't think most piezo emitters will be very effective, because people won't recognize that chirp as an indicator that a bicycle is approaching. What we need are just plain automotive horns, whose sole drawback is their physical properties-- they're big and heavy. But people know what they mean. Airzound is close to fulfilling my needs, but has too many caveats for how often I actually need to use it.