i have an air horn -- the air bottle fits into a water bottle holder and the switch mounts on the handlebar. it is VERY loud - not sure the decibel but on the same level as a car horn. you can pump it up with a regular bike pump.

although you only get maybe 20 seconds of sound and then you have to pump it up and like tires it discharges with time --- once i pushed the botton and it was empty.

and it is NOT good for warning pedstrians as it is very loud and (in my opnion) evr rude and often scares them out of their skin (they think they are about to be run over by a truck!)

but i quit using it mainly b/c for me it was not so easy to push the button --- in many situations i needed both hands on the brakes and not on the horn... i currently ride with a bell (for pedestrians) and also yell. (i also currently in Germany in general have more "troubles " with pedestrains than autos, but i have thought of remounting mine for those auto-encounters)