Bells, Horns, and Whistles
Compressed-air and loud battery-powered horns are perfect for communicating with motorists in those few crisis, in-traffic, commuting instances when other means (hand signals no, not that hand signal or a shout) might not work. They are also good for female commuters who wish to alert a neighborhood or someone on a street far away about a threat from someone nearby. (Of course, no horn by itself will bring the same response as will a scream. The combination of an ear-splitting horn and female screams will cause every head to turn. Some pepper spray in the face and a knee lower down will get the mugger's attention as well.)
Performance Bicycle offers a nine-volt-battery-operated MegaHorn that puts out a whopping 100-decibel shriek, as well as a Delta AirZound Horn, which, at 115 decibels, is reportedly the "world's loudest bike horn." Interestingly, it uses no batteries. The rider compresses the air with a bike pump and the air reservoir either fits into a water-bottle cage or mounts to the top tube. While this is one more thing that a commuting cyclist must either worry about being stolen (if left on a bike that's locked outside) or haul inside with all one's other cycling gear, it does have the advantage of an adjustable volume. Whether it goes low enough to be sufficiently inoffensive to pedestrians is a question one should answer before making a purchase.
One innovative bike products company, CycleAware, has developed a "multi-signal electronic bicycle horn" that is powered by a nine-volt battery and emits your choice of a "friendly passing signal," an emergency siren, or a very loud blast. To my ears, however, this loudest squawk is a somewhat less blood-curdling blast than that produced by compressed-air horns. I like the quick-release handlebar mounting bracket and that it can be activated either by depressing the button on the horn itself or by a remote button on the grip (a feature shared by some other horns). This can be crucial when making an emergency stop, for it means you don't have to let go of your brake handle to hit the horn. The CycleAware "friendly" signal is, unfortunately, still not as friendly as a bell, but that's a price perhaps worth paying in order to have the louder-blast option at your fingertip.
Details mentioned in this article were accurate at the time of publication