As it is a bit late for April 1, we have to assume that the announcement of bicycle horn that is literally deafening is for real.
The ironically-named Airchime horn comes from an American locomotive and is said to be louder than an F14 on take off with reheat. At a distance of 50 feet the F14 produces 130 decibels, but the bicycle, called the Hornster, can produce 140 decibels. In comparison, typical city traffic is 78 decibels.
If you are thinking 140 decibels is only about double 78 decibels, think again. Decibels are measured on a logarithmic scale, so an an increase of 10 decibels means a sound 10 times louder. In order to produce such a loud noise, the bicycle has to be fitted with a scuba diver's tank to pressurise the horn.
We can see one slight problem with the safety concept, which comes from the Environmental Transport Association (although the "Environ" in Environmental may be silent). That horn might well scare off an oncoming truck and save an accident. However, any motorists or pedestrians that had just had their eardrums blasted might be inclined to take direct action against the cyclist - and the weight of the tank is going to slow down any attempted escape.