Some jurisdictions require a bicyclist to have an audible warning device, commonly a bell or horn.
My bicycle has an audible warning device. Well, actually, I have only one warning device which I move to whatever bicycle I happen to be riding: my operatically-trained voice. Though less courteous than a bell or horn to warn pedestrians, my voice can transmit messages, not only a warning — “bicycle behind you”; “passing on your left.” My voice can be modulated from a whisper to a shout, which is more likely to be audible inside motor vehicles than any bell or horn commonly sold for use on a bicycle (except an air horn, which would startle pedestrians right out of their shoes every time). Bells and little squeeze-bulb horns make the most sense on multi-use paths, but they are rather pointless when riding in the street.
My voice also operates without my having to remove a hand from a brake lever.
I live and ride in Massachusetts, USA. which requires an audible warning when overtaking a pedestrian, but doesn’t specify a device. That works for me.
Some bicyclists don’t like to install a bell on their bicycle; one claim is that there is limited “dashboard” space on drop bars.
I think that a more common objection is that these bicyclists don’t want to look geeky or add the horrifying couple of ounces of extra weight. After all, if there’s room for a bicycle computer one side of the handlebar stem, there’s room for a bell on the other side — or on the stem itself.
Bell and horn requirements could possibly be invoked by a police officer with nothing better to do, to cite bicyclists who are not committing any other offenses. If I had to defend myself against this charge in court, I’d bring little ding-ding bell, give a demonstration of it and of my voice, and ask whether the court could tell me seriously that the bell was a more effective warning device.
…and hope I didn’t have laryngitis on the trial date.
If I regularly rode where a bell is required by law, I suppose that I would install one. But I’d still use my voice most of the time.