Is it rude to use speakerphone in public?

The case against is stronger than the defenders admit, and it rests on a real cognitive finding rather than fussiness. Overheard speech, particularly a halfalogue where only one side is audible, is measurably more distracting than a full conversation or than background noise of the same volume. Researchers attribute this to unpredictability: the brain automatically tries to fill in the missing half, which consumes attention involuntarily. Studies have found halfalogues degrade performance on concentration tasks in ways that equivalent two-sided conversation does not. So the complaint that a speakerphone call is worse than two people talking is literally correct.

There are legitimate exceptions and they are narrower than practice suggests. Accessibility is the real one: people with hearing loss, limited hand mobility, or certain disabilities may genuinely need speaker mode, and that deserves accommodation without comment. Brief functional use, checking an address while carrying groceries, is fine. What is not defensible is a sustained personal call in a waiting room, a train car, or a restaurant, and the video-call-in-public variant is the same offense with a second audio channel. The workable standard is simple: if the people around you cannot leave, use the handset or step outside, and treat the volume of your own voice on speaker as the tell, since people reliably talk louder without realizing it.