A bullhorn is a cone you hold up to your mouth so your voice carries much farther. You speak into the small end; the wide end throws the sound out. Coaches, tour guides, protest leaders, and police use one in a noisy street or a big open space. It is a handheld thing with its own little speaker — not a tiny microphone plugged into a huge sound system, though people mix those up. The word is especially American; in Britain people more often say megaphone. There is no bull involved. The “bull” is only the old habit of using that animal’s name to mean big and loud.