A megaphone in the strict sense is a simple cone that channels the voice with no electronics. A bullhorn is battery-powered, with a microphone and amplifier. In practice megaphone is used for both, and bullhorn — chiefly American — is the more specific word for the powered version.
The core difference
A shape and a powered device, with one word doing double duty.
- megaphone — strictly the unpowered cone; commonly used for both types.
- bullhorn — the powered kind, with microphone, amplifier and battery. Mainly American usage.
How the unpowered one works
A plain cone amplifies nothing. It improves how efficiently the voice couples to the air and directs the sound forward instead of letting it spread, which makes it louder in front and quieter behind. Cheerleaders' cones and old ships' speaking trumpets work on that principle alone.
Figurative use
Megaphone is the one that carries figuratively — "megaphone diplomacy", "a megaphone for their views". Bullhorn rarely does, which is a clue to which word feels more general.
