From Longman Dictionary of Contemporary Englishmegaphonemeg‧a‧phone /ˈmeɡəfəʊn $ -foʊn/ noun 1 
TPGO[countable] a piece of equipment like a large horn which you talk through to make your voice sound louder, when you are speaking to a crowd2 → megaphone diplomacy
TPGO[countable] a piece of equipment like a large horn which you talk through to make your voice sound louder, when you are speaking to a crowd2 → megaphone diplomacyExamples from the Corpus
megaphone• Perhaps conscious that he had gained a megaphone reputation, Meacher had settled down to his own social security review.• The clerk touched each elector on the head, and counted each poll aloud over a megaphone.• An inspector spoke through a megaphone while the armed squad kept watch.• A lot of them had megaphones.• They were directed back to their positions by the first assistant and his megaphone, and a second take began.• After some further conversation, he raised his megaphone to announce a break.• He has always looked for one more megaphone.• Sure enough, there he was, Cord Shay, at the megaphone.Origin megaphone (1800-1900) Greek megas “large” + phone “voice, sound” 