From Longman Dictionary of Contemporary Englishloudhailerloud‧hail‧er /ˌlaʊdˈheɪlə $ -ər/ noun [countable] 
British EnglishTC a piece of equipment with a microphone, that you can hold in your hand and speak through to make your voice louder SYN megaphone, bullhorn American English
British EnglishTC a piece of equipment with a microphone, that you can hold in your hand and speak through to make your voice louder SYN megaphone, bullhorn American EnglishExamples from the Corpus
loudhailer• Her reverie was broken by the sound of a loudhailer coming from the street.• The attackers then called by loudhailer on those in the house to surrender.• The organizers, loudhailers in hand, were corralling the crowd into marching formation when the first group of skinheads appeared. 