From Longman Dictionary of Contemporary Englishbullhornbull‧horn /ˈbʊlhɔːn $ -hɔːrn/ noun [countable] 
TD American English old-fashioned a piece of equipment that you hold up to your mouth to make your voice louder SYN megaphone British English
TD American English old-fashioned a piece of equipment that you hold up to your mouth to make your voice louder SYN megaphone British EnglishExamples from the Corpus
bullhorn• I grabbed a bullhorn and yelled, in my most commanding flight attendant voice, for them to back up.• Certain sounds may seem to come out of a bullhorn.• The visible ones grow larger as the fog reflects up at them; their voices become bullhorns.• The cop is saying through his bullhorn that he wants to take the baby. 