a directional, high-powered, electrical loudspeaker or megaphone.
Also, bull horn.

1950–55; bull1 + horn

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World English Dictionary
bullhorn (ˈbʊlˌhɔːn)
(US), (Canadian) Also called (in Britain and certain other countries): loud-hailer a portable loudspeaker having a built-in amplifier and microphone

Collins English Dictionary - Complete & Unabridged 10th Edition
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Example sentences
When you pull out a bullhorn, even if people don't change their politics, they
  cannot help but listen.
He had a bullhorn but for some reason he wasn't using it.
In one of the videos, an official uses a bullhorn to warn the crowd.
The device can be equipped with either a headset or an adjustable gooseneck
  microphone and has a bullhorn jack.
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