Vociferous describes loudmouths, such as the vociferous mob at the soccer game.
Vociferous is from the Latin vociferari, meaning "to shout, yell." If you break it down to the first part, take vox, meaning "voice" and add it to ferre, meaning "to carry," then vociferous describes voices that carry; you can hear a vociferous person from across the room at a dance party. Vociferous isn't just loud, but annoying, too, like when the vociferous fans of the opposing team chant insults in unison. Try yanking a cookie out of a little kid's hand if you want to hear a vociferous reaction.