English: A public address horn speaker in a train station in Smíchov, Prague, the Czech Republic. This type of speaker, called a reflex or reentrant horn, or bullhorn, is a type of horn loudspeaker. Horns are widely used as speakers in public address systems because they are more efficient, typically able to produce 10 times the sound energy of cone speakers from a given amplifier output. In this type of speaker, a small metal diaphragm is vibrated by an attached coil of wire in a magnetic field, creating sound waves, which are conducted to the outside air through a flaring acoustic horn duct. Since the horn must be several feet long, to make it more compact the sound path is "folded" into a zigzag pattern, with the sound passing through a narrow central duct to a concentric backward duct, and then out through the flaring outer horn. Bullhorns have very poor frequency response, typically from 300-400 Hz to 5 kHz. However this can actually be an advantage in public address systems, since it matches the frequency distribution of the human voice, any background noise outside this bandwidth is attenuated.