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A tweeter is a small loudspeaker. Tweeters are designed to reproduce high-pitched sounds (such as the sound of a whistle or a bird singing). It can mean sound's frequency going up to 20kHz. Humans can't hear frequencies larger than 20 000 Hz. Low frequencies would destroy small speakers because it is not designed to create larger movements. This is what a subwoofer is for, to play low frequencies.