Atweeter is a small or rather tinyloudspeaker. Tweeters are designed to reproduce high-pitched sounds (such as the sound of awhistle or abird singing). It can mean sound'sfrequency 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 asubwoofer is for, to play low frequencies.
