The electrodynamic loudspeaker today comes in several types in order to produce a quality sound for a given frequency range:
Tweeter - 2kHz - 20kHz. It is used to produce all high end frequencies. There are many ways to construct a tweeter today. Most are electrodynamic (magnetic) speakers however there are piezo-electric, electrostatic and plasma tweeters.
Mid-range speaker - 300 - 5kHz. This speaker covers most human voice along with most instruments.
Woofer - 40 - 1 kHz. Low frequencies. The woofer goes back to work done in the 1930s at Bell Labs.
Subwoofer - 20 - 200 Hz. Very low frequencies. The human ear can only hear down to 20Hz. low frequency sound is unidirectional. This means that it can be placed anywhere in a room and be heard from any point with the same sound quality. Subwoofers also produce sound waves that penetrate walls easily. Noise from this type of speaker is even known to penetrate vertically through 5+ floors of concrete apartment buildings. Needless to say it is easy to get in trouble with local noise ordinances. If you plan your next loud event you may want to think about frequencies and the ability of objects to absorb/stop or reflect sound. Subwoofers were developed in the 1960s.
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