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|Sound & Hearing
A subwoofer is a special kind of woofer, designed to reproduce the very lowest frequencies in the audible range. Typically these would be in the range of the one to two lowest octaves that can still be considered "sound," that is, from about 12 - 16 Hz to roughly 50 - 60 Hz. (An octave represents a doubling of frequency)
Due to the nature of these tones, generally a subwoofer must be able to move a lot of air to play them accurately. To do this they are either quite large in diameter or have very long throw capabilities. They also should handle a lot of power, since the bass tones tend to require most of the energy expended in reproducing music.
A true subwoofer can be set up in mono, since the wavelengths associated with these frequencies are so long that not only is the ear insensitive to their directionality, but a single wave would not even fit inside a typical room without reflecting several times to do it.
In order to work reasonably efficiently, you can also expect a subwoofer to use a rather large enclosure (just as the bass instruments in real life tend to be big).
I wouldn't even begin to consider something a subwoofer unless it had accurate, linear output down to at least 20 Hz and handled at least 250 watts. What would be the point, when a well designed pair of stereo speakers in "bookshelf" size boxes can easily handle 150 watts each and get down to 30-40 Hz without any help?