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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?
As the years have gone by, we are seeing at least four different implementations of varying degrees of subwoofer applications, each with their own limitations and requirements.
One is still for music - real music, reproduced accurately. This would be a subwoofer typically used to improve low end extension of a small speaker system that sounds very good but lacks the bottom octaves. (The more extreme version of this is a larger system with a dedicated box intended to produce the low end).
Another has been around for a while, and is the one everybody hates - the car subwoofer. These are usually designed to play about three notes very loudly, with no particular emphasis on sound quality. Used with music that features a very heavy, steady, pounding beat, they are used by young males to signal to others in their pack that they are "down".
Then there is the home theater subwoofer, as commonly used. Just as people prefer a stretch image over not using part of their HD widescreen TV, they want to know that their subwoofer was worth buying. The goal here is chest crushing special effects, whether they be explosions, earthquakes, or the ominous hum of an alien spaceship. Many, many modern movie scores are engineered to accomodate this lust for gratification.
Then, finally, but not last in the sequence, are the "subwoofers" included in various "home theater in a box" and computer speaker systems. These probably don't go very low, and are quite colored, and are used to too high a frequency (often up to 2-300 Hz) to be considered "real" subwoofers, but they allow for the main (and rear, center, etc.) speakers to be miniscule. Often there is an amplifier for all channels built in to them, especially for computer applications.
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