The need to reproduce these frequencies has increased since older formats, such as LP, have been displaced by digital formats, such as CD, and particularly 5.1 formats such as Dolby Digital, in which the ".1" channel is dedicated to very loud bass information.
Subwoofers typically use conventional drivers (woofer) with cones as wide as 27", but 10" or 12" is typical, and sometimes 6.5" or less. Width tends to be advantageous because low frequencies involve shifting a great deal of air; a recent trend has been for high excursion, i.e., how far the cone can move from stand still; for example, some can move much as 1.5" in or out, yielding an overall displacement of 3". They usually powered by a high power amplifier, and often an electronic crossover ensures that higher frequencies will not be directed to the subwoofer.
It should be noted that a subwoofer does not necessarily provide superior bass performance to large conventional loudspeakers; they are merely subwoofers because they ought to reproduce only the lowest frequencies. A conventional woofer may reproduce frequencies up to 200, 300, or in a two-way speaker a mid-woofer (paired with a tweeter) also handles midrange, up to 3,000 Hertz or more.
Rather, the intention may be to use small main ("satellite") speakers (of which there are 2 for stereo, and 5 or more for surround sound) and locate ("hide") the subwoofer elsewhere; to augment an existent speaker to relieve it of reproducing bass and gain output level and/or quality; or because high levels of low bass are required and using a dedicated amplifier and speaker provides the output level and quality required. Thus, subwoofers may be part of a package that includes satellite speakers, purchased separately, or built into the cabinet of a conventional loudspeaker. (e.g., some speakers include a subwoofer in the lower portion of the cabinet.)
Physical separation of subwoofer and "satellite" speakers not only enables placement in an inconspicuous location, but since sub-bass frequencies are particularly sensitive to location (e.g., due to room resonance), the best position of the subwoofer may not be where the "satellite" speakers are located. (e.g., it has been suggested subwoofer(s) be placed in the corner of the room.) This is possible since low bass frequencies have a long wavelength; hence there is little difference between the information reaching our left and right ears, and we are unable to easily locate their direction. Harmonics at higher frequencies (from the satellites) can then be used, by the auditory system, to calculate the directional information. Note that only one subwoofer need be used, even when using 5 or more satellite speakers—e.g., for surround sound. All low frequency information is sent to the subwoofer. (Such "bass management" is common among equipment such as surround surround amplifiers/receivers.)
The physically separate subwoofer/satellite arrangement has been popularised by lifestyle systems, such as those manufactured by Bose, and multimedia speakers, examples of which include the Klipsch ProMedia. Particularly among low cost systems, however, it may be little more than a marketing device: it's not likely that a small woofer in a compact cabinet will have better performance than good "bookshelf" speakers; as mentioned, the term "subwoofer" is no guarantee of particular bass performance. On the other hand, high end domestic subwoofers are manufactured by companies such as M&K and REL. These can be purchased separately, to be added to an existent system or when considering a set of speakers; or as part of a high-end speaker package. All of these subwoofers tend to have in-built amplification.
Subwoofers are often found in professional applications such as movie theatres, various sound reinforcement applications (ranging from nightclubs to theme restaurants) and studios. Some of these applications require subwoofers designed for very high sound levels, such as the JBL 4645—certified for THX movie theatres—which uses an 18" driver (woofer). Note that movie theatre speakers (situated behind a perforated screen) typically use 15" drivers (woofers), so the use here is only to reproduce the lowest frequencies at high sound pressure levels. Electro-Voice subwoofers sometimes employ their Manifold Technology configuration, intended for applications such as concert sound reinforcement: this arrangement (using, for example, 18" drivers (woofers)) is capable of very high output levels—after all, concert venues may seat 10,000s of individuals—a lot of air to shift! (Note that music applications generally require less capability than movie soundtracks in the very lowest octave, and this is reflected in the design trade-offs of such speakers; the goal is very high output, not very low bass.) An even more extreme device is the ServoDrive ContraBass, where the driver's cone is moved using a belt-drive coupled servo motor! It can be found in venues such as theme park rides.
The automobile is ideal for the "hidden" subwoofer approach, due to space limitations of locations such as doors. (Ignoring the acoustic problems of a car interior.) Typically, subwoofers are installed in the trunk. Curiously, some car stereo enthusiasts seem intent on producing ultra-high sound pressure levels in the confines of their vehicle's cabin. In international car stereo competitions, up to 64 subwoofers driven by some 100,000 watts have been used to generate up to 175 decibels! Naturally, these sound levels are not safe for humans.
Indeed, hearing loss is one concern, alongside space considerations and neighbour relations. Since much bass is felt, sub-bass can be augmented using tactile transducers. These have recently emerged as a device that attaches to furniture, such as one's seat, via which vibrations are transmitted to the body; they can be connected to an amplifier as per a normal loudspeaker.
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