Editing Subwoofer (section)

==Applications==

===Home audio===
The use of a subwoofer augments the bass capability of the main speakers, and allows them to be smaller without sacrificing low-frequency capability. A subwoofer does not necessarily provide superior bass performance in comparison to large conventional loudspeakers on ordinary music recordings due to the typical lack of very low frequency content on such sources. However, there are recordings with substantial low-frequency content that most conventional loudspeakers are ill-equipped to handle without the help of a subwoofer, especially at high playback levels, such as music for [[pipe organ]]s with 32' (9.75 meter) bass pipes (16 Hz), very large bass drums on symphony orchestra recordings and electronic music with extremely low [[synth bass]] parts, such as bass tests or bass songs.

Frequencies which are sufficiently low are not easily [[Sound localization|localized]] by humans, hence many stereo and multichannel audio systems feature only one subwoofer channel and a single subwoofer can be placed off-center without affecting the perceived sound stage, since the sound that it produces will be difficult to localize. The intention in a system with a subwoofer is often to use small main speakers (of which there are two for stereo and five or more for surround sound or movie tracks) and to hide the subwoofer elsewhere (e.g. behind furniture or under a table), or to augment an existing speaker to save it from having to handle woofer-destroying low frequencies at high levels. This effect is possible only if the subwoofer is restricted to quite low frequencies, usually taken to be, say, 100 Hz and below—still less localization is possible if restricted to even lower maximum frequencies. Higher upper limits for the subwoofer (e.g. 120 Hz) are much more easily localized, making a single subwoofer impractical. Home-cinema systems typically use one subwoofer cabinet (the "1" in [[5.1 surround sound]]). However, to "improve bass distribution in a room that has multiple seating locations, and prevent [[Null (physics)|nulls]] with weakened bass response, some home-cinema enthusiasts use 5.2- or 7.2- or 9.2-channel surround sound systems with two subwoofer cabinets in the same room.<ref name=HTSBG />

Some users add a subwoofer because high levels of low-frequency bass are desired, even beyond what is in the original recording, as in the case of [[house music]] enthusiasts. Thus, subwoofers may be part of a package that includes satellite speakers, may be purchased separately, or may be built into the same cabinet as a conventional speaker system. For instance, some floor-standing tower speakers include a subwoofer driver in the lower portion of the same cabinet. Physical separation of subwoofer and satellite speakers not only allows placement in an inconspicuous location, but since [[sub-bass]] frequencies are particularly sensitive to room location (due to room resonances and reverberation 'modes'), the best position for the subwoofer is not likely to be where the satellite speakers are located.

Higher end home-cinema systems and enthusiasts may also opt to take low-frequency bass reproduction even further by incorporating two or more external subwoofers.<ref name=DellaSala2009 /> Having two subwoofers placed around the room ensures even distribution of bass, reducing subwoofer localization and pressurizing the room with low frequency notes that can be felt, just like the cinemas.<ref name=ExpertsAgree />

For greatest efficiency and best coupling to the room's air volume, subwoofers can be placed in a corner of the room, far from large room openings, and closer to the listener. This is possible since low bass frequencies have a long [[wavelength]]; hence there is little difference between the information reaching a listener's left and right ears, and so they cannot be readily localized. All low-frequency information is sent to the subwoofer. However, unless the sound tracks have been carefully mixed for a single subwoofer channel, it is possible to have some cancellation of low frequencies if bass information in one channel's speaker is out of phase with another.

The physically separate subwoofer/satellite arrangement, with small satellite speakers and a large subwoofer cabinet that can be hidden behind furniture, has been popularized by multimedia speaker systems such as [[Bose Acoustimass Home Entertainment Systems]], [[Polk Audio]] RM2008 Series and [[Klipsch Audio Technologies]] ProMedia, among many others.<ref name=PolkAudio /><ref name=ProMediaUltra />

[[File:Subwoofer Genius 2.1 360(2).jpg|thumb|upright|A small subwoofer cabinet designed for use with a home computer]]

Low-cost HTIB systems advertise their integration and simplicity. Particularly among lower cost HTIB systems and with [[boombox]]es, however, the inclusion of a subwoofer may be little more than a marketing technique. It is unlikely that a small woofer in an inexpensively-built compact plastic cabinet will have better bass performance than well-designed conventional (and typically larger) speakers in a plywood or MDF cabinet. Mere use of the term "subwoofer" is no guarantee of good or extended bass performance. Many multimedia subwoofers might better be termed "mid-bass cabinets" (80 to 200 Hz), as they are too small to produce deep bass in the typical 20 to 100 Hz range.<ref name=AbsoluteBest />

Further, poorly-designed systems often leave everything below about 120 Hz (or even higher) to the subwoofer, meaning that the subwoofer handles frequencies which the ear can use for sound source localization, thus introducing an undesirable subwoofer "localization effect". This is usually due to poor crossover designs or choices (too high a crossover point or insufficient crossover slope) used in many computer and home-cinema systems; localization also comes from port noise<ref name=Garcia2017 /> and from typically large amounts of harmonic distortion in the subwoofer design.<ref name="subwooferSurSnd" /> Home subwoofers sold individually usually include [[Audio crossover|crossover]] circuitry to assist with the integration of the subwoofer into an existing system.

===Car audio===
[[File:Multiple subwoofers in a car hatchback.jpg|thumb|Multiple subwoofers in a hatchback car]]

Automobiles are not well suited for the "hidden" subwoofer approach due to space limitations in the passenger compartments. It is not possible, in most circumstances, to fit such large drivers and enclosures into doors or dashboards, so subwoofers are installed in the trunk or back seat space. Some [[car audio]] enthusiasts compete to produce very high [[Sound#Sound Pressure Level|sound pressure]] levels in the confines of their vehicle's cabin; sometimes dangerously high sound pressure levels. The "SPL wars" have drawn much attention to subwoofers in general, but subjective competitions in sound quality ("SQ") have not gained equivalent popularity. Top SPL cars are not able to play normal music, or perhaps even to drive normally as they are designed solely for competition. Many non-competition subwoofers are also capable of generating high levels in cars due to the small volume of a typical car interior. High sound levels can cause hearing loss and [[tinnitus]] if one is exposed to them for an extended period of time.<ref name=Passchier2000 />

In the 2000s, several car audio manufacturers produced subwoofers using non-circular shapes, including Boston Acoustic, Kicker, Sony, Bazooka, and X-Tant. Other major car audio manufacturers like Rockford Fosgate did not follow suit since non-circular subwoofer shapes typically carry some sort of distortion penalties.<ref name=AutoOnA /><ref name=12vhndbk /> In situations of limited mounting space they provide a greater cone area and assuming all other variables are constant, greater maximum output. An important factor in the "square sub vs round sub" argument is the effects of the enclosure used. In a sealed enclosure, the maximum displacement is determined by

<math>V_\mathrm{d} = x_\mathrm{max} \times S_\mathrm{d}</math>

where
* <math>V_\mathrm{d}</math> is the volume of displacement (in m<sup>3</sup>)
* <math>x_\mathrm{max}</math> is the amount of linear excursion the speaker is mechanically capable of (in m)
* <math>S_\mathrm{d}</math> is the cone area of the subwoofer (in m<sup>2</sup>).
These are some of the [[Thiele/Small]] parameters which can either be measured or found with the driver specifications.

===Cinema sound===
After the introduction of Sensurround, [[movie theater]] owners began installing permanent subwoofer systems. [[Dolby Stereo#Dolby Stereo 70 mm Six Track|Dolby Stereo 70 mm Six Track]] was a six-channel film sound format introduced in 1976 that used two subwoofer channels for stereo reproduction of low frequencies. In 1981, [[Altec Lansing|Altec]] introduced a dedicated cinema subwoofer model tuned to around 20 Hz: the 8182. Starting in 1983, [[THX]] certification of the cinema sound experience quantified the parameters of good audio for watching films, including requirements for subwoofer performance levels and enough isolation from outside sounds so that noise did not interfere with the listening experience.<ref name=THXOverview /> This helped provide guidelines for multiplex cinema owners who wanted to isolate each individual cinema from its neighbors, even as louder subwoofers were making isolation more difficult. Specific cinema subwoofer models appeared from [[JBL (company)|JBL]], [[Electro-Voice]], [[Eastern Acoustic Works]], Kintek, [[Meyer Sound Laboratories]] and [[BGW Systems]] in the early 1990s. In 1992, [[Dolby Digital]]'s six-channel film sound format incorporated a single LFE channel, the "point one" in 5.1 surround sound systems.

Tom Horral, a Boston-based acoustician, blames complaints about modern movies being too loud on subwoofers. He says that before subwoofers made it possible to have loud, relatively undistorted bass, movie sound levels were limited by the distortion in less capable systems at low frequency and high levels.<ref name=Barron1998 />
===Sound reinforcement===
[[File:EAW KF850s + SB1000s (Blacklight Party).jpg|thumb|upright|Each stack of speakers in this sound reinforcement setup consists of two EAW SB1000 slanted baffle subwoofers (each contains two 18-inch drivers) and two EAW KF850 full range cabinets for the mid and high frequencies.]]

Professional audio subwoofers used in rock concerts in stadia, DJ performances at dance music venues (e.g. [[electronic dance music]]) and similar events must be capable of very high bass output levels, at very low frequencies, with low distortion. This is reflected in the design attention given in the 2010s to the subwoofer applications for sound reinforcement, [[public address systems]], dance club systems and concert systems. Cerwin-Vega states that when a subwoofer cabinet is added to an existing full-range speaker system, this is advantageous, as it moves the "...lowest frequencies from your main [full-range] PA speakers" thus "...eliminat[ing] a large amount of the excess work that your main top [full-range] box was trying to reproduce. As a result, your main [full-range] cabinets will run more efficiently and at higher volumes."<ref name="CerwinVegaUnderstanding" /> A different argument for adding subwoofer cabinets is that they may increase the "level of clarity" and "perceived loudness" of an overall PA system, even if the SPL is not actually increased.<ref name=AASTYPA /> ''Sound on Sound'' states that adding a subwoofer enclosure to a full-range system will reduce "cone excursion", thus lowering distortion, leading to an overall cleaner sound.<ref name="Purton" />

Consumer applications (as in home use) are considerably less demanding due to much smaller listening space and lower playback levels. Subwoofers are now almost universal in professional sound applications such as live concert sound, churches, nightclubs, and theme parks. [[Movie theater]]s certified to the [[THX]] standard for playback always include high-capability subwoofers. Some professional applications require subwoofers designed for very high sound levels, using multiple 12-, 15-, 18- or 21-inch drivers (30&nbsp;cm, 40&nbsp;cm, 45&nbsp;cm, 53&nbsp;cm respectively). Drivers as small as 10-inch (25&nbsp;cm) are occasionally used, generally in horn-loaded enclosures.

The number of subwoofer enclosures used in a concert depends on a number of factors, including the size of the venue, whether it is indoors or outdoors, the amount of low-frequency content in the band's sound, the desired volume of the concert, and the design and construction of the enclosures (e.g. direct-radiating versus horn-loaded). A tiny coffeehouse may only need a single 10-inch subwoofer cabinet to augment the bass provided by the full-range speakers. A small bar may use one or two direct-radiating 15-inch (40&nbsp;cm) subwoofer cabinets. A large dance club may have a row of four or five twin 18-inch (45&nbsp;cm) subwoofer cabinets, or more. In the largest stadium venues, there may be a very large number of subwoofer enclosures. For example, the 2009–2010 [[U2 360° Tour]] used 24 [[Clair Brothers]] BT-218 subwoofers (a double 18-inch (45&nbsp;cm) box) around the perimeter of the central circular stage, and 72 proprietary Clair Brothers cardioid S4 subwoofers placed underneath the ring-shaped "B" stage which encircles the central main stage.<ref name=Backstage /><ref name=Becka2009 />

[[File:Caissons grave DSC 5563EC.jpg|thumb|left|upright=1.1|A row of subwoofer cabinets in front of the stage of a rock concert. One enclosure out of every stack of three is turned back&shy;ward to make a cardioid output pattern.]]

The main speakers may be 'flown' from the ceiling of a venue on chain hoists, and 'flying points' (i.e. attachment points) are built into many professional loudspeaker enclosures. Subwoofers can be flown or stacked on the ground near the stage. One of the reasons subwoofers may be installed on the ground is that on-the-ground installation can increase the bass performance,  particularly if the subwoofer is placed in the corner of a room (conversely, if a subwoofer cabinet is perceived as too loud, alternatives to on-the-ground or in-corner installation may be considered). There can be more than 50 double-18-inch (45&nbsp;cm) cabinets in a typical rock concert system. Just as consumer subwoofer enclosures can be made of [[medium-density fibreboard]] (MDF), [[oriented strand board]] (OSB), [[plywood]], plastic or other dense material, professional subwoofer enclosures can be built from the same materials.<ref name=Wayland2010 /><ref name=Altec9880 /> MDF is commonly used to construct subwoofers for permanent installations as its density is relatively high and weatherproofing is not a concern. Other permanent installation subwoofers have used very thick plywood: the [[Altec Lansing|Altec]] 8182 (1981) used 7-ply 28&nbsp;mm birch-faced oak plywood.<ref name=8000Series /> Touring subwoofers are typically built from 18–20&nbsp;mm thick void-free Baltic birch ([[Betula pendula]] or [[Betula pubescens]]) plywood from Finland, Estonia or Russia; such plywood affords greater strength for frequently transported enclosures.<ref name=JBLPEG /> Not naturally weatherproof, Baltic birch is coated with carpet, thick paint or spray-on [[truck bedliner]] to give the subwoofer enclosures greater durability.<ref name=Growler /><ref name=SolidNRG />

Touring subwoofer cabinets are typically designed with features that facilitate moving the enclosure (e.g. wheels, a "towel bar" handle and recessed handles), a protective grille for the speaker (in direct radiating-style cabinets), metal or plastic protection for the cabinets to protect the finish as the cabinets are being slid one on top of another, and hardware to facilitate stacking the cabinets (e.g. interlocking corners) and for "flying" the cabinets from stage rigging. In the 2000s, many small- to mid-size subwoofers designed for bands' live sound use and DJ applications are "powered subs"; that is, they have an integrated [[power amplifier]]. These models typically have a built-in crossover. Some models have a metal-reinforced hole in which a speaker pole can be mounted for elevating full-frequency range cabinets.

====Use in a full-range system====
In professional concert sound system design, subwoofers can be incorporated seamlessly with the main speakers into a stereo or mono full-range system by using an active [[Audio crossover|crossover]]. The audio engineer typically adjusts the frequency point at which lower frequency sounds are routed to the subwoofer speaker(s), and mid-frequency and higher frequency sounds are sent to the full-range speakers. Such a system receives its signal from the main mono or stereo [[mixing console]] mix bus and amplifies all frequencies together in the desired balance. If the main sound system is stereo, the subwoofers can also be in stereo. Otherwise, a mono subwoofer channel can be derived within the crossover from a stereo mix, depending on the crossover make and model. While 2010-era subwoofer cabinet manufacturers suggest placing subwoofers on either side of a stage (as implied by the inclusion of pole cups for the full-range PA cabinets), Dave Purton argues that for club gigs, having two subwoofer cabinets on either side of a stage will lead to gaps in bass coverage in the venue; he states that putting the two subwoofer cabinets together will create a more even, omnidirectional sub-bass tone.<ref name="Purton" />

{| class="wikitable"
|+ PA systems by size and subwoofer approach<ref name="CerwinVegaUnderstanding" />
|-
! PA system set-up
! Venue size
|-
| Small system: Two pole-mounted mid/high frequency PA speaker cabinets and two small subwoofer cabinets with 15- or 18-inch subwoofers (Note: this would be used in club where jazz, acoustic music, country music or soft rock is played)
| Small club with capacity for up to 300 people
|-
| Small high amplifier power system: 2 high amplifier power-rated mid/high frequency PA speakers with 15-inch woofers and a large horn-loaded tweeter; two high amplifier power-rated subwoofer cabinets with one or two 18-inch subwoofer cabs (front-firing, also known as "front loaded", or manifold-loaded subwoofer cabinets)
| Small club with capacity for up to 500 people
|-
| Mid-size PA system: 4 larger multiwoofer mid/high frequency PA speaker cabs (e.g. each with two 15-inch woofers) and four subwoofer cabinets, either front-firing, manifold loaded or a folded horn
| Large clubs with capacity for 500+ people, small music festivals, fairs
|-
| Large-size PA system: Multiple mid/high frequency PA speakers, possibly "flown" up high in rigging, and a number of subwoofer cabinets (either front firing, manifold loaded or folded horn)
| Large venues with capacity for 1000+ people, larger music festivals
|}

====Aux-fed subwoofers====
Instead of being incorporated into a full-range system, concert subwoofers can be supplied with their own signal from a separate mix bus on the mixing console; often one of the auxiliary sends ("aux" or "auxes") is used. This configuration is called "aux-fed subwoofers", and has been observed to significantly reduce low-frequency "muddiness" that can build up in a concert sound system which has on stage a number of microphones each picking up low frequencies and each having different phase relationships of those low frequencies.<ref name="Young2008" /> The aux-fed subwoofers method greatly reduces the number of sources feeding the subwoofers to include only those instruments that have desired low-frequency information; sources such as [[Bass drum|kick drum]], [[bass guitar]], [[Sampler (musical instrument)|samplers]] and [[keyboard instrument]]s. This simplifies the signal sent to the subwoofers and makes for greater clarity and low punch.<ref name=Boomer /> Aux-fed subwoofers can even be stereo, if desired, using two auxiliary mix buses.

====Directional bass====
[[File:Cardioid Subwoofers.png|thumb|Cardioid dispersion pattern of two end-fire subwoofers placed one in front of the other. The signal feeding the enclosure nearest the listener is delayed by a few milliseconds.{{Efn|Cardioid subwoofer image: The second subwoofer has been delayed a precise amount corresponding to the time it takes sound to traverse the distance between speaker grilles. Image captured from [[Electro-Voice]]'s RACE loudspeaker pattern prediction software. Frequency shown is 60&nbsp;Hz.}}]]

To keep low-frequency sound focused on the audience area and not on the stage, and to keep low frequencies from bothering people outside of the event space, a variety of techniques have been developed in concert sound to turn the naturally omnidirectional radiation of subwoofers into a more directional pattern. Several examples of [[sound reinforcement system]] applications where sound engineers seek to provide more directional bass sound are: [[music festival]]s, which often have several bands performing at the same time on different stages; large [[rave]]s or [[Electronic dance music|EDM]] events, where there are multiple [[DJ]]s performing at the same time in different rooms or stages; and [[Multiplex (movie theater)|multiplex]] [[movie theater]]s, in which there are many films being shown simultaneously in auditoriums that share common walls. These techniques include: setting up subwoofers in a vertical array; using combinations of delay and polarity inversion; and setting up a delay-shaded system. With a cardioid dispersion pattern, two end-fire subwoofers can be placed one in front of the other. The enclosure nearest the listener is delayed by a few milliseconds. The second subwoofer is delayed a precise amount corresponding to the time it takes sound to traverse the distance between speaker grilles.

=====Vertical array=====
Stacking or [[Rigging (theatre)|rigging]] the subwoofers in a vertical array focuses the low frequencies forward to a greater or lesser extent depending on the physical length of the array. Longer arrays have a more directional effect at lower frequencies. The directionality is more pronounced in the vertical dimension, yielding a radiation pattern that is wide but not tall. This helps reduce the amount of low-frequency sound bouncing off the ceiling indoors and assists in mitigating external noise complaints outdoors.

=====Rear delay array=====
[[File:Cardioid Subwoofer Array.png|thumb|CSA: Six subwoofers arranged for less bass energy on stage. Signal going to the reversed enclosures is delayed a few milliseconds.]]

Another cardioid subwoofer array pattern can be used horizontally, one which takes few channels of processing and no change in required physical space. This method is often called "cardioid subwoofer array" or "CSA"<ref name=CSA /> even though the pattern of ''all'' directional subwoofer methods is cardioid. The CSA method reverses the enclosure orientation and inverts the polarity of one out of every three subwoofers across the front of the stage, and delays those enclosures for maximum cancellation of the target frequency on stage. Polarity inversion can be implemented electronically, by reversing the wiring polarity, or by physically positioning the enclosure to face rearward. This method reduces forward output relative to a tight-packed, flat-fronted array of subwoofers, but can solve problems of unwanted low-frequency energy coming into microphones on stage. Compared to the end-fire array, this method has less on-axis energy but more even pattern control throughout the audience, and more predictable cancellation rearward. The effect spans a range of slightly more than one octave.<ref name="CSA"/>

A second method of rear delay array combines end-fire topology with polarity reversal, using two subwoofers positioned front to back, the drivers spaced one-quarter wavelength apart, the rear enclosure inverted in polarity and delayed by a few milliseconds for maximum cancellation on stage of the target frequency.<ref name=Brill /> This method has the least output power directed toward the audience, compared to other directional methods.
{{Clear}}

=====End-fire array=====
[[File:End-fire Subwoofer Array.png|thumb|End-fire array using three rows of subwoofers. Each row is delayed a few milliseconds more than the previous row.]]

The end-fire subwoofer method, also called "forward steered arrays",<ref name="ForwardSteered" /> places subwoofer drivers co-axially in one or more rows, using destructive interference to reduce emissions to the sides and rear. This can be done with separate subwoofer enclosures positioned front to back with a spacing between them of one-quarter wavelength of the target frequency, the frequency that is least wanted on stage or most desired in the audience. Each row is delayed beyond the first row by an amount related to the speed of sound in air; the delay is typically a few milliseconds. The arrival time of sound energy from all the subwoofers is near-simultaneous from the audience's perspective, but is canceled out to a large degree behind the subwoofers because of offset sound wave arrival times. Directionality of the target frequency can achieve as much as 25&nbsp;dB rear attenuation, and the forward sound is coherently summed in line with the subwoofers.<ref name=Kamlet2004 /> The positional technique of end-fire subwoofers came into widespread use in European live concert sound in 2006.<ref name=Stevens2006 />

The end-fire array trades a few decibels of output power for directionality, so it requires more enclosures for the same output power as a tight-packed, flat-fronted array of enclosures. Sixteen enclosures in four rows were used in 2007 at one of the stages of the [[Ultra Music Festival]], to reduce low-frequency interference to neighboring stages.<ref name=Brill2007 /> Because of the physical size of the end-fire array, few concert venues are able to implement it. The output pattern suffers from comb-filtering off-axis, but can be further shaped by adjusting the frequency response of each row of subwoofers.<ref name="ForwardSteered"/>
{{Clear}}

=====Delay-shaded array=====
A long line of subwoofers placed horizontally along the front edge of the stage can be delayed such that the center subwoofers fire several milliseconds prior to the ones flanking them, which fire several milliseconds prior to ''their'' neighbors, continuing in this fashion until the last subwoofers are reached at the outside ends of the subwoofer row ([[beamforming]]). This method helps to counteract the extreme narrowing of the horizontal dispersion pattern seen with a horizontal subwoofer array. Such delay shading can be used to virtually reshape a loudspeaker array.<ref name=Engebretson2007 />

=====Directional enclosure=====
Some subwoofer enclosure designs rely on drivers facing to the sides or to the rear in order to achieve a degree of directionality.<ref name=NexoGeo /><ref name=PSW6 /> End-fire drivers can be positioned within a single enclosure that houses more than one driver.<ref name=ATA218C />

===Variants===
Some less commonly-used bass enclosures are variants of the subwoofer enclosure's normal range, such as the upper-bass cabinet (80–200 Hz) and the infrasonic (extra low) subwoofer (below 20 Hz).

===Enclosure designs===
[[File:Bandpass enclosure.png|thumb|Compound or 4th-order band-pass enclosure]]

Front-loaded subwoofers have one or more subwoofer speakers in a cabinet, typically with a grille to protect the speakers. In practice, many front-loaded subwoofer cabinets have a vent or port in the speaker cabinet, thus creating a [[bass reflex]] enclosure. Even though a bass reflex port or vent creates some additional phase delay, it adds SPL, which is often a key factor in PA and sound reinforcement system applications. As such, non-vented front-firing subwoofer cabinets are rare in pro audio applications.

Horn-loaded subwoofers have a subwoofer speaker that has a pathway following the loudspeaker. To save space, the pathway is often folded, so that the folded pathway will fit into a box-style cabinet. Cerwin-Vega states that its folded horn subwoofer cabinets, "...on average, produce 6{{nbsp}}dB more output at 1 watt than a dual 18[-inch] vented box" giving "four times the output with half the number of drivers".<ref name="CerwinVegaUnderstanding" /> The Cerwin-Vega JE-36C has a five feet long folded horn chamber length in the wooden cabinet.<ref name="CerwinVegaUnderstanding" />

Manifold subwoofers have two or more subwoofer speakers that feed the throat of a single horn. This increases SPL for the subwoofer, at the cost of increased distortion. EV has a manifold speaker cabinet in which four drivers are mounted as close together as practical. This is a different design than the "multiple drivers in one throat" approach. An unusual example of manifold subwoofer design is the [[Thomas Mundorf]] (TM) approach of having four subwoofers facing each other and sitting close together, which is used for [[theater in the round]] shows, where the audience surrounds the performers in a big circle (e.g. [[Metallica]] has used this in some concerts). The TM approach produces an omnidirectional bass sound.<ref name=McCarthy2016 /> Cerwin-Vega defines a manifold enclosure as one in which "...the driver faces into a tuned ported cavity. You hear sound directly from the back of the driver in addition to the sound that emanates out of the port. This type of enclosure design extends the frequency capability of the driver lower than it would reproduce by itself."<ref name="CerwinVegaUnderstanding" />

[[Bandpass]] subwoofers have a sealed cabinet within another cabinet, with the "outer" cabinet typically having a vent or port.

===Bass instrument amplification===
In rare cases, sound reinforcement subwoofer enclosures are also used for [[bass instrument amplification]] by [[electric bass]] players and [[synth bass]] players. For most bands and most small- to mid-size venues (e.g. nightclubs and bars), standard bass guitar speaker enclosures or keyboard amplifiers will provide sufficient sound pressure levels for onstage monitoring. Since a regular electric bass has a low "E" (41&nbsp;Hz) as its lowest note, most standard bass guitar cabinets are only designed with a range that goes down to about 40&nbsp;Hz. However, in some cases, performers wish to have extended sub-bass response that is not available from standard instrument speaker enclosures, so they use subwoofer cabinets. Just as some electric guitarists add huge stacks of guitar cabinets mainly for show, some bassists will add immense subwoofer cabinets with 18-inch woofers mainly for show, and the extension subwoofer cabinets will be operated at a lower volume than the main bass cabinets.

Bass guitar players who may use subwoofer cabinets include performers who play with extended range basses that include a low "B" string (about 31&nbsp;Hz), bassists who play in styles where a very powerful sub-bass response is an important part of the sound (e.g. funk, Latin, gospel, R & B, etc.), and/or bass players who perform in stadium-size venues or large outdoor venues. Keyboard players who use subwoofers for on-stage monitoring include [[Hammond organ|electric organ]] players who use bass [[pedal keyboard]]s (which go down to a low "C" which is about 33&nbsp;Hz) and synth bass players who play rumbling sub-bass parts that go as low as 18&nbsp;Hz. Of all of the keyboard instruments that are amplified onstage, synthesizers can produce some of the lowest pitches, because unlike a traditional electric piano or electric organ, which have as their lowest notes a low "A" and a low "C", respectively, a synth does not have a fixed lowest octave. A synth player can add lower octaves to a patch by pressing an "octave down" button, which can produce pitches that are at the limits of human hearing.

Several concert sound subwoofer manufacturers suggest that their subs can be used for bass instrument amplification. Meyer Sound suggests that its 650-R2 Concert Series Subwoofer, a {{convert|14|sqft|adj=on|sp=us}} enclosure with two 18-inch drivers (45&nbsp;cm), can be used for bass instrument amplification.<ref name=650R2 /> While performers who use concert sound subwoofers for onstage monitoring may like the powerful sub-bass sound that they get onstage, sound engineers may find the use of large subwoofers (e.g. two 18-inch drivers (45&nbsp;cm)) for onstage instrument monitoring to be problematic, because it may interfere with the "Front of House" sub-bass sound.