Editing Subwoofer (section)

====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.
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=====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"/>
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=====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 />