Editing Subwoofer (section)

==Construction and features==

[[File:Speaker-cross-section.svg|thumb|Cross-section of a subwoofer drive unit]]
'''Loudspeaker and enclosure design'''

Subwoofers use speaker [[Loudspeaker|drivers]] ([[woofer]]s) typically between 8-inch (20&nbsp;cm) and 21-inch (53&nbsp;cm) in diameter. Some uncommon subwoofers use larger drivers, and single prototype subwoofers as large as 60-inch (152&nbsp;cm) have been fabricated.<ref name=60InchSub /> On the smaller end of the spectrum, subwoofer drivers as small as 4-inch (10&nbsp;cm) may be used. Small subwoofer drivers in the 4-inch range are typically used in small computer speaker systems and compact home-cinema subwoofer cabinets. The size of the driver and number of drivers in a cabinet depends on the design of the [[loudspeaker enclosure]], the size of the cabinet, the desired sound pressure level, the lowest frequency targeted and the level of permitted distortion. The most common subwoofer driver sizes used for sound reinforcement in nightclubs, raves and pop/rock concerts are 10-, 12-, 15- and 18-inch models (25&nbsp;cm, 30&nbsp;cm, 38&nbsp;cm, and 45&nbsp;cm respectively). The largest available sound reinforcement subwoofers, 21-inch (53&nbsp;cm) drivers, are less commonly seen.{{cn|date=October 2023}}

The reference efficiency of a loudspeaker system in its passband is given by:<ref name=Small_DRLSA /><ref name=Small_CBPt1 /><ref name=Small_VBPt1 /><ref name=Small_PRPt1 />

: <math>\eta_0 = \frac{4 \pi^2}{c^3} \cdot \frac{f_s^3 V_{as}}{Q_{es}}</math>

where <math>c</math> is the speed of sound in air and the variables are [[Thiele/Small]] parameters: <math>f_s</math> is the resonance frequency of the driver, <math>V_{as}</math> is the volume of air having the same acoustic compliance as the driver suspension, and <math>Q_{es}</math> is the driver <math>Q</math> at <math>f_s</math> considering the electrical DC resistance of the driver voice coil. Deep low-frequency extension is a common goal for a subwoofer and small box volumes are also considered desirable, to save space and reduce the size for ease of transportation (in the case of sound reinforcement and DJ subwoofers). [[J. Anton Hofmann|Hofmann]]'s "Iron Law" therefore mandates low efficiency under those constraints, and indeed most subwoofers require considerable power, much more than other individual drivers.{{cn|date=October 2023}}

So, for the example of a closed-box loudspeaker system, the box volume <math>V_{ab}</math> to achieve a given total <math>Q</math> of the system <math>Q_{tc}</math> is proportional to <math>V_{as}</math>:<ref name=Small_CBPt1 />

: <math>V_{ab} = \frac{V_{as}}{\alpha}</math>

where <math>\alpha</math> is the system compliance ratio given by the ratio of the driver compliance and the enclosure compliance, which can be written as:<ref name=Small_CBPt2 />

: <math>\alpha = \frac{Q_{tc}^2}{Q_{ts}^2}-1 = \frac{f_c^2}{f_s^2}-1 </math>

where <math>f_c</math> is the system resonance frequency.

Therefore, a decrease in box volume (i.e., a smaller speaker cabinet) and the same <math>f_3</math> will decrease the efficiency of the subwoofer. The normalized half-power frequency of a closed-box loudspeaker system is given by:<ref name=Small_CBPt2 />

: <math>f_3/ f_c = \left[ \frac{(1/Q_{tc}^2-2)+\sqrt{(1/Q_{tc}^2-2)^2+4}}{2} \right] ^{1/2} </math>

Here we note that if <math>Q_{tc}=1/\sqrt{2} \approx 0.7071</math>, then <math>f_3=f_c</math>.

[[File:Bass reflex spk.PNG|thumb|[[Bass reflex]] [[Loudspeaker enclosure|enclosure]] schematic (cross-section)]]

As the efficiency is proportional to <math>f_s^3</math>, small improvements in low-frequency extension with the same driver and box volume will result in very significant reductions in efficiency. For these reasons, subwoofers are typically very inefficient at converting electrical energy into sound energy. This combination of factors accounts for the higher amplifier power required to drive subwoofers, and the requirement for greater power handling for subwoofer drivers. Enclosure variations (e.g., [[bass reflex]] designs with a port in the cabinet) are often used for subwoofers to increase the efficiency of the driver/enclosure system, helping to reduce the amplifier power requirements. Vented-box loudspeaker systems have a maximum theoretical efficiency that is 2.9&nbsp;dB greater than that of the closed-box system.<ref name=Small_VBPt2 />

[[File:Subwoofer cab internal bracing.jpg|thumb|Heavily braced and built subwoofer enclosure]]

Subwoofers are typically constructed by mounting one or more woofers in a cabinet of medium-density fibreboard (MDF), oriented strand board (OSB), plywood, fiberglass, aluminum or other stiff materials. Because of the high air pressure that they produce in the cabinet, subwoofer enclosures often require internal bracing to distribute the resulting forces.<ref name=FX4021 />

Subwoofers have been designed using a number of enclosure approaches: [[bass reflex]] (with a port or vent), using a subwoofer and one or more [[passive radiator speaker]]s in the enclosure, [[acoustic suspension]] (sealed enclosure), [[Loudspeaker enclosure#Infinite baffle|infinite baffle]], [[Horn loudspeaker|horn-loaded]], [[Loudspeaker enclosure#Tapped horn|tapped horn]], [[Loudspeaker enclosure#Transmission line|transmission line]] and [[Loudspeaker enclosure#Compound or band-pass|bandpass]]. Each enclosure type has advantages and disadvantages in terms of efficiency increase, bass extension, cabinet size, distortion, and cost.<ref name=FX4021/>

Multiple enclosure types may even be combined in a single design, such as in computer audio with the subwoofer design of the [[Labtec]] LCS-2424 (later acquired by [[Logitech]] and used for their Z340/Z540/Z640/Z3/Z4), which is a (primitive) passive radiator bandpass enclosure with a bass reflex dividing chamber.<ref name=FX4021 />

While not necessarily an enclosure type, [[Isobaric loudspeaker|isobaric]] (such as push-pull) coupled loading of two drivers has sometimes been used in subwoofer products of computer,<ref name=FX4021 /> home cinema<ref name=GenesisTech /> and sound reinforcement<ref name=VueAudio /> class, and also DIY versions in automotive applications, to provide relatively deep bass for their size. Self-contained "isobaric-like" driver assemblies have been manufactured since the 2010s.<ref name=W32108 /><ref name=W82022 /><ref name=SubwooferCutaway />

The smallest subwoofers are typically those designed for desktop multimedia systems. The largest common subwoofer enclosures are those used for concert sound reinforcement systems or dance club sound systems.  An example of a large concert subwoofer enclosure is the 1980s-era Electro-Voice MT-4 "Bass Cube" system, which used four 18-inch (45&nbsp;cm) drivers. An example of a subwoofer that uses a bass horn is the Bassmaxx B-Two, which loads an 18-inch (45&nbsp;cm) driver onto an {{convert|11|ft|adj=on|sp=us}} long folded horn.<ref name=Jones2010 /> Folded horn-type subwoofers can typically produce a deeper range with greater efficiency than the same driver in an enclosure that lacks a horn.<ref name=Jones2010 /> However, folded horn cabinets are typically larger and heavier than front-firing enclosures, so folded horns are less commonly used. Some experimental fixed-installation subwoofer horns have been constructed using brick and concrete to produce a very long horn that allows a very deep sub-bass extension.<ref name="Gordon2008" />

Subwoofer output level can be increased by increasing cone surface area or by increasing cone excursion. Since large drivers require undesirably large cabinets, most subwoofer drivers have large excursions. Unfortunately, high excursion, at high power levels, tends to produce more distortion from inherent mechanical and magnetic effects in electro-dynamic drivers (the most common sort).<ref name="LDSG Appendix A" /> The conflict between assorted goals can never be fully resolved; subwoofer designs necessarily involve tradeoffs and compromises. Hofmann's Iron Law (the efficiency of a woofer system is directly proportional to its cabinet volume (as in size) and to the cube of its cutoff frequency, that is how low in pitch it will go) applies to subwoofers just as it does to all loudspeakers.<ref name="LDSG Appendix A"/> Thus, a subwoofer enclosure designer aiming at the deepest-pitched bass will probably have to consider using a large enclosure size; a subwoofer enclosure designer instructed to create the smallest possible cabinet (to make transportation easier) will need to compromise how low in pitch their cabinet will go.<ref name="LDSG Appendix A"/>

===Frequency range and frequency response===
The frequency response specification of a speaker describes the range of frequencies or musical tones a speaker can reproduce, measured in [[hertz]] (Hz).<ref name=Dicomo2022 /> The typical frequency range for a subwoofer is between 20–200 Hz.<ref name="CrutchfieldGlossary" /> Professional concert sound system subwoofers typically operate below 100 Hz,<ref name="Young2008" /> and [[THX]]-certified systems operate below 80 Hz.<ref name="DellaSala" />  Subwoofers vary in terms of the range of pitches that they can reproduce, depending on a number of factors such as the size of the cabinet and the construction and design of the enclosure and driver(s). Specifications of frequency response depend wholly for relevance on an accompanying amplitude value—measurements taken with a wider amplitude tolerance will give any loudspeaker a wider frequency response. For example, the JBL 4688 TCB Subwoofer System, a now-discontinued system which was designed for movie theaters, had a frequency response of 23–350 Hz when measured within a 10-decibel boundary (0&nbsp;dB to &minus;10&nbsp;dB) and a narrower frequency response of 28–120 Hz when measured within a 6-decibel boundary (±3&nbsp;dB).<ref name=JBL4688 />

Subwoofers also vary in regard to the sound pressure levels achievable and the distortion levels that they produce over their range. Some subwoofers, such as The Abyss by [[MartinLogan]] for example, can reproduce pitches down to around 18 Hz (which is about the pitch of the lowest rumbling notes on a huge pipe organ with {{convert|32|ft|adj=on|sp=us}} 16 Hz bass pipes) to 100 Hz (±3&nbsp;dB). Nevertheless, even though the Abyss subwoofer can go down to 18 Hz, its lowest frequency and maximum SPL with a limit of 10% distortion is 35.5 Hz and 79.8 dB at 2 meters.<ref name=Butterworth2009 /> This means that a person choosing a subwoofer needs to consider more than just the lowest pitch that the subwoofer can reproduce.

===Amplification===
[[File:Aktives Subwoofer Modul.jpg|thumb|The internal components of an active (powered) subwoofer, showing the circuitry for the [[power amplifier]]]]

'Active subwoofers' include their own dedicated amplifiers within the cabinet. Some also include user-adjustable [[#Equalization|equalization]] that allows boosted or reduced output at particular frequencies; these vary from a simple "boost" switch, to fully [[parametric equalizer]]s meant for detailed speaker and room correction. Some such systems are even supplied with a calibrated microphone to measure the subwoofer's in-room response, so the automatic equalizer can correct the combination of subwoofer, subwoofer location, and room response to minimize the effects of room modes and improve low-frequency performance.

[[File:Final05jm1.jpg|thumb|left|upright|This rear panel of a powered subwoofer shows the [[heat sink]]s used to cool the [[power amplifier]].]]

'Passive subwoofers' have a subwoofer driver and enclosure, but they do not include an amplifier. They sometimes incorporate internal passive crossovers, with the filter frequency determined at the factory. These are generally used with third-party power amplifiers, taking their inputs from active crossovers earlier in the signal chain. Inexpensive [[Home theater in a box|home-theater-in-a-box]] (HTIB) packages often come with a passive subwoofer cabinet that is amplified by the multi-channel amplifier. While few high-end home-cinema systems use passive subwoofers, this format is still popular in the professional sound industry.<ref name=JBLSSDM />

===Equalization===
[[File:Rear panel of a Polk audio sub-woofer.jpg|thumb|The rear panel of a Polk subwoofer cabinet, showing a low-pass filter adjustment knob]]

Equalization can be used to adjust the in-room response of a subwoofer system.<ref name=BCSS /> Designers of active subwoofers sometimes include a degree of corrective equalization to compensate for known performance issues (e.g. a steeper than desired low end [[roll-off]] rate). In addition, many amplifiers include an adjustable low-pass filter, which prevents undesired higher frequencies from reaching the subwoofer driver. For example, if a listener's main speakers are usable down to 100 Hz, then the subwoofer filter can be set so the subwoofer only works below 100 Hz.<ref name="DellaSala" /> Typical filters involve some overlap in frequency ranges; a steep 4th-order 24&nbsp;dB/octave low-pass filter is generally desired for subwoofers in order to minimize the overlap region. The filter section may also include a high-pass "[[Infrasound|infrasonic]]" or "subsonic" filter, which prevents the subwoofer driver from attempting to reproduce frequencies below its safe capabilities. Setting an infrasonic filter is important on bass reflex subwoofer cabinets, as the bass reflex design tends to create the risk of cone overexcursion at pitches below those of the port tuning, which can cause distortion and damage the subwoofer driver. For example, in a ported subwoofer enclosure tuned to 30 Hz, one may wish to filter out pitches below the tuning frequency; that is, frequencies below 30 Hz.

Some systems use parametric equalization in an attempt to correct for room frequency response irregularities.<ref name=Messenger2004 /> Equalization is often unable to achieve flat frequency response at all listening locations, in part because of the resonance (i.e. [[standing wave]]) patterns at low frequencies in nearly all rooms. Careful positioning of the subwoofer within the room can also help flatten the frequency response.<ref name=Plumb2000 /> Multiple subwoofers can manage a flatter general response since they can often be arranged to excite room modes more evenly than a single subwoofer, allowing equalization to be more effective.<ref name=Welti2002 />

===Phase control===
[[File:Subwoofer downfiring.jpg|thumb|upright|The rear panel of a down-firing, active subwoofer cabinet]]

Changing the relative phase of the subwoofer with respect to the woofers in other speakers may or may not help to minimize unwanted destructive acoustic interference in the frequency region covered by both the subwoofer and the main speakers. It may not help at all frequencies, and may create further problems with frequency response, but even so is generally provided as an adjustment for subwoofer amplifiers.<ref name=Elliot2004 /> Phase control circuits may be a simple polarity reversal switch or a more complex continuously variable circuit.

Continuously variable phase control circuits are common in subwoofer amplifiers, and may be found in crossovers and as [[do-it-yourself]] electronics projects.<ref name=Elliot2004PC /><ref name=Nicholls2002 /><ref name=Slone2001 /><ref name=White2002 /><ref name=Robjohns2007 /> Phase controls allow the listener to change the arrival time of the subwoofer sound waves relative to the same frequencies from the main speakers (i.e. at and around the crossover point to the subwoofer). A similar effect can be achieved with the delay control on many home-cinema receivers. The subwoofer phase control found on many subwoofer amplifiers is actually a polarity inversion switch.<ref name=DellaSala2004 /> It allows users to reverse the polarity of the subwoofer relative to the audio signal it is being given. This type of control allows the subwoofer to either be in phase with the source signal, or 180 degrees out of phase.

The subwoofer phase can still be changed by moving the subwoofer closer to or further from the listening position, however this may not be always practical.

===Servo subwoofers===
Some active subwoofers use a servo feedback mechanism based on cone movement that modifies the signal sent to the voice coil. The servo feedback signal is derived from a comparison of the input signal to the amplifier versus the actual motion of the cone.<ref name=ServoSubs /> The usual source of the feedback signal is a few turns of voice coil attached to the cone or a microchip-based [[accelerometer]] placed on the cone itself.<ref name=ServoSubChip /><ref name=Doucet /> An advantage of a well-implemented servo subwoofer design is reduced distortion making smaller enclosure sizes possible.<ref name=Calabria2004 /> The primary disadvantages are cost and complexity.<ref name=DeutschDD12 />

Servo-controlled subwoofers are not the same as [[Tom Danley]]'s ServoDrive subwoofers, whose primary mechanism of sound reproduction avoids the normal voice coil and magnet combination in favor of a high-speed belt-driven [[servomotor]].<ref name=TECAwards /> The ServoDrive design increases output power, reduces harmonic distortion and virtually eliminates [[power compression]], the loss of loudspeaker output that results from an increase in voice coil impedance due to overheating of the voice coil. This feature allows high-power operation for extended periods of time.<ref name=Danley1983 /><ref name=ServoDrive /><ref name=Danley1986 /> Intersonics was nominated for a [[TEC Award]] for its ServoDrive Loudspeaker (SDL) design in 1986 and for the Bass Tech 7 model in 1990.<ref name=1986TEC /><ref name=1990TEC />