Subwoofer

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a 12" subwoofer driver
a 12" subwoofer driver

A subwoofer is a woofer, or a complete loudspeaker dedicated to the reproduction of bass audio frequencies, typically from 150 Hz down to 20 Hz. Subwoofers, also known as "subs", are used to augment the low frequency playback of main loudspeakers. Subwoofers are constructed by mounting one or more woofers) typically between 8" and 21" in diameter in a well-braced wood or plastic cabinet. Passive subwoofers have a subwoofer driver and enclosure; active subwoofers have a built-in amplifier.

The first subwoofer was developed in the 1960s by the owner of a high-end audio store in Los Angeles whose customers were complaining about a lack of bass response in the store's electrostatic speakers. Subwoofers came into greater popular consciousness in 1974 with the movie Earthquake which used six large subwoofers driven by a pair of 1600-watt amplifiers to recreate the rumbling of a powerful earthquake. With the advent of the compact cassette and consumer digital audio formats in the late 1980s, it became possible to add more low frequency content to recordings. As a result, in the 1990s and 2000s, subwoofers became increasingly popular in home stereo systems, home theater set ups, and car audio. By the 2000s, subwoofers became almost universal in professional sound applications such as live concert sound, churches, nightclubs and theme parks.

Contents

[edit] History

The first subwoofer was developed during the 1960s by Ken Kreisel, former president of the now defunct Miller & Kreisel Sound Corporation in Los Angeles. Kreisel's business partner, Jonas Miller, owned a high-end audio store in Los Angeles, and customers buying some of the high end electrostatic speakers complained about a lack of bass response in the electrostatics, compared to conventional loudspeakers; Kreisel's solution was to design a powered loudspeaker that would reproduce only those frequencies that were too low for the electrostatic speakers to convey and thereby fill in the missing sonic information.[1]. Infinity's full range electrostatic speaker system of about the same time also used a subwoofer to cover the lower frequency range the electrostatic arrays did not handle adequately.

The first use of a subwoofer in a recording session was in 1973 for mixing the Steely Dan album Pretzel Logic when recording engineer Roger Nichols arranged for Kreisel to bring a prototype of his subwoofer to Village Recorders.[2] Further design modifications were made by Kreisel over the next ten years, and in the 1970s and 1980s by engineer John P. D'Arcy; record producer Daniel Levitin served as a consultant and "golden ears" for the design of the crossover network (used to partition the frequency spectrum so that the subwoofer would not attempt to reproduce frequencies too high for its effective range, and so that the main speakers would not need to handle frequencies too low for their effective range).

Subwoofers came into greater popular consciousness in 1974 with the movie Earthquake which was released in Sensurround. Sensurround was initially installed in 17 U.S. theaters. Six very large subwoofers were driven by a pair of 1600 watt amplifiers that were triggered by control tones printed on one of the audio tracks on the film. Four of the subwoofers were positioned in front of the audience under (or behind) the film screen and two more were placed together at the rear of the audience on a platform. Noise energy in the range of 5 Hz to 40 Hz was generated at the level of 110-120 dB(SPL). Much publicity was given to the new low frequency entertainment method and the film was a box office success. More Sensurround systems were assembled and installed. By 1976 there were almost 300 Sensurround systems leapfrogging through select theaters. Further films to use the effect include Midway in 1976 and Rollercoaster in 1977.[3]

With the advent of the compact cassette and consumer digital audio formats, the reproduction of deep, loud bass was no longer limited by the ability of a phonograph record stylus to track a groove.[4] It became possible to add more low frequency content to recordings.[5] Home subwoofers grew in popularity, as they were easy to add to existing multimedia speaker setups and they were easy to position or hide.[6][7]

[edit] Construction and Features

Cross-section of a subwoofer drive unit. Image not to scale.
Cross-section of a subwoofer drive unit. Image not to scale.

Subwoofers use speaker drivers (woofers) typically between 8" and 21" in diameter, but single prototypes as large as 60" have been fabricated, and drivers as small as 4" may be used, depending on the design of the loudspeaker enclosure, the desired sound pressure level, the lowest frequency targeted and the amount of distortion that is allowable.[8] The most common subwoofer driver sizes in professional audio are 10", 12", 15" and 18".

The efficiency of a speaker driver is given by:

\eta_0 = \left(\frac{4 . \pi^2 . F_s^3 . V_{as}}{c^3 . Q_{es}}\right)\times100\ %

Where the variables are Thiele/Small parameters. Deep low frequency extension is a common goal for a subwoofer and small box volumes are also considered desirable. For the example of a sealed speaker box, the box volume to achieve a given Qts is proportional to Vas:

V_b = \frac{V_{as}}{\alpha} Where: \alpha = \frac{Q_{tc}}{Q_{ts}}^2-1

Therefore a decrease in box volume and the same F3 will decrease the efficiency of the sub woofer. Similarly the F3 of a speaker is proportional to Fs:

F_c = \frac{(Q_{ts}F_s)}{Q_{ts}}

As the efficiency is proportional to Fs3, Small decreases in low frequency extension with the same 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 power output of subwoofer amplifiers, and the requirement for greater power handling for subwoofer drivers. Some enclosure types are used for subwoofers to increase the efficiency of the driver/enclosure system, helping to reduce the amplifier power requirement.

Subwoofer mounted in a sealed enclosure
Subwoofer mounted in a sealed enclosure

Subwoofers have been designed using a number of enclosure approaches: bass reflex, acoustic suspension, infinite baffle, horn loaded, tapped horn, transmission line and bandpass. Each enclosure type has advantages and disadvantages in efficiency increase, bass extension, cabinet size, distortion, and cost.

Subwoofers are typically constructed by mounting one or more woofers in a cabinet made from medium-density fibreboard (MDF), oriented strand board (OSB), plywood, plastic or other dense materials. With interior volumes typically greater in size than their companion "main" loudspeakers, subwoofers often incorporate strong internal crossbracing to add strength and reduce box resonances.

[edit] Amplification

[edit] Active

Active subwoofers are built with their own dedicated amplifier. Some also include a user-adjustable equalization that allows boosted or reduced output at particular frequencies; these vary from a simple "boost" switch, to fully parametric equalizers meant for detailed speaker and room correction. Some such systems even include a calibrated microphone to measure the subwoofer's in-room response, so the automatic equalizer can correct the response to some pre-determined performance.

[edit] Passive

Passive subwoofers have a subwoofer driver and enclosure, but do not include an amplifier. They sometimes include passive crossovers inside, with the filter frequency determined at the factory. These are used with third-party power amplifiers, taking their inputs from active crossovers earlier in the signal chain. While few home-theater-in-a-box systems use passive subwoofers, this format is still popular in the professional sound industry.[9]

[edit] Equalization

Equalization can be used to adjust the in-room response of a subwoofer system. Designers of active subwoofers sometimes include a degree of corrective equalization to provide for tailored performance. In addition, many amplifiers include an adjustable low-pass filter, which limits the range of the subwoofer to low frequencies. For example, if a listener's main speakers are usable down to 80 Hz, then the subwoofer filter can be set so the subwoofer only works below 80 Hz. The crossover section may also include a high-pass "infrasonic" filter that prevents the subwoofer driver from attempting to reproduce frequencies below its capabilities. Some systems use parametric EQ to correct for a room's frequency response.[10][11] Equalization is often unable to achieve flat frequency response at all listening locations because of the resonance (ie, standing wave) patterns at low frequencies in nearly all rooms. Careful positioning of the subwoofer within the room can help flatten the frequency response.[12] Multiple subwoofers can manage a flatter general response since they will often excite room modes more evenly than a single subwoofer, allowing equalisation to be more effective.[13]

[edit] Phase control

Changing the relative phase of the subwoofer may or may not help to minimize unwanted destructive acoustic interference in the frequency region covered by both subwoofer and main speakers; It may not help at all frequencies, and may create further problems with frequency response.[14]. Phase control circuits either consist of a simple polarity reversal switch or more complex continuously variable circuits.

[edit] Continuously variable

Continuously variable phase control circuits are common in subwoofer amplifiers, and may be found in crossovers and as do-it-yourself electronics projects.[15][16][17][18][19][20][21][22] Phase controls allow the listener to change the arrival time of the subwoofer sound waves relative to the main speakers. The same effect can be achieved with the delay control on many home theater receivers.

[edit] Polarity

Back panel of a Polk subwoofer. Notice consumer line-level and speaker-level inputs, the polarity switch and the crossover frequency control.
Back panel of a Polk subwoofer. Notice consumer line-level and speaker-level inputs, the polarity switch and the crossover frequency control.

The subwoofer phase control found on many subwoofer amplifiers is simply a polarity inversion switch.[23] It allows a listener to reverse the polarity of the subwoofer system relative to the main speakers. This configuration allows the subwoofer to either be in phase, or 180 degrees out of phase.

[edit] Servo subwoofers

Some active subwoofers use a servo feedback mechanism based on cone movement which modifies the signal sent to the cone. The servo feedback signal is derived from a comparison of the input signal to the amplifier versus the actual motion of the cone. 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.[24][25] An advantage of a well-implemented servo subwoofer design is reduced distortion making smaller enclosure sizes possible.[26][27] The primary disadvantages are cost and complexity.[28]

[edit] Servodrive subwoofers

Note that servo controlled subwoofers are not the same as 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. The Servodrive design increases output power, reduces harmonic distortion and virtually eliminates the loss of loudspeaker output that results from an increase in voice coil impedance due to overheating of the voice coil (called power compression.) This feature allows high power operation for extended periods of time.[29][30][31] Intersonics was nominated for a TEC Award for its Servo Drive Loudspeaker (SDL) design in 1986 and for the Bass Tech 7 model in 1990.[32][33]

[edit] Applications

[edit] 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. Low frequencies are not easily localized; 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 produced is difficult to localize. Port noise and other types of distortion is typically produces sound higher in frequency, and may make localisation possible[citation needed].

The 1987 Bose Acoustimass 5 stereo subwoofer contained one six-inch (152 mm) driver per channel and provided crossover filtering for its two satellites
The 1987 Bose Acoustimass 5 stereo subwoofer contained one six-inch (152 mm) driver per channel and provided crossover filtering for its two satellites[34]

The intention in a system with a subwoofer is often to use small main ("satellite") speakers (of which there are two for stereo and five or more for surround sound or movie tracks) and to locate (i.e. "hide") the subwoofer elsewhere, or to augment an existing speaker to save it from handling low frequencies at high levels, or because high levels of low bass are desired. 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. 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's possible to have some cancellation of low frequencies if bass information in one channel is out of phase with another.

The physically separate subwoofer/satellite arrangement has been popularized by multimedia speaker systems such as Bose Acoustimass Home Entertainment Systems, Polk Audio RM2008 Series and Klipsch Audio Technologies ProMedia.[35][36] Low-cost "home theater in a box" systems advertise their integration and simplicity.

Particularly among low cost systems, however, inclusion of a subwoofer may be little more than a marketing device. It's quite unlikely that a small woofer in an inexpensively built compact cabinet will have better bass performance than well designed conventional (and typically larger) speakers. Mere use of the term "subwoofer" is no guarantee of good or extended bass performance. Many multimedia "subwoofers" might better be termed "bass modules" as they are too small to produce deep bass. Further, poorly designed systems often leave everything below about 120 Hz 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 crossover point or insufficient crossover slope) used in many computer and home theater systems; localization also comes from port noise and from typically large amounts of harmonic distortion in the subwoofer design.

Home subwoofers sold individually usually include crossover circuitry to assist integration into an existing system.

[edit] Car audio

Automobiles are well suited to the "hidden" subwoofer approach due to space limitations in the passenger compartments. It's not possible, in most circumstances, to fit such large drivers and enclosures into doors or dashboards. Typically, subwoofers are installed in the trunk or back seat space.

A constellation of subwoofers in a car hatchback
A constellation of subwoofers in a car hatchback

Some car audio enthusiasts compete to produce very high sound pressure levels in the confines of their vehicle's cabin; sometimes dangerously high. 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 subwoofers are 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.[37].

[edit] Non-circular subwoofers

Within the last few years, car audio has seen a few subwoofers using non-circular shapes from manufacturers such as Kicker, Sony, Bazooka, X-Tant, etc. These shapes typically carry some distortion penalties.[38][39][40] 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

V_\mathrm{d} = x_\mathrm{max} \times S_\mathrm{d}

where

  • Vd stands for volume of displacement (in m^3)
  • xmax to the amount of linear excursion the speaker is mechanically capable of (in m)
  • Sd to the cone area of the sub woofer (in m2).

These are some of the Thiele/Small parameters which can either be measured or found with the driver specifications.

[edit] Cinema sound

After the introduction of Sensurround, movie theater owners began installing permanent subwoofer systems. 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 introduced a dedicated cinema subwoofer model tuned to 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 didn't interfere with the listening experience.[41] 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, 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 low-frequency effects (LFE) channel, the "point one" in 5.1 surround sound.

Tom Horral, a Boston-based acoustician, blames subwoofers for louder cinema sound in general. He says that before subwoofers made it possible to have loud, relatively undistorted bass, movie sound levels were limited by the lesser capabilities of full-range loudspeakers.[42]

[edit] Concert sound

Professional audio subwoofers must be capable of very high output levels. This is reflected in the design attention given in recent years to the subwoofer applications for sound reinforcement, public address, and concert systems. 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. Specifically, movie theatres 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" drivers. Drivers as small as 10" are occasionally used, generally in horn loaded enclosures.

The main speakers may be 'flown' from the ceiling of a venue on chain hoists, and 'flying points' (ie, attachment points) are built into many professional loudspeaker enclosures. Subwoofers can be flown or stacked on the ground near the stage. There can be more than 50 double-18-inch cabinets in a typical concert system.

Just as consumer subwoofers 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.[43][44] MDF is commonly used to construct subwoofers for permanent installations as its density is very high and weatherproofing is not a concern. Other permanent installation subwoofers have used very thick plywood: the Altec 8182 (1981) used 7-ply 28 mm birch-faced oak plywood.[45] Touring subwoofers are typically built from 18-20 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.[46] Not naturally weatherproof, Baltic birch is coated with carpet, thick paint or spray-on truck bedliner to give the subwoofers greater durability.[47][48]

[edit] 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 crossover. 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.

[edit] 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 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.[49] The aux-fed subs method greatly reduces the number of sources feeding the subwoofers to include only those instruments that have desired low frequency information; sources such as kick drum, bass guitar, samplers and keys. This simplifies the signal sent to the subwoofers and makes for greater clarity and low punch.[50] Aux-fed subs can even be stereo, if desired, using two auxiliary mix buses.

[edit] Directional subwoofers

In order to keep low frequency energy 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.

[edit] Vertically arrayed subwoofers

Stacking or flying 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 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.

[edit] End-fired subwoofers
Cardioid dispersion pattern of two subwoofers placed one in front of the other. The enclosure nearest the listener must be delayed by a few milliseconds
Cardioid dispersion pattern of two subwoofers placed one in front of the other. The enclosure nearest the listener must be delayed by a few milliseconds[51]

Some subwoofer arrays and individual subwoofer designs rely on drivers facing to the sides or to the rear in order to achieve a degree of directionality.[52][53] Other designs and techniques place subwoofer drivers coaxially 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 or within a single enclosure that houses more than one driver.[54] Delay adjustments and polarity reversals are among the tools used by advocates of end-fired subs. Directionality is typically centered around a selected target frequency, and can achieve as much as 25 dB rear attenuation.[55] The positional technique of end-fired subwoofers came into limited regional use in live concert sound in 2006.[56]

[edit] Delay shaded subwoofers

A long line of subwoofers placed horizontally along the front edge of the stage can be delayed such that the center subs 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. This method helps to counteract the extreme narrowing of horizontal dispersion pattern seen with a horizontal subwoofer array. Such delay shading can be used to virtually reshape a loudspeaker array.[57]

[edit] Bass shakers

Since much very low bass is felt, sub-bass can be 'augmented' using tactile transducers. These have recently emerged as a device class, called variously "bass shakers", "butt shakers" and "throne shakers". They are attached to a seat, for instance a drummer's stool ("throne") or gamer's chair, car seat or home theater seating, and the vibrations of the driver are transmitted to the body then to the ear in a manner similar to bone conduction.[58][59] They connect to an amplifier like a normal subwoofer. They can be attached to a large flat surface (for instance a floor or platform) to create a large low frequency conduction area, although the transmission of low frequencies through the feet isn't as efficient as the seat.[60]

The advantage of tactile transducers used for low frequencies is that they allow a listening environment that isn't filled with loud low frequency waves. This helps the concert drummer to monitor his or her kick drum performance without "polluting" the stage with powerful low frequency waves from a 15" subwoofer monitor. By not having a subwoofer monitor, a bass shaker also enables a drummer to lower the sound pressure levels that they are exposed to during a performance. Bass shakers help the user avoid disturbing others nearby and help to augment the headphone listening experience. However, some feel that the felt vibrations are disconnected from the auditory experience, that music is less satisfying than sound effects and that the bass shaker itself can rattle and distract the listener.[61]

[edit] See also

[edit] References

  1. ^ Levitin, D. J. (February 1996). "The Audio Interview: M&K's Ken Kreisel". Audio 80: 28-34. 
  2. ^ Stereophile, March 1997. Wes Phillips. Audio Odyssey: Ken Kreisel of M&K
  3. ^ About Sensurround
  4. ^ AES E-Library: Tracking Ability Specifications for Phonograph Cartridges by Kogen, J. H.
  5. ^ Record-Producer.com, April 12, 2007. Mastering for vinyl vs. mastering for CD
  6. ^ Bag End. Infra Sub 18
  7. ^ Rutgers. Matthew Rodriguez et al. Experiences Designing and Building A Subwoofer Amplifier
  8. ^ Incredible 60-inch Subwoofer
  9. ^ JBL Sound System Design Manual, part two
  10. ^ http://bfdguide.ws/ BFD Parametric Equalizer
  11. ^ SoundStage! Bang & Olufsen Beolab 5 Loudspeakers
  12. ^ ETF Acoustic. Audio Measurement Basics
  13. ^ Todd Welti, Research Acoustician, Harman International Industries, Inc Subwoofers: Optimum Number and Locations
  14. ^ Rod Elliott Phase Correction - Myth or Magic
  15. ^ Rod Elliot Subwoofer Phase Controller
  16. ^ Sound On Sound Choosing & Installing a Subwoofer
  17. ^ Slone, G. Randy (2001). The Audiophile's Project Sourcebook: 80 High-Performance Audio Electronics Projects. McGraw-Hill Professional, 361. ISBN 0071379290. 
  18. ^ White, Joel (2002). Home Theater Solutions. Thomson Course Technology, 232. ISBN 192968567X. 
  19. ^ the professional answer Common Subwoofer Myths
  20. ^ diyAudio.com Subwoofer variable phase control - statement that variable phase controls are common on commercial subwoofers
  21. ^ Sound On Sound All You Wanted to Know About Subwoofers
  22. ^ YC's Hi-Fi Pages SVS 25-31 PC+ Review
  23. ^ Audioholics. Gene DellaSala. The Marriage Between the Subwoofer and Tower Speakers
  24. ^ Rythmik Audio Technology
  25. ^ Analog Devices. Doucet et al. Using an Accelerometer to Make an Active Subwoofer
  26. ^ Home Theater Hi-Fi. Review of Paradigm Servo-15
  27. ^ Home Theater Hi-Fi. Review of Rythmik Direct Servo kit
  28. ^ Ultimate AV. Review of Velodyne DD
  29. ^ AES E-Library: A High Efficiency Servo-Motor Driven Subwoofer by Danley, Thomas J.; Rey, Charles A.; Whymark, Roy R
  30. ^ Servodrive Bass Tech 7
  31. ^ AES E-Library: The Elimination of Power Compression in Servo Drive Loudspeakers by Danley, Thomas J.; Whymark, Roy R.; Rey, Charles A
  32. ^ 1986 TEC Awards
  33. ^ 1990 TEC Awards
  34. ^ Bose Acoustimass 5 user manual (1987)
  35. ^ Polk Audio RM2008 Series
  36. ^ Klipsch ProMedia Ultra 5.1
  37. ^ Passchier-Vermeer W, Passchier WF (2000). "Noise exposure and public health". Environ. Health Perspect. 108 Suppl 1: 123–31. PMID 10698728. 
  38. ^ AutoQnA, Are square subs better or worse than round subs?
  39. ^ Polk Audio, Tips, Tweaks and Common Sense About Car Audio
  40. ^ diyAudio.com, "A tricky question from a novice" - discussion about non-circular speakers
  41. ^ THX Company Overview
  42. ^ New York Times. By James Barron. Ideas & Trends: Shhhhh! The Audience Is Listening.; When Seeing a Movie Is a Pain in the Ear May 31, 1998
  43. ^ Speaker City. Bob Wayland for American Woodworker. Spectacular Speakers
  44. ^ Altec-Lansing 9880-8A 18" Subwoofer
  45. ^ Altec-Lansing 8000 Series Loudspeaker Systems
  46. ^ JBL Professional Enclosure Guide
  47. ^ JTR Growler with Line-X coating
  48. ^ Solid-NRG DS-1802SQ with Line-X coating
  49. ^ ProSoundWeb. Tom Young. A Detailed Explanation Of The Aux Fed Subwoofer Technique
  50. ^ Peavey. Don Boomer, Product Manager, Sound Reinforcement Division. AUX FED SUBS
  51. ^ 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 Hz.
  52. ^ Nexo Geo-D subwoofer
  53. ^ Meyersound. PSW-6 : High-Power Cardioid Subwoofer
  54. ^ EONA ADRaudio ATA 218C HH directional subwoofer
  55. ^ JBL Application Guide. Rick Kamlet. Spaced SB210 FSA Forward Steered Low-Bass Array. January 2004.
  56. ^ Dave Stevens: A Barking Dog. July 18, 2006. Pinging In From ES and Gear Notes Rotterdam, Wuppertal and Finkenstein
  57. ^ QSC. Mark Engebretson. Advanced Loudspeaker Tuning Techniques. QSC Intrinsic Correction™ (2007)
  58. ^ O'Reilly, 2004. Brett McLaughlin. Home Theater Hacks: 100 Industrial-Strength Tips & Tools
  59. ^ Home Theater. Robert Silva. Subwoofers: What You Need To Know Page 3: Subwoofer Alternatives.
  60. ^ Home Theater Hi-Fi, June 2002. Evan Upchurch. Product Review - ButtKicker 2 Low Frequency Shaker
  61. ^ ExtremeTech, September 8, 2005. Jeremy Atkinson. Get Your Butt Kicked

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