A subwoofer is a
woofer, or a complete loudspeakerdedicated to the reproduction of bass audio frequencies, from perhaps 150 Hz down as far as 20 Hz, or in rare cases lower. Also known as "subs", these are used to augment the low frequency performance 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. Subwoofers have been designed using a number of speaker enclosure designs, including bass reflex(in which a port or tube is made in the cabinet), infinite baffle, horn-loaded, and bandpass designs, each of which has advantages and disadvantages in efficiency, cabinet size, distortion, and cost. Passive subwoofers have a subwoofer driver and enclosure and they are powered by an external amplifier. Active subwoofers are a speaker cabinet with a built-in amplifier.
The first subwoofers were developed in the 1960s by the owner of a high-end audio store in Los Angeles whose customers complained about a lack of bass response in the store's
electrostatic speakers. Subwoofers came into greater popular consciousness in the 1970s with the introduction of Sensurroundin movies such as "Earthquake", which played back loud low-frequency sounds through large subwoofers. With the advent of the compact cassetteand the compact discin the 1980s, the reproduction of deep, loud bass was no longer limited by the ability of a phonograph recordstylus to track a groove. [ [http://www.aes.org/e-lib/browse.cfm?elib=905 AES E-Library: Tracking Ability Specifications for Phonograph Cartridges by Kogen, J. H.] ] , and producers could add more low frequency content to recordings. As well, during the 1990s, DVDs were increasingly recorded with " surround sound" processes that included a Low Frequency Effects (LFE) channel, which could be heard using the subwoofer in home theatersystems. During the 1990s, subwoofers also became increasingly popular in home stereo systems, custom car audioinstallations, and in PA systems. By the 2000s, subwoofers became almost universal in sound reinforcement systems in nightclubs and concert venues.
The first subwoofer was developed during the 1960s by Ken Kreisel, the former president of the
Miller & Kreisel Sound Corporationin Los Angeles. When Kreisel's business partner, Jonas Miller, who owned a high-end audio store in Los Angeles, told Kreisel that some purchasers of the store's high-end electrostatic speakershad complained about a lack of bass response in the electrostatics, Kreisel designed a powered woofer that would reproduce only those frequencies that were too low for the electrostatic speakers to convey. [cite journal | author= Levitin, D. J. | title=The Audio Interview: M&K's Ken Kreisel | journal = Audio | volume = 80 | month = February | year = 1996 | pages = 28–34] . Infinity's full range electrostatic speaker system that was developed during the 1960s also used a woofer to cover the lower frequency range that its electrostatic arrays did not handle adequately.
The first use of a subwoofer in a recording session was in 1973 for mixing the
Steely Danalbum " Pretzel Logic" when recording engineerRoger Nichols arranged for Kreisel to bring a prototype of his subwoofer to Village Recorders. [ [http://www.stereophile.com/interviews/136/ Stereophile, March 1997. Wes Phillips. "Audio Odyssey: Ken Kreisel of M&K"] ] 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 Levitinserved as a consultantand " 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 received a great deal of publicity in 1974 with the movie "Earthquake" which was released in
Sensurround. Initially installed in 17 U.S. theaters, the Sensurround system used large subwoofers which were driven by racks of 500 watt amplifiers which 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. Powerful noise energy in the range of 5 Hz to 40 Hz was generated at the level of 110-120 dB(SPL). The new low frequency entertainment method helped the film become a box office success. More Sensurround systems were assembled and installed. By 1976 there were almost 300 Sensurround systems leapfrogging through select theaters. Other films to use the effect include the WW II naval battle epic "Midway" in 1976 and "Rollercoaster" in 1977. [ [http://www.in70mm.com/newsletter/2004/69/sensurround/about.htm About Sensurround] ]
With the advent of the
compact cassetteand consumer digital audioformats, the reproduction of deep, loud bass was no longer limited by the ability of a phonograph recordstylus to track a groove. [ [http://www.aes.org/e-lib/browse.cfm?elib=905 AES E-Library: Tracking Ability Specifications for Phonograph Cartridges by Kogen, J. H.] ] It became possible to add more low frequency content to recordings. [ [http://www.record-producer.com/learn.cfm?a=44 Record-Producer.com, April 12, 2007. "Mastering for vinyl vs. mastering for CD"] ] Home subwoofers grew in popularity, as they were easy to add to existing multimedia speaker setups and they were easy to position or hide. [ [http://www.aboutgizmos.com/bagendinsub11.html Bag End. "Infra Sub 18"] ] [ [http://www.osd.rutgers.edu/gs/06papers/Subwoof.pdf Rutgers. Matthew Rodriguez et al. "Experiences Designing and Building A Subwoofer Amplifier"] ]
Construction and Features
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 level of permitted distortion. [ [http://www.techeblog.com/index.php/tech-gadget/incredible-60-inch-subwoofer "Incredible 60-inch Subwoofer"] ] The most common subwoofer driver sizes in professional audio are 10", 12", 15" and 18".
The efficiency of a speaker driver is given by:
Where the variables are
Thiele/Smallparameters. Deep low frequency extension is a common goal for a subwoofer and small box volumes are also considered desirable. Hoffman's Iron Laws therefore mandate low efficiency under those constraints, and indeed most subwoofers require considerable power, much more than other individual drivers.
So for the example of a sealed speaker box, the box volume to achieve a given Qts is proportional to Vas:
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:
As the efficiency is proportional to Fs3, 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 power output of subwoofer amplifiers, and the requirement for greater power handling for subwoofer drivers. Enclosure variations (e.g.,
bass reflexdesigns) are sometimes used for subwoofers to increase the efficiency of the driver/enclosure system, helping to reduce the amplifier power requirement.
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 of medium-density fibreboard (MDF), oriented strand board (OSB), plywood, plastic or other dense materials. Because of the high power they use, subwoofers often require strong internal crossbracing to add strength and reduce box resonances.
'Active subwoofers' include their own dedicated amplifiers. Some also include 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 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 effects of room modes and improve low frequency performance.
'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. While few high-end home-theater systems use passive subwoofers, this format is still popular in the professional sound industry. [ [http://www.jblpro.com/pub/manuals/pssdm_2.pdf JBL Sound System Design Manual, part two] ]
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 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 higher frequencies the subwoofer cannot reproduce adequately from reaching the driver at all. 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. Realizable filter behavior does not permit such sharp cutoffs, so some overlap is to be expected and must be compensated for.
Digitalcrossover filters can produce sharper and more precise cutoff characteristics than analogfilters.
The crossover section may also include a high-pass "infrasonic" filter which prevents the subwoofer driver from attempting to reproduce frequencies below its safe capabilities.
Some systems use parametric equalization in an attempt to correct for room frequency response irregularites. [ [http://bfdguide.ws/ http://bfdguide.ws/ "BFD Parametric Equalizer"] ] [ [http://www.ultraaudio.com/equipment/bang_olufsen_beolab_5.htm SoundStage! "Bang & Olufsen Beolab 5 Loudspeakers"] ] 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. [ [http://www.etfacoustic.com/basics.html ETF Acoustic. "Audio Measurement Basics"] ] 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 equalisation to be more effective. [ [http://www.harman.com/wp/pdf/multsubs.pdf Todd Welti, Research Acoustician, Harman International Industries, Inc " Subwoofers: Optimum Number and Locations"] ]
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
interferencein 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, but is even so generally provided as an adjustment for subwoofer amplifiers. [ [http://sound.westhost.com/pcmm.htm Rod Elliott "Phase Correction - Myth or Magic"] ] . Phase control circuits may be a simple polarity reversal switch or a more complex continuously variable circuits.
Continuously variable phase control circuits are common in subwoofer amplifiers, and may be found in crossovers and as do-it-yourself electronics projects. [ [http://sound.westhost.com/project103.htm Rod Elliot "Subwoofer Phase Controller"] ] [ [http://www.soundonsound.com/sos/jul02/articles/subwoofers.asp Sound On Sound "Choosing & Installing a Subwoofer"] ] [cite book
last = Slone
first = G. Randy
title = The Audiophile's Project Sourcebook: 80 High-Performance Audio Electronics Projects
publisher = McGraw-Hill Professional
date = 2001
pages = 361
isbn = 0071379290] [cite book
last = White
first = Joel
title = Home Theater Solutions
publisher = Thomson Course Technology
date = 2002
pages = 232
isbn = 192968567X] [ [http://theprofessionalanswer.com/WordPress/?p=18 the professional answer "Common Subwoofer Myths"] ] [ [http://www.diyaudio.com/forums/showthread/t-1700.html diyAudio.com "Subwoofer variable phase control" - statement that variable phase controls are common on commercial subwoofers] ] [ [http://www.soundonsound.com/sos/apr07/articles/subwoofers.htm Sound On Sound "All You Wanted to Know About Subwoofers"] ] [ [http://kingfisher.clubsnap.org/hifi/reviews/Review_svs.htm YC's Hi-Fi Pages "SVS 25-31 PC+ Review"] ] 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 theater receivers.
The subwoofer phase control found on many subwoofer amplifiers is actually a polarity inversion switch. [ [http://www.audioholics.com/tweaks/get-good-bass/the-marriage-between-the-subwoofer-and-tower-speakers Audioholics. Gene DellaSala. "The Marriage Between the Subwoofer and Tower Speakers"] ] 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.
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
accelerometerplaced on the cone itself. [ [http://www.rythmikaudio.com/technology.html Rythmik Audio "Technology"] ] [ [http://www.analog.com/en/content/0,2886,760%255F788%255F7004,00.html Analog Devices. Doucet et al. "Using an Accelerometer to Make an Active Subwoofer"] ] An advantage of a well-implemented servo subwoofer design is reduced distortion making smaller enclosure sizes possible. [ [http://www.hometheaterhifi.com/volume_12_3/paradigm-servo-15-subwoofer-7-2005-part-1.html Home Theater Hi-Fi. Review of Paradigm Servo-15] ] [ [http://www.hometheaterhifi.com/volume_11_2/diy-14-rythmik-directservo-subwoofer-4-2004.html Home Theater Hi-Fi. Review of Rythmik Direct Servo kit] ] The primary disadvantages are cost and complexity. [ [http://ultimateavmag.com/speakersystems/1204velodyne/ Ultimate AV. Review of Velodyne DD] ]
Note that servo controlled subwoofers are not the same as
Servodrivesubwoofers 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. [ [http://www.aes.org/e-lib/browse.cfm?elib=11721 AES E-Library: A High Efficiency Servo-Motor Driven Subwoofer by Danley, Thomas J.; Rey, Charles A.; Whymark, Roy R] ] [ [http://www.servodrive.com/basstech7.html Servodrive Bass Tech 7] ] [ [http://www.aes.org/e-lib/browse.cfm?elib=5023 AES E-Library: The Elimination of Power Compression in Servo Drive Loudspeakers by Danley, Thomas J.; Whymark, Roy R.; Rey, Charles A] ] 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. [ [http://mixfoundation.org/tec/1986.html 1986 TEC Awards] ] [ [http://mixfoundation.org/tec/1990.html 1990 TEC Awards] ]
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).
The conflict between assorted goals can never be fully resolved; subwoofer designs are necessarily compromises. Hoffman's Iron Law (the efficiency of a woofer system is directly proportional to its cabinet volume and to the cube of its cutoff frequency) applies to subwoofers just as to all loudspeakers. [ [http://ldsg.snippets.org/appdx-a.php DIY Loudspeaker Designer's Selection Guide (the LDSG). Appendix A - Loudspeaker Design. "Hoffman's Iron Law":"] ]
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 typically produce sounds higher in frequency, and may make localisation possibleFact|date=April 2008.
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-bassfrequencies 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 AudioRM2008 Series and Klipsch Audio TechnologiesProMedia. [ [http://www.polkaudio.com/homeaudio/products/systems/subsatsystems/series/RM2008/ Polk Audio RM2008 Series] ] [ [http://www.klipsch.com/products/details/promedia-ultra-5-1.aspx Klipsch ProMedia Ultra 5.1] ] 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.
Automobiles are well suited to 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 audioenthusiasts 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 tinnitusif one is exposed to them for an extended period of time.cite journal |author=Passchier-Vermeer W, Passchier WF |title=Noise exposure and public health |journal=Environ. Health Perspect. |volume=108 Suppl 1 |issue= |pages=123–31 |year=2000 |pmid=10698728 |doi=] .
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. [ [http://www.autoqna.com/auto-audio/2192-auto-audio-6.html AutoQnA, Are square subs better or worse than round subs?"] ] [ [http://www.polkaudio.com/downloads/12vhndbk.pdf Polk Audio, Tips, Tweaks and Common Sense About Car Audio"] ] [ [http://www.diyaudio.com/forums/showthread.php?threadid=80386 diyAudio.com, "A tricky question from a novice" - discussion about non-circular speakers"] ] 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
* Vd stands for volume of displacement (in m^3)
* xmax to the amount of linear excursion the speaker is mechanically capable of (in mm)
* Sd to the cone area of the sub woofer (in ).These are some of the
Thiele/Smallparameters which can either be measured or found with the driver specifications.
After the introduction of Sensurround,
movie theaterowners 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, Altecintroduced a dedicated cinema subwoofer model tuned to 20 Hz: the 8182. Starting in 1983, THXcertification 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. [ [http://www.thx.com/company/index.html THX Company Overview] ] 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 Laboratoriesand BGW Systemsin 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 distortion in less capable systems at low frequency and high levels. [ [http://query.nytimes.com/gst/fullpage.html?res=9A04E1DB1E38F932A05756C0A96E958260&sec=&spon=&pagewanted=all 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] ]
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 systems, 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 THXstandard 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' (i.e., 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. [ [http://www.speakercity.com/Merchant2/merchant.mvc?Screen=CTGY&Store_Code=SC&Category_Code=Article Speaker City. Bob Wayland for American Woodworker. "Spectacular Speakers"] ] [ [http://www.altecpro.com/pdfs/vintage/SpeakerAndMics/systems/9880-8A%20LF%20Speaker%20System.pdf Altec-Lansing 9880-8A 18" Subwoofer] ] 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 Altec8182 (1981) used 7-ply 28 mm birch-faced oak plywood. [ [http://www.altecpro.com/pdfs/vintage/SpeakerAndMics/systems/8000%20Series%20LF%20Speaker%20Systems.pdf Altec-Lansing 8000 Series Loudspeaker Systems] ] Touring subwoofers are typically built from 18-20 mm thick void-free Baltic birch ( Betula pendulaor Betula pubescens) plywood from Finland, Estonia or Russia; such plywood affords greater strength for frequently transported enclosures. [ [http://www.jblpro.com/pub/manuals/enclgde.pdf JBL Professional Enclosure Guide] ] Not naturally weatherproof, Baltic birch is coated with carpet, thick paint or spray-on truck bedlinerto give the subwoofers greater durability. [ [http://www.jtrspeakers.com/ JTR Growler with Line-X coating] ] [ [http://www.solid-nrg.com/ds1802sq.htm Solid-NRG DS-1802SQ with Line-X coating] ]
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 consolemix 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.
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. [ [http://www.prosoundweb.com/install/tech_corner/auxsubs.shtml ProSoundWeb. Tom Young. "A Detailed Explanation Of The Aux Fed Subwoofer Technique"] ] 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. [ [http://peavey.com/support/technotes/concepts/AUX_SUBs.pdf Peavey. Don Boomer, Product Manager, Sound Reinforcement Division. "AUX FED SUBS"] ] Aux-fed subs can even be stereo, if desired, using two auxiliary mix buses.
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.
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.
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. [ [http://pro-light-news.com/html/16/e60419ne.htm Nexo Geo-D subwoofer] ] [ [http://www.meyersound.com/products/concertseries/psw-6/ Meyersound. "PSW-6 : High-Power Cardioid Subwoofer"] ] 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. [ [http://www.adraudio.com/Products/ATA218C.html EONA ADRaudio "ATA 218C HH directional subwoofer"] ] 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. [ [http://www.jblpro.com/pages/install/Spaced%20SB210%20FSA%20Forward%20Steered%20Bass%20Array.pdf JBL Application Guide. Rick Kamlet. "Spaced SB210 FSA Forward Steered Low-Bass Array". January 2004.] ] The positional technique of end-fired subwoofers came into limited regional use in live concert sound in 2006. [ [http://rex.roaddog.com/mt/blog/2006/07/ Dave Stevens: A Barking Dog. "July 18, 2006. Pinging In From ES and Gear Notes Rotterdam, Wuppertal and Finkenstein"] ]
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. [ [http://www.qscaudio.com/products/dsp/SC28/intrinsic_correction_whitepaper_2007.pdf QSC. Mark Engebretson. "Advanced Loudspeaker Tuning Techniques. QSC Intrinsic Correction™" (2007)] ]
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. [ [http://www.oreilly.com/catalog/htheaterhks/ O'Reilly, 2004. Brett McLaughlin. "Home Theater Hacks: 100 Industrial-Strength Tips & Tools"] ] [ [http://hometheater.about.com/cs/loudspeakers/a/aasubwoofera_3.htm Home Theater. Robert Silva. "Subwoofers: What You Need To Know" Page 3: Subwoofer Alternatives.] ] 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. [ [http://www.hometheaterhifi.com/volume_9_2/buttkicker-two-6-2002.html Home Theater Hi-Fi, June 2002. Evan Upchurch. "Product Review - ButtKicker 2 Low Frequency Shaker"] ]
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 he or she is 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. [ [http://www.extremetech.com/article2/0,1697,1857002,00.asp ExtremeTech, September 8, 2005. Jeremy Atkinson. "Get Your Butt Kicked"] ]
* [http://www.hometheaterhifi.com/features/technical-topics/big-bass-in-small-places---a-question-of-fundamentals.html Big Bass in Small Places - A Question of Fundamentals] Secrets of Home Theater and High Fidelity. Technical article detailing subwoofers.
* [http://www.hometheatershack.com/forums/subwoofer-tests/ Subwoofer Tests] Subwoofer Tests performed by Ilkka Rissanen. Detailed measurements, graphs, and a discussion forum.
* [http://www.hometheatershack.com/forums/diy-subwoofers/ DIY Subwoofer Forum] Discussion forum for Sealed, Ported, SonoSub, Infinite Baffle, and more. Complete with projects, tips, and guides.
* [http://www.subwoofertests.com AV Talk Subwoofer Tests] In-depth, subwoofer testing
* [http://www.bigpicturebigsound.com/article_560.shtml Setting up a Subwoofer for Home Theater] - detailed guide on Big Picture Big Sound
* [http://www.diysubwoofers.org The Subwoofer DIY Page]
* [http://www.diyaudio.com DIY Audio] The forum for DIY enthusiasts (includes subwoofer section).
* [http://www.rythmikaudio.com/technology.html Rythmik Servo Technology] Information on how servo subwoofers work, including an [http://www.rythmikaudio.com/technology.html#demo animated example] .
* [http://www.realmofexcursion.com Realm of Excursion] A site and forum dedicated to audio, including a large number of videos featuring high excursion speakers/subwoofers.
* [http://www.subwoofer-builder.com Sub woofer builder.com] Bill Collison's sub woofer design site
* [http://picasaweb.google.com/lh/viewPhoto?uname=HBrillJr&aid=5046282588229999185&iid=5046282712784050802 Harry Brill, Jr.'s photos of large-scale concert subwoofers in end-fired cardioid positioning]
* [http://srforums.prosoundweb.com/index.php/m/313424/72/#msg_313424 Ales Dravinec. R&D manager, EONA ADRaudio. Cardioid subwoofer photograph example. ProSoundWeb Live Audio Board]
* [http://srforums.prosoundweb.com/index.php/m/29332/3954/0///449/#msg_29332 Mike Babcock (www.themonitorguy.com). Cardioid subwoofer prediction software examples with charts and photos. ProSoundWeb Live Audio Board]
* [http://www.yourmissus.com/subwoofer/ Isobaric Subwoofer Design] A diy 6th Order Isobaric Series Tuned Subwoofer Project
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