A
subwoofer is a
wooferWoofer is the term commonly used for a loudspeaker driver designed to produce low frequency sounds, typically from around 40 hertz up to about a kilohertz or higher. The name is from the onomatopoeic English word for a dog's bark, "woof"...
, or a complete
loudspeakerA loudspeaker is an electroacoustic transducer that converts an electrical signal into sound. The speaker pulses in accordance with the variations of an electrical signal and causes sound waves to propagate through a medium such as air or water.Loudspeakers are the most variable elements in a...
, dedicated to the reproduction of
bassBass , when used as an adjective, is used to describe tones of low frequency or range. Played in an ensemble/orchestra, such notes are frequently used to provide a counterpoint or counter-melody, in a harmonic context either to outline or juxtapose the progression of the chords, or with percussion...
audio
frequenciesFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency....
, from 80
HzThe hertz is a unit of frequency. It is defined as the number of complete cycles per second. It is the basic unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts...
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
wooferWoofer is the term commonly used for a loudspeaker driver designed to produce low frequency sounds, typically from around 40 hertz up to about a kilohertz or higher. The name is from the onomatopoeic English word for a dog's bark, "woof"...
s in a well-braced wood or plastic enclosure. Subwoofers have been designed using a number of speaker enclosure designs, including
bass reflexA Bass reflex system is a type of loudspeaker enclosure that uses the sound from the rear side of the diaphragm to increase the efficiency of the system at low frequencies as compared to a typical closed box loudspeaker or an infinite baffle mounting.Reflex port is the distinctive feature of a...
(with a port or tube in the enclosure), infinite baffle,
horn-loadedA horn loudspeaker is a complete loudspeaker or loudspeaker element which uses a horn to increase the overall efficiency of the driving element, typically a diaphragm driven by an electromagnet. The horn itself is a passive component and does not amplify the sound from the driving element as such,...
, and bandpass designs, each of which has advantages and disadvantages in efficiency, size, distortion, cost, and
power handlingIn audio electronics, there are several methods of measuring power output and power handling capacity...
. Passive subwoofers have a subwoofer driver and enclosure and they are powered by an external amplifier. Active subwoofers include a built-in amplifier.
The first subwoofers were developed in the 1960s to add bass response to home stereo systems. Subwoofers came into greater popular consciousness in the 1970s with the introduction of
SensurroundSensurround is a process developed in the 1970s by Universal Studios to enhance the audio experience during the presentation of theatrical movies...
in movies such as
EarthquakeEarthquake is a 1974 American disaster film that achieved huge box-office success, continuing the disaster film genre of the 1970s where recognizable all-star casts attempt to survive life or death situations...
, which produced loud low-frequency sounds through large subwoofers. With the advent of the
compact cassetteThe Compact Cassette, often referred to as audio cassette, cassette tape, cassette, or simply tape, is a magnetic tape sound recording format. Although originally designed for dictation, improvements in fidelity led the Compact Cassette to supplant reel-to-reel tape recording in most...
and the
compact discA Compact Disc is an optical disc used to store digital data. It was originally developed to store sound recordings exclusively, but later it also allowed the preservation of other types of data. Audio CDs have been commercially available since October 1982...
in the 1980s, the reproduction of deep, loud bass was no longer limited by the ability of a phonograph record stylus to track a groove, and producers could add more low frequency content to recordings. As well, during the 1990s, DVDs were increasingly recorded with "
surround soundSurround sound encompases a range of techniques for enriching the sound reproduction quality of an audio source with audio channels reproduced via additional, discrete speakers. The three-dimensional sphere of human hearing can be virtually achieved with audio channels above and below the listener...
" processes that included a Low Frequency Effects (LFE) channel, which could be heard using the subwoofer in
home theaterA home theater Is a theater built in a home, designed to mimic commercial theater performance.*Home cinema*AV receiver, often referred to as home theater systems....
systems. During the 1990s, subwoofers also became increasingly popular in home stereo systems, custom
car audioCar audio/video , mobile audio, 12-volt and other terms are used to describe the sound or video system fitted in an automobile. Such devices aren't necessarily limited to automobiles, and can be used, marketed, or manufactured for marine, aviation, and mass transit...
installations, and in PA systems. By the 2000s, subwoofers became almost universal in
sound reinforcement systemA sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers that makes live or pre-recorded sounds louder and may also distribute those sounds to a larger or more distant audience...
s in nightclubs and concert venues.
History
The first subwoofer was developed during the 1960s by Ken Kreisel, the former president of the Miller & Kreisel Sound Corporation in
Los AngelesLos Ángeles is the capital of the province of Biobío, in the municipality of the same name, in Region VIII , in the center-south of Chile. It is located between the Laja and Biobío rivers. The population is 123.445 inhabitants...
. 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 speakers had 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.. 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 DanSteely Dan is an American jazz-rock band centered on core members Walter Becker and Donald Fagen. The band reached a peak of popularity in the late 1970s, with the release of seven albums blending elements of jazz, rock, funk, R&B, and pop...
album
Pretzel LogicPretzel Logic is the third studio album by the American jazz-rock band Steely Dan, originally released in 1974. The album's opening song, "Rikki Don't Lose That Number", became the band's biggest hit, reaching #4 on the charts soon after the release of the album. The album itself went gold, and...
when recording engineer Roger Nichols arranged for Kreisel to bring a prototype of his subwoofer to Village Recorders. Further design modifications were made by Kreisel over the next ten years, and in the 1970s and 1980s by engineer
John P. D'ArcyJohn Paul D'Arcy, was an American electrical engineer and inventor closely associated with designing commercial subwoofers and lasers for Photorefractive keratectomy ....
;
record producerIn the music industry, a record producer has many roles, among them controlling the recording sessions, coaching and guiding the musicians, organizing and scheduling production budget and resources, and supervising the recording, mixing and mastering processes...
Daniel LevitinProfessor Daniel J. Levitin, Ph.D. is an American cognitive psychologist, neuroscientist, record producer, musician, and writer...
served as a
consultantA consultant is a professional who provides advice in a particular area of expertise such as management, accountancy, the environment, entertainment, technology, law , human resources, marketing, medicine, finance, life management, economics,...
and "
golden earA golden ear is a term in audio circles referring to a person who believes that they possess special talents in hearing. Golden ears claim to be able to discern subtle differences in audio reproduction that most inexperienced and untrained listeners cannot, much like trained wine experts can...
s" for the design of the
crossover networkAudio crossovers are a class of electronic filters designed specifically for use in audio applications, especially hi-fi. Commonly used loudspeaker drivers are incapable of covering the entire audio spectrum with acceptable volume and lack of distortion by themselves...
(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
EarthquakeEarthquake is a 1974 American disaster film that achieved huge box-office success, continuing the disaster film genre of the 1970s where recognizable all-star casts attempt to survive life or death situations...
which was released in
SensurroundSensurround is a process developed in the 1970s by Universal Studios to enhance the audio experience during the presentation of theatrical movies...
. 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 17 Hz to 120 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
MidwayMidway is a war film directed by Jack Smight and produced byWalter Mirisch from a screenplay by Donald S. Sanford. The music score was by John Williams and the cinematography by Harry Stradling, Jr...
in 1976 and
RollercoasterRollercoaster is a summer 1977 disaster-suspense film directed by James Goldstone. It was one of four films created in Sensurround by Universal Studios, along with Midway, Earthquake, and the theatrical version of Battlestar Galactica....
in 1977.
With the advent of the
compact cassetteThe Compact Cassette, often referred to as audio cassette, cassette tape, cassette, or simply tape, is a magnetic tape sound recording format. Although originally designed for dictation, improvements in fidelity led the Compact Cassette to supplant reel-to-reel tape recording in most...
and consumer
digital audioDigital audio uses digital signals for sound reproduction. This includes analog-to-digital conversion, digital-to-analog conversion, storage, and transmission. In effect, the system commonly referred to as digital is in fact a discrete-time, discrete-level analog of a previous electrical analog...
formats, the reproduction of deep, loud bass was no longer limited by the ability of a phonograph record stylus to track a groove. It became possible to add more low frequency content to recordings. Home subwoofers grew in popularity, as they were easy to add to existing multimedia speaker setups and they were easy to position or hide.
Construction and Features
Subwoofers use speaker
driversA loudspeaker is an electroacoustic transducer that converts an electrical signal into sound. The speaker pulses in accordance with the variations of an electrical signal and causes sound waves to propagate through a medium such as air or water.Loudspeakers are the most variable elements in a...
(
wooferWoofer is the term commonly used for a loudspeaker driver designed to produce low frequency sounds, typically from around 40 hertz up to about a kilohertz or higher. The name is from the onomatopoeic English word for a dog's bark, "woof"...
s) 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 enclosureA loudspeaker enclosure is a cabinet designed to transmit sound to the listener via mounted loudspeaker drive units. The major role of the enclosure is to prevent the out-of-phase sound waves from the rear of the speaker from combining with the in-phase sound waves from the front of the speaker...
, the desired sound pressure level, the lowest frequency targeted and the level of permitted distortion. 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/Small"Thiele/Small" commonly refers to a set of electromechanical parameters that define the specified low frequency performance of a loudspeaker driver. These parameters are published in specification sheets by driver manufacturers so that designers have a guide in selecting off-the-shelf drivers for...
parameters. 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 Q
ts is proportional to Vas:
Where:
Therefore a decrease in box volume and the same F
3 will decrease the efficiency of the sub woofer. Similarly the F
3 of a speaker is proportional to Fs:
As the efficiency is proportional to Fs
3, 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 reflexA Bass reflex system is a type of loudspeaker enclosure that uses the sound from the rear side of the diaphragm to increase the efficiency of the system at low frequencies as compared to a typical closed box loudspeaker or an infinite baffle mounting.Reflex port is the distinctive feature of a...
designs) 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 reflexA Bass reflex system is a type of loudspeaker enclosure that uses the sound from the rear side of the diaphragm to increase the efficiency of the system at low frequencies as compared to a typical closed box loudspeaker or an infinite baffle mounting.Reflex port is the distinctive feature of a...
,
acoustic suspensionThe acoustic suspension woofer is a type of loudspeaker that reduces bass distortion caused by non-linear, stiff mechanical suspensions in conventional loudspeakers...
, infinite baffle,
horn loadedA horn loudspeaker is a complete loudspeaker or loudspeaker element which uses a horn to increase the overall efficiency of the driving element, typically a diaphragm driven by an electromagnet. The horn itself is a passive component and does not amplify the sound from the driving element as such,...
, 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.
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.
Amplification
'Active subwoofers' include their own dedicated amplifiers within the cabinet. 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. Using a passive subwoofer adds flexibility for the user, because the user can select which type of amplifier (Class AB or
Class DThe term Class D may refer to:* Class D, a planet classification from the fictional Star Trek universe* Class-D amplifier or switching amplifier* Class D IP addresses on a Classful network* Class D Airspace class as defined by the ICAO...
, for example); brand of amplifier; or features (e.g., limiting to prevent distortion) that they want to use with their speaker or speakers.
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 compensate for known performance issues (e.g., a steeper than desired low end
roll-offRoll-off is a term commonly used to describe the steepness of a transmission function with frequency, particularly in electrical network analysis, and most especially in connection with filter circuits in the transition between a passband and a stopband...
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. Realizable filter behavior does not permit such sharp cutoffs, so some overlap is to be expected and must be compensated for.
DigitalA digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
crossover filters can produce sharper and more precise cutoff characteristics than
analogAnalogue electronics are those electronic systems with a continuously variable signal. In contrast, in digital electronics signals usually take only two different levels. The term "analogue" describes the proportional relationship between a signal and a voltage or current that representes the...
filters. The crossover section may also include a high-pass "
infrasonicInfrasound is sound that is lower in frequency than 20 Hz or cycles per second, the normal limit of human hearing. Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infrasound, the sound pressure must be sufficiently high...
" 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. Equalization is often unable to achieve flat frequency response at all listening locations in part because of the resonance (i.e.,
standing waveA standing wave, also known as a stationary wave, is a wave that remains in a constant position. This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling in opposite...
) patterns at low frequencies in nearly all rooms. Careful positioning of the subwoofer within the room can also help flatten the frequency response. 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.
Phase control
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 physics, interference is the addition of two or more waves that results in a new wave pattern.Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same...
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, but is even so generally provided as an adjustment for subwoofer amplifiers.. 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. 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. 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.
Servo subwoofers
Some active subwoofers use a servo feedback mechanism based on cone movement which 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. The usual source of the feedback signal is a few turns of voice coil attached to the cone or a microchip-based
accelerometerAn accelerometer measures the acceleration it experiences relative to freefall.Single- and multi-axis models are available to detect magnitude and direction of the acceleration as a vector quantity, and can be used to sense orientation, vibration and shock...
placed on the cone itself. An advantage of a well-implemented servo subwoofer design is reduced distortion making smaller enclosure sizes possible. The primary disadvantages are cost and complexity.
Servo controlled subwoofers are not the same as
ServodriveServodrive is most commonly known as a manufacturer of an unusual professional subwoofer. Rather than the cone being moved by a piston-like coil as normal it instead is moved by a motor. This, the manufacturer claims, allows it to be considerably more efficient than a traditional subwoofer and "the...
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. 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.
Home audio
The use of a subwoofer augments the bass capability of the main speakers, and allows them to be smaller without sacrificing low frequency capability. A subwoofer does not necessarily provide superior bass performance in comparison to large conventional loudspeakers on ordinary music recordings due to the typical lack of very low frequency content on such sources. However, there are recordings with substantial low frequency content that most conventional loudspeakers are ill-equipped to handle without the help of a subwoofer, especially at high playback levels, such as music for
pipe organThe pipe organ is a musical instrument that produces sound by driving pressurized air through pipes selected via a keyboard. Because each organ pipe produces a single pitch, the pipes are provided in sets called ranks, each of which has a common timbre and loudness throughout the keyboard compass...
s with 32' bass pipes (16 Hz) and some recordings with extremely low synth bass parts.
Low frequencies are not easily
localizedSound localization is a listener's ability to identify the location or origin of a detected sound in distance and direction or the methods in acoustical engineering to simulate the placement of an auditory cue in a virtual 3D space ....
; 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. 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 hide the subwoofer elsewhere (e.g. behind furniture or under a table), or to augment an existing speaker to save it from having to handle woofer-destroying low frequencies at high levels.
Some users add a subwoofer because high levels of low bass are desired, as in the case of
house musicHouse is a style of electronic dance music that originated in Chicago, Illinois, USA. It was initially popularized in mid-1980s discothèques catering to the African-American and Latino American communities, first in Chicago, then in New York City, New Jersey, Detroit and Miami...
enthusiasts. Thus, subwoofers may be part of a package that includes satellite speakers, may be purchased separately, or may be built into the same cabinet as a conventional speaker system. For instance, some floor standing tower speakers include a subwoofer driver in the lower portion of the same cabinet. Physical separation of subwoofer and "satellite" speakers not only allows placement in an inconspicuous location, but since
sub-bassSub-bass is a term used to describe audible sounds below 90 Hz and extending downward to include the lowest frequency humans can hear, typically 20 Hz. Sound systems often feature one or more subwoofer loudspeakers that are dedicated solely to amplifying sounds in the sub-bass range...
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
wavelengthIn physics, the wavelength of a sinusoidal wave is the spatial period of the wave – the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
; 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 SystemsThe Bose Acoustimass Home Entertainment Systems are Bose's line of surround sound speakers. The original Acoustimass speaker set was introduced in 1989, as the Bose Acoustimass 5 Series I. This was a stereo speaker system. It had two direct-reflecting speakers that connected to a passive bass then...
,
Polk AudioPolk Audio is a manufacturer of audio products best known for their home and automobile speakers. The company also produces a wide range of other audio products such as amplifiers and FM tuners...
RM2008 Series and Klipsch Audio Technologies ProMedia. Low-cost
"home theater in a box"A "home theater in a box" is an integrated home theater package which "bundles" together a combination DVD player/ multi-channel amplifier , speaker wires, connection cables, a remote control, a set of five or more surround sound speakers and a low-frequency...
systems advertise their integration and simplicity.
Particularly among low cost "
Home Theater in a BoxA "home theater in a box" is an integrated home theater package which "bundles" together a combination DVD player/ multi-channel amplifier , speaker wires, connection cables, a remote control, a set of five or more surround sound speakers and a low-frequency...
" systems and with "boom boxes", however, inclusion of a subwoofer may be little more than a marketing device. It is unlikely that a small woofer in an inexpensively-built compact plastic cabinet will have better bass performance than well-designed conventional (and typically larger) speakers in a plywood or MDF cabinet. Mere use of the term "subwoofer" is no guarantee of good or extended bass performance. Many multimedia "subwoofers" might better be termed "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
crossoverAudio crossovers are a class of electronic filters designed specifically for use in audio applications, especially hi-fi. Commonly used loudspeaker drivers are incapable of covering the entire audio spectrum with acceptable volume and lack of distortion by themselves...
circuitry to assist integration into an existing system.
Car audio
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 audioCar audio/video , mobile audio, 12-volt and other terms are used to describe the sound or video system fitted in an automobile. Such devices aren't necessarily limited to automobiles, and can be used, marketed, or manufactured for marine, aviation, and mass transit...
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
tinnitusTinnitus is the perception of sound within the human ear in the absence of corresponding external sound....
if one is exposed to them for an extended period of time..
In the 2000s, several car audio manufacturers have produced subwoofers using non-circular shapes from manufacturers, including Kicker, Sony, Bazooka, and X-Tant. These shapes typically carry some distortion penalties. 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
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 ).
These are some of the
Thiele/Small"Thiele/Small" commonly refers to a set of electromechanical parameters that define the specified low frequency performance of a loudspeaker driver. These parameters are published in specification sheets by driver manufacturers so that designers have a guide in selecting off-the-shelf drivers for...
parameters which can either be measured or found with the driver specifications.
Cinema sound
After the introduction of Sensurround,
movie theaterA movie theater, movie theatre, picture theatre, film theater or cinema is a venue, usually a building, for viewing motion pictures ....
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,
THXTHX is a trade name of a high-fidelity sound reproduction standard for movie theaters, screening rooms, home theaters, computer speakers, gaming consoles, and car audio systems. THX stands for Tomlinson Holman's eXperiment and is a throwback to George Lucas's first feature film, THX 1138...
certification of the cinema sound experience quantified the parameters of good audio for watching films, including requirements for subwoofer performance levels and enough isolation from outside sounds so that noise did not interfere with the listening experience. 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
JBLJBL is an American audio electronics company currently owned by Harman International. It was founded in 1946 by James Bullough Lansing. Their primary products are loudspeakers and associated electronics. There are two independent divisions within the company — JBL Consumer and JBL Professional...
,
Electro-VoiceElectro-Voice is a manufacturer of audio equipment, including microphones, amplifiers, and loudspeakers. A subdivision of Telex Communications Inc., Electro-Voice markets its products for use in small or large concert venues, broadcasting, houses of worship, and in retail situations.-History:On...
, Eastern Acoustic Works, Kintek,
Meyer Sound LaboratoriesMeyer Sound Laboratories is an American company based in Berkeley, California that manufactures self-powered loudspeakers, multichannel audio show control systems, electroacoustic architecture, and audio analysis tools for the professional sound reinforcement, fixed installation, and sound...
and
BGW SystemsBGW Systems is a designer and manufacturer of audio power amplifiers based in Southern California in the United States. The company also manufactures other audio electronics designs as well as computer systems and sheet metal products.-History:...
in the early 1990s. In 1992,
Dolby DigitalDolby Digital is the marketing name for a series of data/audio compression technologies developed by Dolby Laboratories.- Versions :Dolby Digital includes several similar technologies, which include Dolby Digital EX, Dolby Digital Live, Dolby Digital Plus, Dolby Digital Surround EX, Dolby...
'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.
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 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
THXTHX is a trade name of a high-fidelity sound reproduction standard for movie theaters, screening rooms, home theaters, computer speakers, gaming consoles, and car audio systems. THX stands for Tomlinson Holman's eXperiment and is a throwback to George Lucas's first feature film, THX 1138...
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' (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 subwoofer enclosures can be made of
Medium-density fibreboardMedium-density fiberboard is an engineered wood product formed by breaking down hardwood or softwood residuals into wood fibres, often in a defibrator, combining it with wax and a resin binder, and forming panels by applying high temperature and pressure.It is made up of separated fibers, but can...
(MDF),
Oriented strand boardOriented strand board, or OSB, or waferboard, or Sterling board or SmartPly is an engineered wood product formed by layering strands of wood in specific orientations. In appearance it may have a rough and variegated surface with the individual strips Oriented strand board, or OSB, or waferboard,...
(OSB),
plywoodPlywoodis a type of engineered wood made from thin sheets of wood, called plies or wood veneers. The layers are glued together so that adjacent plies have their grain at right angles to each other for greater strength. There are usually an odd number of plies, as the symmetry makes the board less...
, plastic or other dense material, professional subwoofer enclosures can be built from the same materials. MDF is commonly used to construct subwoofers for permanent installations as its density is relatively high and weatherproofing is not a concern. Other permanent installation subwoofers have used very thick plywood: the Altec 8182 (1981) used 7-ply 28 mm birch-faced oak plywood. 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. Not naturally weatherproof, Baltic birch is coated with carpet, thick paint or spray-on
truck bedlinerTruck bedliners are coverings that protect the inner-side of a truck bed from damage and can prevent fine-grained cargo such as sand from sifting between the sides and the rear door...
to give the subwoofer enclosures greater durability.
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
crossoverAudio crossovers are a class of electronic filters designed specifically for use in audio applications, especially hi-fi. Commonly used loudspeaker drivers are incapable of covering the entire audio spectrum with acceptable volume and lack of distortion by themselves...
. Such a system receives its signal from the main mono or stereo
mixing consoleIn professional audio, a mixing console, or audio mixer, also called a sound board or soundboard, is an electronic device for combining , routing, and changing the level, timbre and/or dynamics of audio signals. A mixer can mix analog or digital signals, depending on the type of mixer...
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.
"Aux"-fed subwoofers
Instead of being incorporated into a full-range system, concert subwoofers can be supplied with their own signal from a separate mix bus on the mixing console; often one of the auxiliary sends ("aux" or "auxes") is used. This configuration is called "aux-fed subwoofers", and has been observed to significantly reduce low frequency "muddiness" that can build up in a concert sound system which has on stage a number of microphones each picking up low frequencies and each having different phase relationships of those low frequencies. 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 drumA bass drum is a relatively large drum that produces a note of low definite or indefinite pitch. The bass drums are of variable sizes and are used in several musical genres . Three major types of bass drums can be distinguished: the large orchestral bass drum, the smaller kick' drum, and the...
,
bass guitarThe electric bass guitar is a stringed instrument played primarily with the fingers or thumb , or by using a plectrum....
,
samplersA sampler is an electronic musical instrument closely related to a synthesizer. Instead of generating sounds from scratch, however, a sampler starts with multiple recordings of different sounds added by the user, and then plays each back based on how the instrument is configured...
and
keysA keyboard instrument is any musical instrument played using a musical keyboard. The most common of these is the piano. Other widely used keyboard instruments include various types of organs as well as other mechanical, electromechanical and electronic instruments...
. This simplifies the signal sent to the subwoofers and makes for greater clarity and low punch. Aux-fed subs can even be stereo, if desired, using two auxiliary mix buses.
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.
Vertically arrayed subwoofers
Stacking or
flyingA fly system is a system of ropes, counterweights, pulleys, and other such tools within a theatre designed to allow a technical crew to quickly move set pieces, lights, and microphones on and off stage by 'flying' them in from a large opening above the stage known as a flyspace, flyloft, fly tower,...
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.
End-fired subwoofers
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. 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. Delay adjustments and polarity reversals are among the tools used by advocates of end-fired subs. Directionality is typically centered on a selected target frequency, and can achieve as much as 25 dB rear attenuation. The positional technique of end-fired subwoofers came into limited regional use in live concert sound in 2006.
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.
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 conductionBone conduction is the conduction of sound to the inner ear through the bones of the skull.Bone conduction is the reason why a person's voice sounds different to him/her when it is recorded and played back...
. 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.
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 is exposed to during a performance. For home cinema or videogame use, bass shakers help the user avoid disturbing others in nearby apartments or rooms, because even powerful sound effects such as explosion sounds in a war videogame or the simulated rumbling of an earthquake in an adventure film will not be heard by others. However, some critics argue that the felt vibrations are disconnected from the auditory experience, and they claim that that music is less satisfying with the "butt shaker" than sound effects. As well, critics have claimed that the bass shaker itself can rattle during loud sound effects, which can distract the listener.
See also
- Bass management
The fundamental principle of bass management in surround sound replay systems is that bass content in the incoming signal, irrespective of channel, should be directed only to loudspeakers capable of handling it, whether the latter are the main system loudspeakers or one or more special...
- Bass test
A bass test is an audio recording, usually composed mainly of synthesized low frequency bass notes often played with a simple, repeating melody. Bass tests are used to test the low end, or bass response, of audio systems, particularly loudspeakers and amplifiers...
- Mid-range speaker
A loudspeaker driver that produces the frequency range from approximately 300–5000 hertz is known as a mid-range. They are also called, less commonly, squawkers....
- Super Tweeter
A super tweeter is a speaker driver designed to produce ultra high frequencies. Its purpose is to recreate a larger, more realistic sound field. Super tweeters are usually found in Hi-Fi speaker systems and home theatre systems...
- Rotary Woofer
The Rotary Woofer is a subwoofer-style loudspeaker which reproduces very low frequency content by using a conventional speaker voice coil's motion to change the pitch of a set of fan blades rotating at a constant speed...
- Thiele/Small
"Thiele/Small" commonly refers to a set of electromechanical parameters that define the specified low frequency performance of a loudspeaker driver. These parameters are published in specification sheets by driver manufacturers so that designers have a guide in selecting off-the-shelf drivers for...
- Tweeter
A tweeter is a loudspeaker designed to produce high frequencies, typically from around 2,000 Hz to 20,000 Hz . Some tweeters can manage response up to 45 kHz...
- Woofer
Woofer is the term commonly used for a loudspeaker driver designed to produce low frequency sounds, typically from around 40 hertz up to about a kilohertz or higher. The name is from the onomatopoeic English word for a dog's bark, "woof"...
External links
- Big Bass in Small Places - A Question of Fundamentals Secrets of Home Theater and High Fidelity. Technical article detailing subwoofers.
- Subwoofer Tests Subwoofer Tests performed by Ilkka Rissanen. Detailed measurements, graphs, and a discussion forum.
- DIY Subwoofer Forum Discussion forum for Sealed, Ported, SonoSub, Infinite Baffle, and more. Complete with projects, tips, and guides.
- AV Talk Subwoofer Tests In-depth, subwoofer testing
- Setting up a Subwoofer for Home Theater - detailed guide on Big Picture Big Sound
- The Subwoofer DIY Page
- DIY Audio The forum for DIY enthusiasts (includes subwoofer section).
- Rythmik Servo Technology Information on how servo subwoofers work, including an animated example.
- Realm of Excursion A site and forum dedicated to audio, including a large number of videos featuring high excursion speakers/subwoofers.
- Sub woofer builder.com Bill Collison's sub woofer design site
- Harry Brill, Jr.'s photos of large-scale concert subwoofers in end-fired cardioid positioning
- Ales Dravinec. R&D manager, EONA ADRaudio. Cardioid subwoofer photograph example. ProSoundWeb Live Audio Board
- Mike Babcock (www.themonitorguy.com). Cardioid subwoofer prediction software examples with charts and photos. ProSoundWeb Live Audio Board
- Isobaric Subwoofer Design A diy 6th Order Isobaric Series Tuned Subwoofer Project
- IB Subwoofer FAQ Pages Infinite Baffle Subwoofer (a subwoofer configuration using another room as "the box"): FAQ page with illustrated guide and links to forum
- http://www.avforums.com/forums/subwoofers UK's largest subwoofer forums discussing all technologies used in home theatres and home cinemas
- Free Speaker Plans Community oriented DIY loudspeaker design plans, general resources and forum.