Information about Subwoofer

A subwoofer refers to either a woofer, or a complete loudspeaker dedicated to the reproduction of bass audio frequencies, typically from 150 Hz down to 20 Hz. In the case of a rotary woofer, it is possible to reproduce frequencies down to 1 Hz. Bass frequencies below about 80Hz become increasingly difficult for typical floorstanding or bookshelf loudspeakers to reproduce at high playback levels (100 dB or more) because they do not have large enough drivers to generate the required amount of air displacement. Therefore, subwoofers are useful for augmenting main loudspeakers by providing playback of low bass frequencies at high levels.

History

The first subwoofer was developed during the 1960s by Ken Kreisel, currently president of 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 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. Further design modifications were made by Kreisel over the next ten years (and continuing to the present day), and in the 1970s and 1980s by engineer John P. D'Arcy; record producer Daniel Levitin served as a consultant and "golden ear" 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 1600w 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. Energy in the range of 5 Hz to 40Hz was generated at the level of 110-120 dB. 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. ^[2]

Overview

Subwoofers use drivers (woofers) (typically between 8" and 18" in diameter), but some have been as large as 60". Some drivers as small as 4" may be referred to as subwoofers but drivers less than 8" generally have neither the cone area nor excursion ability necessary to produce deep bass, especially in practical sized enclosures. The most common subwoofer sizes in professional concert audio are 12", 15" and 18".

All loudspeakers characteristically exhibit trade-offs between frequency response and response time (transient response) depending on the characteristics of the voice coil, properties of the magnetic motor, the cone mass, and the nature of the enclosure. While cone size and voice coil inductance are major factors, a recent study ^[3] indicates that the role of inductance is more important than was previously believed. The effects of the cone mass are not significant at sub-bass frequencies.

With the advent of the CD, the demand to reproduce deep, clean, loud bass increased, and had increased again with the introduction of digital audio Surround sound formats like Dolby Digital and DTS, which make extensive use of subwoofers for special effects. In modern music, the subwoofer 'channel' is often used aggressively by producers to enhance the listening experience.

Subwoofers are available in several formats, including bass reflex, acoustic suspension, infinite baffle, horn loaded, tapped horn and bandpass types. Each enclosure topology design has advantages and disadvantages in efficiency, bass extension, and cabinet size. Bass-reflex enclosures are the most common type in domestic hi-fi and professional sound.

The fundamental difficulty for subwoofers is to couple the motion of the cone to the listening environment, while limiting cone excursion to safe levels, cooling of the voice coil, while minimizing distortion, and maximizing output. Since the human ear is increasingly insensitive as frequencies decrease, these requirements are hard to meet simultaneously.

Active and passive

Generating high output at low bass frequencies is a demanding task, so the driver, enclosure, and amplifier are designed to work together as a package. These subwoofers with a built-in amplifier are called Active or powered subwoofers. The amplifiers in these units have built in crossover, phase, and sometimes equalizer capabilities.

Some amplifiers include an adjustable low-pass crossover, 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 crossover 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 reproducing frequencies below its capabilities. The subwoofer phase control allows a listener to adjust the phase (time) alignment of the subwoofer system relative to the main speakers. Doing so eliminates or minimizes the acoustic cancellation between the subwoofer system and main speakers, the result of them not being in phase within the crossover region as heard at the listener's ear. Phase settings may be a two-position switch (0° or 180°) or a continuous adjustment between these two values.

Some subwoofers are built with their own internal amplifier. Designers of this type can add a degree of corrective equalization to ensure optimum performance. Some also include a user-adjustable equalizer that allows boost or cut output at certain frequencies, and these vary from a simple "boost" switch, to elegant parametric equalizers for detailed speaker and room correction. Some 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.

Passive subwoofers have a subwoofer driver and enclosure, but do not include an amplifier. They sometime include passive crossovers inside, with the filter frequency determined by 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.

Servo subwoofers

Some active subwoofers are also servo controlled, using a feedback mechanism which generates a signal based on the movement of the cone. The signal is used to correct for any deviations from the motion intended. Several manufacturers make servo subwoofer systems, usually accelerometer-based. The advantage of servo subwoofers is reduced distortion. The primary disadvantages are cost and complexity. A successfully implemented servo system addresses the limitations of conventional subwoofers:

• mechanical non-linearities
• thermal non-linearities
• magnetic non-linearities

Reviews of servo subwoofers

• Martin Logan Descent
• Velodyne DD
• Paradigm Servo-15
• Rythmik Direct Servo kit

Servodrive subwoofers

Note that servo controlled subwoofers are not the same as Servodrive subwoofers whose primary mechanism of sound reproduction drops the normal voice coil/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.^[4][5][6]

Applications

Professional audio

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 15", 18" or 21" drivers. Drivers as small as 10" are occasionally used, generally in horn loaded enclosures.

People accustomed to bass in home audio systems and car audio may think the subwoofers in a concert system don't have much low frequency output, since the human ear recognizes loudness (ie, sound pressure) logarithmically. Concert subs can be 10 times the power rating which equates to 10 more deciBels, but will only be twice as loud. Sound intensity obeys the inverse-square law in relation to distance from the source, and at outdoor events the crowd is many metres away from. The human ear is less sensitive to sound at lower frequencies, so very high levels are needed to hear 20 or 25Hz sounds.

The main speakers may be 'flown' from the ceiling of a venue on chain hoists, and 'flying points' (ie, attachement points) are built into many enclosures. Since subwoofers are larger, heavier, and least localizable components, they can be stacked on the ground near the stage. These enclosures may contain several 18-inch drivers, and the enclosures are typically built from Baltic Birch, sourced from Finland, Estonia or Russia. This material is void free, has many plies, and is exceptionally durable. There can be more than 50 double-18-inch cabinets in a typical concert system.

Subwoofers may or may not be fed with signal from the main program mix, instead having kick drum, bass and keyboards routed to them. This avoids the low frequency 'drone' of feedback that can occur if the vocal microphone signals are reproduced by to the subwoofer system. Some concert sound companies use subwoofer systems made by the same manufacturer of the flying speakers, but many others select subwoofers from other manufacturers, better suited to their truck-pack, cost, or other criteria.

Cinema

An early example of the use of sub-woofers in cinemas came with the release of MCA Universal's Earthquake in 1974. It used a system called 'Sensurround' (with speakers made by Cerwin-Vega) to create a sensation akin to that of an earthquake. This involved the addition of a rented set of sub-woofers, and electronics designed to generate bass noise infrasound that could be felt as well as heard. It was not popular with multiplex cinema operators, since the low frequency energy would bleed from cinema to cinema, annoying patrons watching other movies. Subwoofer systems are used in some theme park rides, such as "Days of Thunder," which uses the sound energy to simulate a physical impact.

Home and Car Audio

Most home and car systems have much smaller and unmatched drivers at low frequencies; they attempt to reproduce very low frequencies with large cone motions instead. This is less than ideal, since large excursion is associated with higher output distortion. Intermodulation distortion increases when cone motion is large, and decreases when farther from the mechanical limits of cone motion. Thus very low frequencies (especially at high levels) from systems with small bass drivers (especially when not well matched by the enclosure to the air — the typical case) are characteristically distorted. If there are high levels of harmonic distortion, listeners will still get some sense of very low frequency sound, even if it is not significantly physically present. It is a psycho-acoustic effect, and many listeners have become used to this as 'proper' low frequency performance. Systems which are designed to avoid such distortion levels will sound 'light' at low frequencies to those accustomed to relatively high distortion bass output.

Home audio

A subwoofer does not necessarily provide superior bass performance in comparison to large conventional loudspeakers on ordinary music recordings due to the lack of very low frequency content on such sources. However, in almost every music genre, there are recordings with expanded fidelity which contain substantial low frequency content that most conventional loud speakers are ill equipped to handle without the help of a subwoofer, especially at high playback levels.

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 required and using a dedicated amplifier and speaker output level and quality are desired. Thus, subwoofers may be part of a package that includes satellite speakers, may be purchased separately, or perhaps 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 resonance and reverberation 'modes'), the best position for the subwoofer is not likely to be where the "satellite" speakers are located. It has been suggested that subwoofer(s) be placed in the 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 placed in space. Harmonics at higher frequencies (reproduced by the satellites) can be, and even low frequency sound sources in the real world can be, located that way. In principle, only one subwoofer need be used since it usually can't be localized. This is true, even when using five or more satellite speakers as in surround or theater sound designs. 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 if low frequency information in one channel is out of phase with another. ("Bass management" or "Small" mode for the satellite speakers is common among equipment such as Dolby Digital surround processors/receivers for those without an installed subwoofer.)

The physically separate subwoofer/satellite arrangement has been popularized by low-cost "home theater in a box" systems, and multimedia speakers, examples of which include the Klipsch ProMedia. One of the earliest (ca 1971) such systems was produced by Infinity, with large electrostatic panels handling the mid and high frequencies and an ottoman sized subwoofer handling low frequencies.

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 really 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 somewhat uses 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) used in many computer and home theater systems; the localization effect can be reduced by lower crossover point and steeper slopes.

High-quality domestic subwoofers are manufactured by companies such as Martin-Logan, Polk Audio, Bowers and Wilkins, Sunfire, M&K, HSU Research, Velodyne, SVS, Wharfedale, and REL, as well as many mainstream loudspeaker manufacturers. These can be purchased separately to be added to an existing system, or when considering a set of speakers. Or even as part of a high-end speaker package. Many domestic subwoofers have in-built amplification, and are referred to as "active subwoofers." Home subwoofers sold individually usually also include crossover 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's not possible, in most circumstances, to fit such drivers and enclosures into doors or dashboards. In addition, there are acoustic problems in most car interiors (e.g., too reflective, too small, too noisy at many frequencies, ...). Typically, subwoofers are installed in the trunk or back seat space.

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.

Even normal consumer-grade subwoofers are often capable of generating exceptionally high levels in cars due to the small volume of the typical car interior. High sound levels can cause hearing loss and tinnitus if one is exposed to them for an extended period of time.

Since much 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 or car seat, and the vibrations of the driver are transmitted to the body then to the ear via bone conduction. They connect to an amplifier like a normal subwoofer. They can be attached to a large flat surface (for instance a floor or window) to create a large low frequency conduction area, though 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 low frequency waves from a subwoofer monitor. 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.

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. There is no audible advantage to these shapes, and they may carry some distortion penalties. They can, in some situations (limited mounting space, for instance) give a slightly greater cone area and so slightly higher output at low frequencies. The intent of non-round subwoofers is to increase the surface area of the cone. In enclosures of a particular size, more subwoofer cone area is possible with some of these shapes, although the difference may be trivial. The increased cone area moves more air, producing higher sound pressure levels. However, these drivers are not optimized for maximum output level, so the advantage is largely theoretical. The effect of corners in their surrounds is to increase distortion.

An important factor in the "square sub vs round sub" argument is the effects of the enclosure used. In a sealed enclosure, all displacement is determined by



where V_d stands for volume of displacement, x_max to the amount of linear excursion the sub is mechanically capable of (in mm), and S_d to the cone area of the subwoofer (in ). These are some of the Thiele/Small parameters which can often be found in the owner's manual or driver specifications. Because of the multiplication, excursion is as important as cone area to generate SPL. However, the importance of these parameters shifts if the subwoofer is used in a ported enclosure. When a subwoofer produces frequencies near the tuning frequency of the ported enclosure, excursion decreases and cone area becomes more important.

Stereo separation

Stereo separation cannot be heard in very low-frequency sound coming from a speaker like a subwoofer; hence many audio systems feature only one subwoofer. The subwoofer can be placed off-center without affecting the perceived sound stage, since the sounds it makes can't be localized.

This omnidirectionality is often misdescribed as an inability of human hearing to perceive stereo separation at low frequencies. Actually, human ears and brain can perceive stereo separation at any audible frequency, provided the two ears receive different sound levels at that frequency. In practice however, typical subwoofer frequencies can be delivered differently to each ear only via headphones or earphones because their wavelengths are so long. When from speakers or other relatively distant sources, low-frequency (ie, large wavelength) sound waves flows smoothly around the human head, reaching both ears with equal strength at the same time regardless of the direction in which the wave is propagating. Higher frequency (small wavelength) sound waves are physically blocked by the size of the human head, thus allowing one ear to receive the wave more strongly than the other. As well, there are phase differences which are used for localization at higher frequencies. These are unavailable at subwoofer frequencies.

References

See also

• Mid-range speaker
• Tweeter
• Woofer
• Rotary Woofer
• Thiele/Small

External links

• Subwoofer Tests The Infamous Subwoofer Tests performed by Ilkka Rissanen... complete with 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, unbiased subwoofer testing
• Setting up a Subwoofer for Home Theater - detailed guide on Big Picture Big Sound
• The Subwoofer DIY Page Build your own subwoofer!
• 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 comprehensive sub woofer design site
• Loudspeaker Components, LLC manufacturer of speaker cones, dust caps and spiders for subwoofers as well