Subwoofer

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

A subwoofer is a type of loudspeaker dedicated to the reproduction of bass frequencies, typically from about 20 Hz to about 200 Hz. It is difficult for small loudspeakers to reproduce frequencies below 40 Hz, especially above 100 dB, and so it is often advantageous to use a loudspeaker intended specifically for this task.

History

The first commercial subwoofer was developed during the 1970s by Ken Kreisel, current president of M&K Sound/Miller & Kreisel Corporation in Los Angeles. Kreisel's business partner, Jonas Miller, owned a high-end audio store in Los Angeles, and customers of some of the highest quality electrostatic speakers complained about a reduction 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. The first known use of a subwoofer in a recording session was for the mixing of 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 ears" for the design of the crossover network (used to partition the frequency spectrum so that the subwoofer would not attempt to reproduce frequencies that were too high for its effective range, and so that the main speakers would not need to process frequencies that were too low for their effective range).

Overview

Subwoofers use drivers (woofer) with cones typically coming in 10" or 12" sizes, but can be as large as 34", and as small as 4".

Larger diameters tend to be advantageous because low frequencies involve moving a great deal of air. A recent trend has been for high excursion. Excursion is defined as how far the cone can linearly travel from its resting position. Some newer models can move as much as +/-2.5" , yielding an overall controlled displacement of 5" with the voice coil of the driver staying inside the magnetic field.

Subwoofers are usually powered by a high power amplifier, and often an electronic crossover with a Low-pass filter is used to ensure that higher frequencies will not be directed to the subwoofer.

The need to reproduce these frequencies has increased since older formats, such as vinyl records, have been replaced by digital formats, such as CDs, and particularly 5.1 formats such as Dolby Digital and DTS, in which the ".1" channel is dedicated solely to the subwoofer. The .1 channel is usually dedicated to extended bass frequencies, for example, the low frequencies of a gunshot, double bass, or thunder. This track is often used aggressively by mixing artists.

Applications

Professional audio

Subwoofers are found in professional applications such as live concerts, movie theatres, various other sound reinforcement applications (ranging from nightclubs to theme restaurants) and studios. Some of these applications require subwoofers designed for very high sound levels, such as the JBL 4645 or the Electro-Voice TL440 which use a larger than average 18" driver and are certified for use in THX movie theatres. Most movie theatre speakers (situated behind a perforated screen) typically use 15" drivers, so the larger subwoofers are used only to reproduce the lowest frequencies at high sound pressure levels.

Large concert sound systems always use subwoofers (referred to as "subs" by the engineers and crew). The bulk of the sound system is usually "flown" (suspended from the ceiling by chain hoists) and the subs are usually stacked on the stage or the ground in front of the stage to the left and right of the performance space.

An unusual example of the use of sub-woofers came with the release of Earthquake in 1974 where they used a system called Sensurround to create a feeling of an earthquake. This was simply a set of large sub-woofers designed to put out infrasound(felt but not heard). Similar systems are used in theme park rides, such as "Days of Thunder," which uses sound to simulate a physical impact.

Many times the subs are not part of the entire sound mix but are specifically fed just kick drum, bass guitar and other low-frequency content from a separate output on the main mixing console. Popular sub systems in use currently are made by companies such as EAW but usually the subs will be made by the manufacturer of the rest of the PA system.

The 18-inch woofer driver is the primary majority device for pro audio applications. They are usually direct radiating in a ported enclosure built of 13-ply birch. For electronic music events with at least a thousand audience members there are often more than 20 double-18-inch cabinets on either side of the stage. 12-inch drivers in very large folded horns are also becoming popular now. One of the most powerful subs manufactured can play as low as 25 Hz and can cover thousands of feet and uses two 12-inch woofers on a 13-foot (4 m) long folded horn .

Pro Audio subs have to be capable of very high output levels – after all, concert venues may seat 10,000s of individuals outdoors. On average, music applications generally require less capability than movie soundtracks in the very lowest octave, but modern popular music is changing this preconception and this is reflected in the design attention given to the subwoofer section of the PA system nowadays compared to a few decades ago. People who are accustomed to bass in home audio systems and car audio many times think that the subs in a concert PA system aren't putting out that much. As sound pressure is measured in decibels which are a logarithmic scale PA subs can be 10 times more powerful yet only measure a 2 more decibels. Also sound intensity obeys the inverse-square law in relation to distance from the sub and at outdoor events the crowd are many meters away from the PA equipment.

Home audio

Subwoofer mounted in a sealed enclosure

It should be noted that a subwoofer does not necessarily provide superior bass performance to large conventional loudspeakers. Subwoofers merely ought to reproduce only the lowest frequencies. A conventional woofer may reproduce frequencies up to 200, 300, or in a two-way speaker a mid-woofer (paired with a tweeter) also handles midrange, up to 3,000 hertz or more.

Rather, the intention may be to use small main ("satellite") speakers (of which there are 2 for stereo, and 5 or more for surround sound) and locate ("hide") the subwoofer elsewhere; to augment an existent speaker to relieve it of reproducing bass and gain output level and/or quality; or because high levels of low bass are required and using a dedicated amplifier and speaker provides the output level and quality required. Thus, subwoofers may be part of a package that includes satellite speakers, purchased separately, or built into the cabinet of a conventional loudspeaker. (e.g., some speakers include a subwoofer in the lower portion of the cabinet.)

Physical separation of subwoofer and "satellite" speakers not only enables placement in an inconspicuous location, but since sub-bass frequencies are particularly sensitive to location (e.g., due to room resonance and 'modes'), the best position of the subwoofer may not be where the "satellite" speakers are located. (e.g., it has been suggested 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 our left and right ears, and we are unable to easily locate their direction below 60 Hz. Harmonics at higher frequencies (from the satellites) can then be used, by the auditory system, to calculate the directional information. Note that only one subwoofer need be used, even when using 5 or more satellite speakers—e.g., for surround sound. All low frequency information is sent to the subwoofer. ("Bass management" or "Small" mode for the satellite speakers is common among equipment such as Dolby Digital surround processors/receivers.)

The physically separate subwoofer/satellite arrangement has been popularised by lifestyle systems, such as those manufactured by Bose Corporation, and multimedia speakers, examples of which include the Klipsch ProMedia. Particularly among low cost systems, however, it may be little more than a marketing device: it's not likely that a small woofer in a compact cabinet will have better bass performance than good speakers; as mentioned, the term "subwoofer" is no guarantee of particular bass performance. Most multimedia "subwoofers" should probably have been called "woofers". They are too weak to play any real deep bass. Further, small satellites systems typically cross over the bass above 120 Hz, introducing the subwoofer "localization effect". On the other hand, high-end domestic subwoofers are manufactured by companies such as Martin-Logan, M&K, HSU, Velodyne, SVS, and REL. These can be purchased separately, to be added to an existent system or when considering a set of speakers; or as part of a high-end speaker package. All of these subwoofers tend to have in-built amplification.

Car audio

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Profile view of an Alpine subwoofer.

The automobile is ideal for the "hidden" subwoofer approach due to space limitations of locations such as doors (ignoring the acoustic problems of a car interior). Typically, subwoofers are installed in the trunk. Curiously, some car stereo enthusiasts seem intent on producing ultra-high sound pressure levels in the confines of their vehicle's cabin. Such high sound levels can cause severe hearing loss if the subject is exposed to them for an extended period of time. Such "SPL wars" have drawn much attention to subwoofers in general, but subjective competitions in sound quality ("SQ") have not gained similar popularity. This can be compared to computer overclockers competing to achieve the highest CPU frequency, amateur drag racers trying to achieve the fastest quarter-mile time, etc. Generally it is clear that a "bigger, better, faster" mindset characterises this scene.

Indeed, hearing loss is one concern, alongside space considerations and neighbour relations. Since much bass is felt, sub-bass can be augmented using tactile transducers. These have recently emerged as a device, called a tactile transducer by at least one brand, that attaches to furniture, such as one's seat, via which vibrations are transmitted to the body; they can be connected to an amplifier as per a normal loudspeaker. However, some feel that the vibrations are a little disembodied from the rest of the auditory experience. These devices typically don't provide as much vibration per dollar as simply placing a reasonably priced subwoofer against the back of the furniture, so at their current prices are most useful for not disturbing anyone than vibrating to audio. They can be placed on a large flat surface to create a large, though poor quality, loudspeaker. Some waterproof models work underwater, or placed on the underside of a deck, to create sound.

Non-round subwoofers

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Profile views of a Non-round Sony Subwoofer

Within the last few years, the car audio field has seen a variety of subwoofers using non-round shapes. Stillwater Designs, nicknamed Kicker, released a square subwoofer several years ago, the Solo-Baric series, and recently, the Solo X. Bazooka has introduced a triangular subwoofer. Xtant has introduced a hexagonal subwoofer. Other companies, such as Sony, have jumped on the bandwagon, producing non-round subwoofers of their own. There is no known auditory advantage to these shapes. However, they can be useful in creating unique 'looks' in a car audio system that is meant to be seen as much as heard, and to create brand distinction and recognition.

The intent of a square subwoofer is to increase the surface area of the woofer's cone. In enclosures of the same size, it is possible to achieve more subwoofer cone surface area with a square design than a circular design. The increased surface area translates into moving more air and higher sound pressure levels. What is not considered here is the effects of excursion in the overall displacement equation. A circular speaker which undergoes larger displacement (moves forward and rearward further from rest position) than a square subwoofer may overcome the advantage in cone area that a square subwoofer holds. Furthermore, due to the nonlinear nature of all non-circular subwoofers, these subs are not capable of excurting as far as a round speaker without becoming nonlinear (introducing distortion into the sound heard). Evidence of this can easily be seen by looking at any of Kicker's square subwoofer designs: they have attempted to aid in cone flexing at the corners by physically adding more material at the corners, helping to decrease non-linearities. However, a triangular design decreases the surface area of the woofer cone relative to a circular or square design in an enclosure of the same size. The intent of the triangular design does not seem to be motivated by sound quality or intensity, but rather to encourage creativity in installation and provide flexibility for otherwise awkward positions. Sony claims that its pentagon shape reduces standing waves on the surface of the cone as well as improves the overall strength of the cone structure, reducing "cone flex".

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

$Vd=2\times xmax\times sd$

where Vd stands for volume of displacement. xmax, which refers to the amount of linear excursion the sub is capable of measured in mm, and sd, which refers to the cone area of the subwoofer measured in $cm^{2}$. These are some Thiele/Small parameters that can often be found in the owners manual produced by most manufacturers. As you can see, excursion is every bit as important as cone area. However, the importance of these parameters does shift slightly if the subwoofer is used in a ported enclosure. When a subwoofer plays frequencies near the tuning frequency of the ported enclosure, excursion decreases and cone area becomes much more important.

Stereo separation

Stereo separation (or five-channel separation) cannot be heard in very low-frequency sound coming from a speaker; hence most audio systems feature only one subwoofer. This subwoofer can be placed off-center without hurting the feeling of balance, since the sounds it makes are perceived as omnidirectional.

This omnidirectionality is often misdescribed as an inability of human hearing to perceive stereo separation at low frequencies. Actually, human hearing can perceive stereo separation at any audible frequency, provided the two ears receive differently strengthed soundwaves at that frequency. In practice however, subwoofer-range frequencies can be delivered differently to each ear only via headphones or earphones. When emanating from speakers or other relatively distant sources, low-frequency (large wavelength) soundwaves tend to flow smoothly around the human head, impacting both ears with equal strength regardless of the direction in which the wave is propagating. Higher frequency (small wavelength) soundwaves are physically blocked by the size of the human head, which allows for one ear to receive the wave more strongly than the other.

See also

• Mid-range speaker.
• Tweeter.
• Woofer.
• MTX Audio JackHammer.

External links

• AV Talk Subwoofer Tests In-depth, unbiased subwoofer testing