Subwoofer Totally Explained

Everything about Subwoofer totally explained

A subwoofer is a woofer, or a complete loudspeaker dedicated to the reproduction of bass audio frequencies, typically from 150 Hz down to 20 Hz. Subwoofers, also known as "subs", are used to augment the low frequency playback of main loudspeakers. Subwoofers are constructed by mounting one or more woofers) typically between 8" and 21" in diameter in a well-braced wood or plastic cabinet. Passive subwoofers have a subwoofer driver and enclosure; active subwoofers have a built-in amplifier.
The first subwoofer was developed in the 1960s by the owner of a high-end audio store in Los Angeles whose customers were complaining about a lack of bass response in the store's electrostatic speakers. Subwoofers came into greater popular consciousness in 1974 with the movie Earthquake which used six large subwoofers driven by a pair of 1600-watt amplifiers to recreate the rumbling of a powerful earthquake. With the advent of the compact cassette and consumer digital audio formats in the late 1980s, it became possible to add more low frequency content to recordings. As a result, in the 1990s and 2000s, subwoofers became increasingly popular in home stereo systems, home theater set ups, and car audio. By the 2000s, subwoofers became almost universal in professional sound applications such as live concert sound, churches, nightclubs, and theme parks.

History

The first subwoofer was developed during the 1960s by Ken Kreisel, former president of the now defunct Miller & Kreisel Sound Corporation in Los Angeles. Kreisel's business partner, Jonas Miller, owned a high-end audio store in Los Angeles, and customers buying some of the high end electrostatic speakers complained about a lack of bass response in the electrostatics, compared to conventional loudspeakers; Kreisel's solution was to design a powered loudspeaker that would reproduce only those frequencies that were too low for the electrostatic speakers to convey and thereby fill in the missing sonic information.. 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 didn't handle adequately.
   The first use of a subwoofer in a recording session was in 1973 for mixing the Steely Dan album Pretzel Logic when recording engineer Roger Nichols arranged for Kreisel to bring a prototype of his subwoofer to Village Recorders. Further design modifications were made by Kreisel over the next ten years, and in the 1970s and 1980s by engineer John P. D'Arcy; record producer Daniel Levitin served as a consultant and "golden ears" for the design of the crossover network (used to partition the frequency spectrum so that the subwoofer wouldn't attempt to reproduce frequencies too high for its effective range, and so that the main speakers wouldn't need to handle frequencies too low for their effective range).
   Subwoofers came into greater popular consciousness in 1974 with the movie Earthquake which was released in Sensurround. Sensurround was initially installed in 17 U.S. theaters. Six very large subwoofers were driven by a pair of 1600 watt amplifiers that were triggered by control tones printed on one of the audio tracks on the film. Four of the subwoofers were positioned in front of the audience under (or behind) the film screen and two more were placed together at the rear of the audience on a platform. Noise energy in the range of 5 Hz to 40 Hz was generated at the level of 110-120 dB(SPL). Much publicity was given to the new low frequency entertainment method and the film was a box office success. More Sensurround systems were assembled and installed. By 1976 there were almost 300 Sensurround systems leapfrogging through select theaters. Further films to use the effect include Midway in 1976 and Rollercoaster in 1977.
   With the advent of the compact cassette and consumer digital audio formats, the reproduction of deep, loud bass was no longer limited by the ability of a phonograph record stylus to track a groove. 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 drivers (woofers) typically between 8" and 21" in diameter, but single prototypes as large as 60" have been fabricated, and drivers as small as 4" may be used, depending on the design of the loudspeaker enclosure, the desired sound pressure level, the lowest frequency targeted and the amount of distortion that's allowable. The most common subwoofer driver sizes in professional audio are 10", 12", 15" and 18".
The efficiency of a speaker driver is given by:
ť

eta_0 = left(frac

where

V_d stands for volume of displacement (in m^3)
x_max to the amount of linear excursion the speaker is mechanically capable of (in m)
S_d to the cone area of the sub woofer (in $m^2$ ). These are some of the Thiele/Small parameters which can either be measured or found with the driver specifications.

Cinema sound

After the introduction of Sensurround, movie theater owners began installing permanent subwoofer systems. Dolby Stereo 70 mm Six Track was a six channel film sound format introduced in 1976 that used two subwoofer channels for stereo reproduction of low frequencies. In 1981, Altec introduced a dedicated cinema subwoofer model tuned to 20 Hz: the 8182. Starting in 1983, THX certification of the cinema sound experience quantified the parameters of good audio for watching films, including the requirement of subwoofer loudspeakers and enough isolation from outside sounds so that noise doesn't 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 JBL, Electro-Voice, Eastern Acoustic Works, Kintek, Meyer Sound Laboratories and BGW Systems in the early 1990s. In 1992, Dolby Digital's six-channel film sound format incorporated a single low-frequency effects (LFE) channel, the "point one" in 5.1 surround sound.
Tom Horral, a Boston-based acoustician, blames subwoofers for louder cinema sound in general. He says that before subwoofers made it possible to have loud, relatively undistorted bass, movie sound levels were limited by the lesser capabilities of full-range loudspeakers.

Concert sound

Professional audio subwoofers must be capable of very high output levels. This is reflected in the design attention given in recent years to the subwoofer applications for sound reinforcement, public address, and concert systems. Consumer applications (as in home use) are considerably less demanding due to much smaller listening space and lower playback levels. Subwoofers are now almost universal in professional sound applications such as live concert sound, churches, nightclubs, and theme parks. Specifically, movie theatres certified to the THX standard for playback always include high capability subwoofers. Some professional applications require subwoofers designed for very high sound levels, using multiple 12", 15", 18" or 21" drivers. Drivers as small as 10" are occasionally used, generally in horn loaded enclosures.
The main speakers may be 'flown' from the ceiling of a venue on chain hoists, and 'flying points' (ie, attachment points) are built into many professional loudspeaker enclosures. Subwoofers can be flown or stacked on the ground near the stage. There can be more than 50 double-18-inch cabinets in a typical concert system.
Just as consumer subwoofers can be made of Medium-density fibreboard (MDF), Oriented strand board (OSB), plywood, plastic or other dense material, professional subwoofer enclosures can be built from the same materials. MDF is commonly used to construct subwoofers for permanent installations as its density is very high and weatherproofing isn't 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 bedliner to give the subwoofers 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 crossover. Such a system receives its signal from the main mono or stereo mixing console mix bus and amplifies all frequencies together in the desired balance. If the main sound system is stereo, the subwoofers can also be in stereo. Otherwise, a mono subwoofer channel can be derived within the crossover from a stereo mix, depending on the crossover make and model.

Aux-fed subwoofers

Instead of being incorporated into a full-range system, concert subwoofers can be supplied with their own signal from a separate mix bus on the mixing console; often one of the auxiliary sends is used. This configuration is called "aux-fed subwoofers", and has been observed to significantly reduce low frequency "muddiness" that can build up in a concert sound system which has on stage a number of microphones each picking up low frequencies and each having different phase relationships of those low frequencies. The aux-fed subs method greatly reduces the number of sources feeding the subwoofers to include only those instruments that have desired low frequency information; sources such as kick drum, bass guitar, samplers and keys. This simplifies the signal sent to the subwoofers and makes for greater clarity and low punch. 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 flying the subwoofers in a vertical array focuses the low frequencies forward to a greater or lesser extent depending on the physical length of the array. Longer arrays have more directional effect at lower frequencies. The directionality is more pronounced in the vertical dimension, yielding a radiation pattern that's 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 around 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 conduction. 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 they're exposed to during a performance. Bass shakers help the user avoid disturbing others nearby and help to augment the headphone listening experience. However, some feel that the felt vibrations are disconnected from the auditory experience, that music is less satisfying than sound effects and that the bass shaker itself can rattle and distract the listener.

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