A subwoofer (or "sub") is a woofer, or a complete loudspeaker typically between 8" and 21" in diameter, which is dedicated to the reproduction of low-pitched audio frequencies (the "bass"). The typical range for a subwoofer is about 20–200 Hz. Subs are used to augment the performance of main loudspeakers. Subwoofers are constructed by mounting one or more woofers in a well-braced wood or plastic enclosure. Subwoofers have been designed using a number of speaker enclosure designs, including bass reflex (with a port or tube in the enclosure), infinite baffle, horn-loaded, and bandpass designs, each of which has advantages and disadvantages in efficiency, size, distortion, cost, and power handling. 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 Sensurround in movies such as Earthquake , which produced loud low-frequency sounds through large subwoofers. With the advent of the compact cassette and the compact disc 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 sound" processes that included a Low Frequency Effects (LFE) channel, which could be heard using the subwoofer in home theater systems. During the 1990s, subwoofers also became increasingly popular in home stereo systems, custom car audio installations, and in PA systems. By the 2000s, subwoofers became almost universal in sound reinforcement systems 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 Angeles. When Kreisel's business partner, Jonas Miller, who owned a high-end audio store in Los Angeles, told Kreisel that some purchasers of the store's high-end electrostatic 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 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 would not attempt to reproduce frequencies too high for its effective range, and so that the main speakers would not need to handle frequencies too low for their effective range).
Subwoofers received a great deal of publicity in 1974 with the movie Earthquake which was released in Sensurround. Initially installed in 17 U.S. theaters, the Sensurround system used large subwoofers which were driven by racks of 500 watt amplifiers which were triggered by control tones printed on one of the audio tracks on the film. Four of the subwoofers were positioned in front of the audience under (or behind) the film screen and two more were placed together at the rear of the audience on a platform. Powerful noise energy in the range of 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 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
Loudspeaker and enclosure design
Subwoofers use speaker drivers (woofers) typically between 8" and 21" in diameter. Some car audio subwoofers have a 22" diameter and single prototype subwoofers as large as 60" have been fabricated. On the smaller end of the spectrum, subwoofer drivers as small as 4" may be used, depending on the design of the loudspeaker enclosure, the desired sound pressure level, the lowest frequency targeted and the level of permitted distortion. The most common subwoofer driver sizes in professional audio are 10", 12", 15" and 18". The largest sound reinforcement subwoofers, 21" drivers, are used for the largest venues, such as stadiums.
The efficiency of a speaker driver is given by:
Where the variables are Thiele/Small 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:
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 reflex 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 reflex, acoustic suspension, infinite baffle, horn loaded, tapped horn, transmission line and bandpass. Each enclosure type has advantages and disadvantages in efficiency increase, bass extension, cabinet size, distortion, and cost. Subwoofers are typically constructed by mounting one or more woofers in a cabinet of medium-density fibreboard (MDF), oriented strand board (OSB), plywood, plastic or other dense materials. Because of the high power they use, subwoofers often require strong internal crossbracing to add strength and reduce box resonances.
There is a great deal of variety in the size of enclosures and, in the case of bass reflex systems, vent designs. When two are more subwoofers are placed in the same enclosure, they work together to move a greater mass of air, resulting in lower frequency extension. For example, if a single 12" subwoofer enclosure can go down to 40 Hz, a larger enclosure with four of these 12" drivers may be able to go to 30 Hz.
The smallest subwoofers are typically those designed for home theater users with limited space. An example of a very small home theater sub is the Velodyne MicroVee, which puts a 6.5" driver, two 6.5" passive radiators and an internal 1000 W RMS amplifier into a 9" x 9" x 9.6" (HxWxD) enclosure. The largest common subwoofer enclosures are those used for concert sound reinforcement systems or nightclub sound systems. An example of a large concert subwoofer enclosure is the 1980s-era ElectroVoice MT-4 "Bass Cube" system, which used four 18" drivers. An example of a subwoofer that uses a bass horn is the Bassmaxx B-Two, which loads an 18" driver onto an 11-foot (3.35 m) long folded horn. Folded horn-type subwoofers can typically produce a deeper range with greater efficiency than the same driver in an enclosure that lacks a horn. Some experimental fixed-installation subwoofer horns have been constructed using brick and concrete to produce a very long horn that allows a very deep sub-bass extension.
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
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