Subwoofer
A subwoofer (or sub) is a loudspeaker designed to reproduce low-pitched audio frequencies, known as bass and sub-bass, that are lower in frequency than those which can be (optimally) generated by a woofer. The typical frequency range that is covered by a subwoofer is about 20–200 Hz for consumer products,[1] below 100 Hz for professional live sound,[2] and below 80 Hz in THX-certified systems.[3] Thus, one or more subwoofers are important for high-quality sound reproduction as they are responsible for the lowest two to three octaves of the ten octaves that are audible. This very low-frequency (VLF) range reproduces the natural fundamental tones of the bass drum, electric bass, double bass, grand piano, contrabassoon, tuba, in addition to thunder, gunshots, explosions, etc.
Subwoofers are never used alone, as they are intended to substitute the VLF sounds of "main" loudspeakers that cover the higher frequency bands. VLF and higher-frequency signals are sent separately to the subwoofer(s) and the mains by a "crossover" network, typically using active electronics, including digital signal processing (DSP). Additionally, subwoofers are fed their own low-frequency effects (LFE) signals that are reproduced at 10 dB higher than standard peak level.[4]
Subwoofers can be positioned more favorably than the main speakers' woofers in the typical listening room acoustic, as the very low frequencies they reproduce are nearly omnidirectional and their direction largely indiscernible. However, much digitally recorded content contains lifelike binaural cues that human hearing may be able to detect in the VLF range, reproduced by a stereo crossover and two or more subwoofers.[5] Subwoofers are not acceptable to all audiophiles, likely due to distortion artifacts produced by the subwoofer driver after the crossover and at frequencies above the crossover.[6]
While the term "subwoofer" technically only refers to the speaker driver, in common parlance, the term often refers to a subwoofer driver mounted in a speaker enclosure (cabinet), often with a built-in amplifier.
Subwoofers are made up of one or more woofers mounted in a loudspeaker enclosure—often made of wood—capable of withstanding air pressure while resisting deformation. Subwoofer enclosures come in a variety of designs, including bass reflex (with a port or vent), using a subwoofer and one or more passive radiator speakers in the enclosure, acoustic suspension (sealed enclosure), infinite baffle, horn-loaded, tapped horn, transmission line, bandpass or isobaric designs. Each design has unique trade-offs with respect to efficiency, low-frequency range, loudness, cabinet size, and cost. Passive subwoofers have a subwoofer driver and enclosure, but they are powered by an external amplifier. Active subwoofers include a built-in amplifier.[7]
The first home audio 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 and loud bass was no longer limited by the ability of a phonograph record stylus to track a groove,[8] 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-cinema (also called 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.
Unlike a system's main loudspeakers, subwoofers can be positioned more optimally in a listening room's acoustic. However, subwoofers are not universally accepted by audiophiles amid complaints of the difficulty of "splicing" the sound with that of the main speakers around the crossover frequency. This is largely due to the subwoofer driver's non-linearity producing harmonic and intermodulation distortion products well above the crossover frequency, and into the range where human hearing can "localize" them, wrecking the stereo "image".
Applications[edit]
Home audio[edit]
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 organs with 32' (9.75 meter) bass pipes (16 Hz), very large bass drums on symphony orchestra recordings and electronic music with extremely low synth bass parts, such as bass tests or bass songs.
Frequencies which are sufficiently low are not easily localized by humans, 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 that it produces will be difficult to localize. The intention in a system with a subwoofer is often to use small main 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. This effect is possible only if the subwoofer is restricted to quite low frequencies, usually taken to be, say, 100 Hz and below—still less localization is possible if restricted to even lower maximum frequencies. Higher upper limits for the subwoofer (e.g. 125 Hz) are much more easily localized, making a single subwoofer impractical. Home-cinema systems typically use one subwoofer cabinet (the "1" in 5.1 surround sound). However, to "improve bass distribution in a room that has multiple seating locations, and prevent nulls with weakened bass response, some home-cinema enthusiasts use 5.2- or 7.2-channel surround sound systems with two subwoofer cabinets in the same room.[82]
Some users add a subwoofer because high levels of low-frequency bass are desired, even beyond what is in the original recording, as in the case of house music 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-bass 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.
Higher end home-cinema systems and enthusiasts may also opt to take low-frequency bass reproduction even further by incorporating two or more external subwoofers.[83] Having two subwoofers placed around the room ensures even distribution of bass, reducing subwoofer localization and pressurizing the room with low frequency notes that can be felt, just like the cinemas.[84]
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 wavelength; 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 is possible to have some cancellation of low frequencies if bass information in one channel's speaker is out of phase with another.
The physically separate subwoofer/satellite arrangement, with small satellite speakers and a large subwoofer cabinet that can be hidden behind furniture, has been popularized by multimedia speaker systems such as Bose Acoustimass Home Entertainment Systems, Polk Audio RM2008 Series and Klipsch Audio Technologies ProMedia, among many others.[85][86]
Bass shakers[edit]
Since infrasonic bass is felt, sub-bass can be augmented using tactile transducers. Unlike a typical subwoofer driver, which produces audible vibrations, tactile transducers produce low-frequency vibrations that are designed to be felt by individuals who are touching the transducer or indirectly through a piece of furniture or a wooden floor. Tactile transducers 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-cinema seating, and the vibrations of the driver are transmitted to the body then to the ear in a manner similar to bone conduction.[119][120] 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 is not as efficient as through the seat.[121]
The advantage of tactile transducers used for low frequencies is that they allow a listening environment that is not filled with loud low-frequency sound waves in the air. This helps the drummer in a rock music band to monitor their kick drum performance without filling the stage with powerful, loud low-frequency sound from a 15-inch (40 cm) subwoofer monitor and an amplifier, which can "leak" into other drum mics and lower the quality of the sound mix. By not having a large, powerful subwoofer monitor, a bass shaker also enables a drummer to lower the sound pressure levels that they are exposed to during a performance, reducing the risk of hearing damage. For home cinema or video game 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 video game 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.[122]
