Special form of speaker used to reproduce only the lower portion of the audible frequency spectrum usually from 80 Hz down to or below 20 Hz. True subwoofers should be able to play useful audible information down close to 20 Hz (the lower limit of human hearing). Most subwoofers feature one or more speaker drivers measuring ten or more inches in diameter (with powerful subwoofers often using drivers of a minimum twelve inches in diameter). here are two primary types of subwoofer – powered and non-powered. Powered subwoofers feature their own built-in amplifier while non-powered models need an outboard amplifier (or may be connected to the amplifier used for the main stereo speakers using a crossover). Powered subwoofers are by far the most popular and most versatile models. ost subwoofers contain some type of passive or active crossover network to separate the low bass signals (usually below 80 Hz) from the rest of the frequency spectrum. The subwoofer then reproduces the low frequency signals and sends the remainder on to the other speakers in an audio system. ithin the realm of subwoofers, there are a number of cabinet designs used including bass reflex or ported enclosures, sealed or acoustic suspension enclosures, isobaric enclosures, and bandpass enclosures. Of these, the two most popular for home audio are the sealed and ported enclosures. ealed enclosure subwoofers feature the lowest extension (play the lowest into the frequency spectrum) and best sound quality, but they are also the least efficient needing additional power to reach the same output levels (volume) as the other designs. Ported subwoofers, on the other hand, use a port (a tube of a particular length allowing air to pass from the inside of the enclosure to the outside through a round hole) to increase efficiency and add around three decibels more output for a given power input versus a sealed enclosure while giving up some ultimate low frequency extension. Both ported and sealed box subwoofers can provide powerful, engaging sound with excellent quality when properly designed and set up. ubwoofers need a large amount of power in order to optimally reproduce low frequency sounds. Low frequency sound waves are very large (about 56 feet long for a single cycle at 20 Hz) and thus require large drivers and large amounts of power to faithfully reproduce them. Most subwoofers feature long-excursion drivers meaning that the speaker drivers travel a long distance back and forth to generate sound waves (a long distance, of course, is relative with most long-throw driver moving up to one inch or so). As drivers become larger and they travel farther, more and more power is required to move them and provide adequate control (damping). Most powered subwoofers, thus, feature amplifiers of 200 watts or more, although some subs use amplifiers of 100 watts and others use more than 1,000 watts. ue to the nature of low frequency sound waves, subwoofers do not need to be used in pairs or located in a specific location relative to the other speakers in a room. Low frequency sound waves are omnidirectional and the human ear cannot locate the origin of sound waves below 80 Hz. This attribute allows subwoofers to be placed anywhere they sound best and allows the use of single subwoofers. Multiple subwoofers provide benefits of increased low frequency power and extension while helping to smooth out frequency response in a given room making two or more subwoofers desirable if possible but certainly not necessary in most cases. ome subwoofers make use of a servo feedback loop allowing the subwoofer to correct its driver motion and reduce distortion. The servo circuit monitors the movement of a subwoofer’s speaker driver and compares it to the input signal (what the subwoofer is supposed to output). If the two do not match, there is distortion. The servo circuit watches for such distortions and then adjusts the amplifier’s output to correct the speaker driver’s motion resulting in reduced distortion. ubwoofers are becoming increasingly important and useful in home audio and home theater. They produce the lowest frequencies with weight and accuracy allowing the other speakers to concentrate on frequencies above 80 Hz or so. Typically this allows the other speakers to provide better sound quality more effortlessly since less of their input power is needed to produce low-frequencies. Additionally, the 5.1 digital surround sound systems allocate a specific low frequency effects channel for subwoofers. Subwoofers produce the lowest frequencies, those below 80 Hz or so, with the greatest detail, accuracy, ease and power adding visceral impact and oomph to almost any audio system.