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".