What is a subwoofer and its function in a 'surround' system?

(Koushik Subramaniam, Coimbatore, Tamil Nadu)

A subwoofer (or ‘sub') is a woofer, or a complete loudspeaker typically between 8 inches and 21 inches in diameter, which is dedicated to the reproduction of low-pitched audio frequencies (the bass).

The typical frequency range for a subwoofer is about 35–200Hz for consumer products, below 100Hz for professional live sound, and below 80Hz in THX-approved systems.

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.

Subwoofers use speaker drivers (woofers) typically between 8 inches and 21 inches in diameter. Some car audio subwoofers have a 22 inches diameter and single prototype subwoofers as large as 60 inches have been fabricated. On the smaller end of the spectrum, subwoofer drivers as small as 4 inches 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.