Subwoofers are useful for augmenting main loudspeakers by providing playback of low bass frequencies at high levels. Subwoofers are available in several formats, including bass reflex, acoustic suspension, infinite baffle, horn loaded, tapped horn and bandpass types. The range of sounds from lowest to highest that the subwoofer is capable of reproducing is called frequency range. The actual frequency range can vary depending upon the type of box, crossover point, and the vehicle the subwoofer is installed in.

All loudspeakers characteristically exhibit trade offs between frequency response and response time or transient response depending on the characteristics of the voice coil, properties of the magnetic motor, the cone mass, and the nature of the enclosure.

The voice coil consists of a bobbin with wire wound around it. The voice coil is attached to the bottom of the cone and extends into the air gap surrounding the pole piece. The voice coil translates the magnet's energy into cone motion. Cone size and voice coil inductance are major factors for a subwoofer.

Subwoofers use drivers woofers typically between 8" and 18" in diameter, but some have been as large as 60". Some drivers as small as 4" may be referred to as subwoofers but drivers less than 8" generally have neither the cone area nor excursion ability necessary to produce deep bass, especially in practical sized enclosures. The most common subwoofer sizes in professional concert audio are 12", 15" and 18".

A brief history

The first subwoofer was developed during the 1960s by Ken Kreisel. Kreisel's solution was to design a powered loudspeaker that would reproduce only those frequencies that were too low for the electrostatic speakers to convey and thereby fill in the missing sonic information. Infinity's full range electrostatic speaker system of about the same time also used a subwoofer to cover the lower frequency range the electrostatic arrays did not handle adequately. Subwoofers came into greater popular attention in 1974.

Applications of subwoofers

Subwoofers are used in various systems. The main areas of application include professional audio, home audio, car audio and many other fields. Subwoofers are now almost universal in professional sound applications such as live concert sound, churches, nightclubs, and theme parks. Professional audio subwoofers must be capable of very high output levels. This is reflected in the design attention given in recent years to the subwoofer applications for sound reinforcement, public address, and concert systems.

A subwoofer does not necessarily provide superior bass performance in comparison to large conventional loudspeakers on ordinary music recordings due to the lack of very low frequency content on such sources. Most home and car systems have much smaller and unmatched drivers at low frequencies; they attempt to reproduce very low frequencies with large cone motions.