Subwoofers are a type of speaker, intended for converting the lowest range of frequencies of audio signals to sound and to produce audible sounds of the quality known as bass. Bass is often characterized as that quality of music that gives it depth and richness. Subwoofer speakers are not meant to be used without other speakers that generate the rest of the frequencies of an audio signal. They are primarily classified according to their power source, enclosure type, and application.
The power source for subwoofer speakers is one way in which these audio components are classified, and subwoofers can have one of two types. Powered subwoofer speakers have their own amplifier, which requires an electric current to operate. Passive subwoofer speakers rely on a separate amplifier and do not require electricity. The type of subwoofer can easily be deduced by the presence or absence of a power cord.
Enclosure type is another way in which subwoofer speakers are classified, and the two main types are sealed enclosures and vented, or ported, enclosures. Sealed enclosures are airtight compartments and are also sometimes called acoustic suspension enclosures. Vented, or ported, enclosures have a vent, or port, designed using specific calculations involving the volume of the enclosure and the characteristics of the given subwoofer. The port, if properly designed, causes the air within it to resonate at a particular frequency, enhancing sound quality as well as output.
A third type of enclosure, called a bandpass enclosure, has a double enclosure design, one behind the speaker and one in front. The rear enclosure may be ported or sealed, while the front enclosure is usually ported. Complex variations on this design involving multiple speakers and compound enclosures are possible, but all rely on the basic principles of sealed and ported enclosure designs.
The final way in which subwoofer speakers are sometimes classified has more to do with application than design. Subwoofers are generally intended for use in the home or in car audio systems. Home audio subwoofers are almost always designed with their own enclosure. Car audio subwoofer speakers are sometimes sold with an enclosure but may be designed to be installed without an enclosure. The two types often have different resistance ratings as well, meaning that each type is not usually suitable for the other kind of application.