There are two major types of subwoofers: passive and powered, and the latter is also referred to as active. Passive subwoofers require an external amplifier to function, while powered subwoofers do not. Instead, a powered subwoofer comes with an amplifier built into its enclosure. Additionally, unlike a passive subwoofer, a powered subwoofer must be plugged into a power source, since it contains an amplifier.
Like passive subwoofers, powered subwoofers are designed to reproduce low frequencies that cannot be accurately or clearly reproduced by other types of speakers, such as all-purpose stereo speakers, mids, and tweeters. The frequency response varies between subwoofers and depends on a number of factors, including the size of the subwoofer, the amplifier, the type of enclosure, and the power rating. In general, larger, high-power subwoofers contained in enclosures designed to reproduce low bass notes produce the lowest frequencies. Depending on the design, the frequency response can extend below 20 Hertz (Hz), which is below the range of human hearing.
Similar to other types of speakers, a powered subwoofer's cone can be made from a variety of materials. These include paper, synthetic materials, and combinations of the two. Although paper doesn't last as long as other materials, it does provide a quick response to the audio due to its lower density construction. The cone's material also affects the audio quality of the subwoofer, but cone preference is largely subjective, or it depends on what the listener likes best.
Powered subwoofers are typically organized according to cone size in inches. The cone affects the frequency response and overall response of the subwoofer. In general, large cones are capable of reproducing lower frequencies than small cones, but take slightly longer to respond to the audio due to the increased surface area.
A powered subwoofer can be used in any application where a passive subwoofer is applied. Common applications include automotive sound systems, home audio, music production, and live musical events. For applications requiring the fewest components and those where space is highly limited, a powered subwoofer is a good option because the amplifier is contained inside the enclosure, allowing it to take up less space.
The types of powered subwoofers available vary in cost and style according to their uses. Typically, large subwoofers measuring at least 18 inches (45.72 cm) designed for live events cost more than those designed for home theaters and vehicles. Powered subwoofers designed for music production also tend to cost more than home and car audio subwoofers. The higher costs are attributed to higher power ratings and increased accuracy when reproducing audio, respectively. In terms of style, the enclosures used for car audio subwoofers typically provide more options, including a larger number of colors to choose from and carpeted enclosures.
Like passive subwoofers, it is possible to build a powered subwoofer from components or a subwoofer kit. A kit for a powered subwoofer contains the subwoofer, an amplifier, audio connectors, and hookup wire. The hookup wire connects the subwoofer to the amplifier internally and permanently, while the audio connectors allow the subwoofer to be connected to a sound source, such as an audio interface or stereo receiver. In some cases, additional audio connectors are provided, so the sound can be sent from the subwoofer's internal amplifier to other speakers, such as mids and tweeters that handle higher frequencies.
Some kits also include additional components, such as an enclosure, speaker batting, and a crossover. Speaker batting absorbs audio reflections inside the subwoofer's enclosure, allowing the bass frequencies to be deeper and reproduced with a flatter frequency response. The crossover splits the incoming audio signal into separate frequency bands and sends the lowest frequencies to the subwoofer. If a crossover isn't included, it might be built into the subwoofer's amplifier.