Unlike sealed enclosures, ported subwoofer enclosures use a vent, or port, to increase the low frequency output of the subwoofer. This vent or vents inside these enclosures is typically made from cylindrical components, such as PVC pipes. These speaker boxes tend to be larger than sealed boxes and are more difficult to design and build. Ported, single reflex bandpass, dual reflex bandpass, and labyrinth boxes are four major types of vented enclosures.
The term ported enclosure can refer to any type of box containing a vent, but is most often used to describe a single-chamber enclosure containing one port. This type of vented box is the easiest to design and build. The width, length, and location of the port is determined by the frequency the enclosure is being tuned to. The tuning frequency determines how low the bass can go and provides the low frequency cut-off for the box. Any frequencies below the cut-off frequency will not be heard, and it is possible to damage the subwoofer when playing sounds below this level.
Ported subwoofer enclosures that contain two chambers and a single port are called single reflex bandpass boxes. This style features a subwoofer mounted to the wall inside the enclosure, and this faces the larger chamber in the box. A port is typically placed in the larger chamber and is located at the top of the box. The subwoofer outputs sound efficiently in the tuned frequency band, while other frequencies are not produced as well. This type of enclosure is hard to build, and users should remember that it is easy to damage a subwoofer without realizing it, since distortion is hard to hear.
Dual reflex bandpass boxes consist of two chambers, two ports, and a single subwoofer. The woofer plays into a tuned, ported chamber and reproduces frequencies in the tuned range efficiently. Frequencies outside the tuned range are cut off. These ported subwoofer enclosures are large and difficult to build. Even the smallest construction or design mistake can have a noticeably negative effect, such as changing the tuning of the box.
Ported sound pressure level enclosures, more commonly called ported (SPL) boxes, are useful for narrow-band subwoofer applications. This type of enclosure boosts a specific frequency, while filtering out other frequencies. Ported (SPL) boxes are usually very large and contain a single port and one chamber. Attempting to play a wide range of musical frequencies through a subwoofer in a ported (SPL) box can cause the woofer to fail. Fortunately, subwoofer failure can be avoided by using a crossover, which filters out unwanted frequencies, with these types of ported subwoofer enclosures.
Labyrinth boxes are large and extremely difficult to build. This type of enclosure consists of a single chamber with a maze-like internal port system. Its design allows a particular frequency to be boosted above all others, but there are generally no practical applications for these ported subwoofer enclosures.