|A ported enclosure system
consists of a driver mounted on one side of a box that has an open tunnel or port which
allows the passage of air in and out of the box. At low frequencies, the vent contributes
substantially to the output of the system.
The ported enclosure system is characterised by lower distortion and higher power handling in the system's operating range, and lower cutoff frequency than a sealed enclosure system using the same driver. Distortion rapidly increases below the cutoff frequency however as the driver becomes unloaded, and the transient response of a ported enclosure system is usually inferior to that of a sealed enclosure system using the same driver. However, the lower cutoff frequency and better power handling within the system's passband often makes ported systems the alignment of choice for many speaker builders.
Ported enclosure systems are much more sensitive to misaligned parameters than sealed enclosure systems, which makes their construction more difficult for the beginning DIYer. I advise that you don't attempt to build these systems, unless you're certain that the T/S parameters for the driver that you want to use are correct.
Almost any driver can be used in a ported enclosure system, however, only drivers which have a Qts value between 0.2 to 0.5 will generally give satisfactory results. If the driver has a Qts above 0.4, try using it in a sealed enclosure or single reflex bandpass system instead.
Ported System projects on the Internet
Special thanks to Ron Ennenga for providing ideas and equations to approximate the effects of in-car cabin gain and line-level filtering in the PORTED.XLS spreadsheet.