Vented systems come by many names including bass reflex, tuned and, most commonly, ported enclosures. As most of you reading this already know, the "vent" is what distinguishes this type of bass system from that of a sealed enclosure. These systems appear simple enough on the surface, but can be difficult to design for optimum performance. For this reason they've been described as sloppy, boomy and sound-quality challenged - a reputation undoubtedly a by-product of misalignments in the design. However, a properly built vented enclosure built around a woofer that falls within the desired parameters should exhibit good transients and a linear frequency response. In fact, it is not uncommon for vented enclosures to have better transient response, and certainly a broader frequency range, than an under-damped sealed enclosure. But just as a well-designed enclosure can be made to perform admirably, their characteristically large size and low tuning frequency (long vent size) makes them less appealing for the car audio enthusiast of today.
There are two primary characteristics associated with vented enclosures: efficiency and low-frequency extension. The vent acts much like the piston of a loudspeaker-let me rephrase that, the column of air within the vent does. As the cone of the woofer compresses and rarifies the air in the enclosure itself, the column of air in the vent reacts accordingly, moving forward and back within the vent. The Barometric pressure outside the enclosure essentially forms a barrier over the vent, keeping the column of air intact. As a result, the column acts like a piston when motivated, vibrating the air outside the vent just as the cone of the woofer does. The coupling between the "air piston" and the woofer, in theory, yields a +3dB gain in efficiency over the same woofer in a proper sealed enclosure. Of course, this gain is largely dependent on the area of the vent. And, because more air is moved in lower frequencies, the coupling also results in great low-frequency output.
Another big advantage happens near the tuning frequency of the enclosure - most of the output is generated from the vent while the cone is highly controlled and movement is minimalized. This results in lower distortion at higher volume levels in the frequency range controlled by the vent. But in order for this to work properly, the vent must be of appropriate area, length and shape. I make mention of this because standard size vents can become non-linear quickly resulting in elevated distortion levels and high group delay. Much of this is related to the vent's area being considerably smaller than the cone of the subwoofer. In order to properly tune a vented enclosure, vents generally need to be much smaller than the cone since the length required for a vent of equal proportions is unfeasible. Because there is such a large size differential between the woofer cone and vent area, the inside of the vent becomes highly resistive as the excursion increases and its effective area shrinks. Once this happens, the velocity of the column of air within increases and the vent essentially unloads. Flared or "venture" ports resist this unloading effect to a higher power level, but are still susceptible.