A vented enclosure is not much more complex than a sealed box. It consists, basically, of a box with a hole in it. However, despite its simple design, vented boxes are considerably harder to get good performance from than sealed boxes – although at many times the extra effort can be worth it.
The vent in the enclosure interacts with the volume of air in the cabinet and the driver to help increase output and reduce cone excursion at and around the tuning frequency. In face, at box tuning, almost all the bass is produced by the vent – NOT the woofer.
The trick in building a vented box is to get the right size enclosure and the right size vent. You can’t be too far off on either of these factors or your speaker’s performance will suffer. In particular, using a too-small box or a too-high vent tuning frequency can eliminate bass instead of increasing it. Porting a sealed box that is too small usually does nothing to improve frequency response. The vent’s placement within the enclosure is also important. You must leave at least the equivalent of the vent’s diameter between the vent and any inside wall. For example, you would not place a vent with a 3” diameter within 3” of any wall. The same is true for clearance between the vent opening and the bottom of the enclosure.
1 - Reduced cone excursion and reduced distortion around vent tuning
2 - Increased output capabilities around vent tuning
3 - Vented boxes give you that extra “bump” that is preferred in certain types of music
1 - Total loss of cone control below vent tuning, which can result in high distortion and driver mechanical failure
2 - Midrange sound coming from inside the box through the vent can produce unpleasant sound coloration
3 - Vented enclosures are more sensitive to changes such as temperature, humidity and driver fatigue
4 - Enclosure design is more complex and the enclosure itself must be more solidly constructed because internal pressure at frequencies around vent tuning can be nearly twice as high as a sealed enclosure
5 - Vented enclosures usually don’t sound as fast as sealed boxes because the resonant effects of the vent tuning which is always slightly out of phase with the driver’s output