Ported EnclosuresThe reputation of ported enclosures has not been one of quality sound. Rather, they are often perceived as being sloppy and generally not very accurate. This common misconception is likely a product of a poorly designed or an improperly ported enclosure. It is not uncommon for ported (vented) enclosures to have better transient response than underdamped sealed enclosures with a high Qtc.
As odd as it may sound, ported enclosures have been around longer than the simpler sealed box. The patent on this design was first issued to A.C. Thuras of Bell Labs back in 1932. His work contributed to basic theory of the design, describing in detail the interaction between the driver and vent. It was not until 1961 that A.N. Thiele published a landmark paper that was later (1973) refined by R.H. Small who defined the behavior of a speaker, thus allowing for ported enclosures to be perfected. Their contribution is well known as Theile-Small parameters and most commonly related to ported speaker systems.
Ported enclosures are often referred to by many names such as bass reflex, tuned, or more commonly, vented. What distinguishes a ported enclosure from that of a sealed is the port or duct that allows the back wave to interact with the front wave. The coupling of a port to the air inside the enclosure reinforces the low-frequency response the subwoofer system. By changing either the length or surface area of the port, the resistance to motion of the column of air within the port changes its resonant characteristics, thus causing the tuning frequency of the enclosure to change.
In a ported enclosure, there are three main factors that have to be considered: the parameters of the speaker being used, the volume of air in the enclosure, and the size (diameter and length) of the port. Proper integration of these three factors will have a positive effect on the output of a ported system. The primary goal of integrating these three factors is to delay the rear output wave of the speaker so that as it comes out of the port it is relatively in phase with the wave produced by the front of the speaker. The result is a gain in efficiency of about 3dB compared to a sealed system using the same woofer. It is this low-frequency reinforcement that is one of the big advantages of a well-designed ported system. Another big advantage is that near the tuning frequency, almost all output is from the vent so cone movement is controlled to the point of almost no movement. That results in lower distortion at higher volume levels in the frequency range controlled by the port. But not everything is perfect with ported enclosures. They suffer from one main drawback: the response of the speaker below the tuning frequency. The speaker no longer has the restoring force of the air in the enclosure, and therefore the cone acts as though it were not enclosed at all. This can lead to serious damage to the speaker itself and leave you stranded without sound.
The laws managing the performance and proper design of ported speaker systems are considerably more complex than those for sealed enclosures. It is very easy to screw up a ported system, so make it a good practice to follow the manufacturer' advice, or the advice of an experienced system designer. Guessing at the port dimensions will likely create a bad sounding enclosure and may also damage your expensive speaker.