There is another underlying flaw with vented enclosures. As the frequency drops below the tuning frequency of the enclosure, 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.
Designing a vented enclosure rests on three main factors: the parameters of the speaker; the volume of air in the enclosure; and the size (diameter and length) and shape of the vent. Proper integration of these three factors will have a positive effect on the potential of a vented system.
Starting with the correct speaker is unquestionably the best way to yield stellar results. A subwoofer featuring a Qts parameter between 0.2 and 0.45 is typically best suited for this style of system. Drivers with a higher Qts will still operate in vented enclosures, but are generally accompanied by ripple in their frequency response, and poor transient performance brought on by high group delay of the bass system. While the Qts is a quick way to determine a speaker's compliance for work in this application, using the Efficiency Bandwidth Product (EBP) equation can be more exacting (see sidebar).
Coming up with the enclosure volume and vent dimensions can be difficult. Tuning a vented system is accomplished by selecting the proper length and surface area of the vent to match a predetermined alignment, which also indicates box volume. These alignments can be met by using different methods based on the Theile/Small parameters of the driver. Fortunately, several computer programs handle the mathematics for you. If you don't have access to one of these programs, follow the manufacturer's recommendation. These are often available in the supplied manual or online, and are typically tried and true designs so you can be assured the response will be desirable.
The laws managing the performance and proper design of vented speaker systems are considerably more complex than those for sealed enclosures. It's very easy to mess up a vented system, so make it a good practice to follow the manufacturer's or an experienced system designer's advice. Guessing at the vent dimensions will likely create a bad-sounding enclosure and may also damage your speaker.
BandpassStarting in the early '90s, they became the rage of the car audio world. But bandpass enclosures are far from being a new design concept. It would probably surprise most people to know that these designs have been around since the 1930s with the original patent for a single reflex bandpass filed in 1934 by Andre d'Alton. Just over five decades later, the methodology of two French designers, Augris and Santens, was published. Their hand-calculator design system for single reflex bandpass speakers set in motion the eruption of popularity of this design amongst manufacturers as well as consumers.
So where did the term "bandpass" derive from? This is really pretty simple. The term refers to the fact that the enclosure will only allow a certain frequency range (band) to play (pass) into the listening environment without the aid of a crossover. In order for this to happen, the frequency response of the speaker is acoustically limited by the enclosure.