This low-frequency reinforcement is one of the big advantages of a well-designed ported system. Another big advantage is 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's 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.
Bandpass EnclosuresBy now, most car audio enthusiasts are probably familiar with bandpass enclosures. Starting 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 these designs have been around since the 1930s. The original patent for a single-reflex bandpass was filed in 1934 by Andre d'Alton. Just over five decades later, Bose was granted a patent on dual-reflex bandpass enclosures (1985).
Three years later, Jean Margerand republished the methodology of two French designers, Augris and Santens. Their hand-calculator design system for single-reflex bandpass speakers is likely responsible for the eruption of popularity among manufacturers, as well as consumers. In the last ten years, considerable progress has been made in defining the behavior of these speaker systems in both home and car audio.
So where did the term "bandpass" derive from? This is really pretty simple. The term refers to the fact 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 to for this to happen, the frequency response of the speaker must be acoustically limited by the enclosure.
In a bandpass enclosure, the woofer resides between two separate chambers. Instead of playing directly into the listening area, the entire output of the speaker system is formed through port(s). These ports act as an acoustic filter and limit the frequency response of the speaker to a specific bandwidth. Adjusting the size of the enclosure and/or the dimensions of the ports, the frequency response (bandwidth) can be greatly altered.
But why would anyone want to use a bandpass enclosure over a sealed or ported? Well, besides the natural filter acting as a crossover, a bandpass enclosure can deliver considerable performance benefits in terms of efficiency and low frequency extension that would likely not be possible in conventional designs of similar size. This of course is a trade-off with regards to the parameters of the enclosure.