Bandpass EnclosuresBy now, most car audio enthusiasts are probably familiar with bandpass enclosures. Starting in the early '90's, 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 1930's. 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 beyond that, Jean Margerand republished the methodology of two French designers, Augris and Santens. It was their hand-calculator design system for single reflex bandpass speakers that is likely responsible for the eruption of popularity amongst 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 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 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. When enclosure parameters are attuned for a narrower bandwidth, gains of up to 8-10dB in efficiency are not uncommon within the bandwidth of the subwoofer system. Adjusting the box parameters for wider bandwidths, low frequency extension can greatly be improved, but at the expense of efficiency and good transient response. Using an intermediate bandwidth can create a compromise between these characteristics.
Bandpass enclosures come in two flavors: single and dual reflex. The more popular of these is the single reflex bandpass, otherwise referred to as a sealed rear chamber bandpass. In a single reflex bandpass, the rear of the speaker resides in a sealed chamber while the front fires into a chamber that is tuned by a port. This front chamber acts as a low-pass filter, which acoustically limits the high frequency response of the speaker. The roll off of this type of bandpass is typically 12dB per octave, much like that of the common sealed enclosure with better overall transient response.