Matt 34 has invited a thread on band pass, more correctly called coupled cavity subs.
This came into vogue in the 1980s. KEF produced a number of designs, like this.
These speakers certainly extended the bass of the 10 inch drivers especially because the bass roll of is second order rather than the fourth order roll off of the ubiquitous Qb4 ported box. They really did not reduce enclosure volume. However this is
what they achieved. Not bad.
Bose of course started their long running series of bass modules.
An in depth discussion and theory behind coupled cavity subs appeared in speaker builder six/88.
Since the eighties interest in band pass bass sections for reference systems has declined. The technique has largely found its place in car audio, computer sound system and of course Bose systems.
The 1998 article encouraged me to build a number of these designs, of which only three were any good. However I learned a lot a long the way.
The eighties, was an era where pushing bass extension very much came into vogue. The B & W answer to the KEF was to make their flagship model a Qb6 box with a 15" woofer. A standard Qb6 box alignment has to include an active equalizer. The speaker consumed gobs of power, the driver could be made to bottom and this design was short lived and largely forgotten.
So what are the issues with coupled cavity designs.
If an isobarik design is chosen a low F3 can be obtained from a relatively small enclosure.
The designer has total control of Qt. For me that was a big attraction.
The low pass crossover from the bass unit can potentially be acoustic.
Bass roll off for the single tuned band pass box is second order and obviously helps extend the the bass response. The roll off is second order on the top and bottom end.
The down sides are decreasing efficiency as the band pass response is extended.
This is especially true for Single tuned Band Pass Box, like this.
This single tuned band pass box is like the KEF solution and like one of the successful boxes I built.
This improves efficiency because of the use of two drivers with their own sealed space.
This is the isobarik version.
My other two succesful designs were like this.
This obviously lowers efficiency but halves enclosure volume for the same F3.
These designs can work quite well. The phase shift is never more than 360 degrees and so is comparable to the vent output of a standard Qb4 box.
The big problem is efficiency, as high output can only be obtained over a very narrow range of frequencies. This leads to the charge so often leveled against these designs, of being "one note wonders."
If response is extended over at least one and a half octaves efficiency will be poor.
Drivers unload below tuning just like a Qb4.
All output comes from the port and so cone radiation is lost.
Port design is difficult unless you use a low pass filter and increase the order of the higher cut off. If you don't cut off the response an octave or so above the upper band pass roll off you will get port resonances stating around 900 Hz or so, or lower if the port is longer. So port artifacts are a problem. A large diameter or vent port makes the use of a crossover at the upper turnover frequency mandatory. This problem negates one of the advantages of a band pass design in my view.
So lets move on to the double tuned band pass box.
This comes in two forms, the parallel configuration.
And the series configuration.
Efficiency is improved by 6 db for the parallel alignment but only 3 db for the series, so the parallel arrangement is to be preferred.
The low frequency of the band pass rolls off at 24 db per octave and the upper end at 12 db per octave.
The big issue is transient response as the phase shift has to be at least 450 degrees and can be as high as 540 degrees. Distortion also rises as you sequence ports.
I'm not aware that any high end designs have used these higher order alignments. This brings me to the issue of nomenclature, where there is confusion. Different authors refer to the first series of enclosures variously as second or fourth order, and the second as fourth and sixth order. So I have used a less confusing and more descriptive nomenclature.
I personally favor the order to designate the order of the bass roll off.
If high sound quality is the goal, it is my opinion you need to stop at the first series of enclosures and reserve orders above that for car use, compact computer systems and Bose. That is pretty much the way it has turned out.
Narrow band pass units with the band pass centered around 50 Hz have proved popular for that high powered bass lick, but I'm sure that need not concern any of us.
