I used to do a lot with ABC boxes right around the time I was doing sub engineering - I even have a 15 year old prototype XBL^2 ten in a prototype compact ABC box... which I'd generally call a failure, because Hoffman's Iron Law still holds true. The frequency response wasn't bad, but didn't have much bottom octave, and wasn't all that loud because I really wanted to see how small I can push things... this was my "that's enough, then" smallest box, end of experiment.
Still have it...
If you think of how a port works, you can see how both the coupling port and the rear port work (and why all the group delay). It might be easier to picture how passive radiators work - they work the same as ports.
I won't step on Justin's toes, I'd love to see a good description myself...
To me it makes sense - the front chamber pretty much working like a traditional vented box - but also stimulating a column of air in the coupling vent which in turn works like a subwoofer stimulating that second chamber (again, maybe easier to imagine a passive radiator), which then has it's own port to the outside world.
But yes, the tuning - why "equal port lengths" and why the result? That's definitely worth diving into, and even though the whole daisy-chain of sub-box-vent-box-vent to me makes intuitive sense "why big group delay?", the tuning aspect is more of a mystery.
I would presume (but never dove into it) that if you designed an ABC box using the "three equal ports" as a starting point - that'll at least get you to a known place.
Then - let's say you take some measurements in-car, and decided that... I don't know, you have a very small car, so you are perfectly happy with your SPL around 60hz, but you are still seeing a slight rise from there to 40hz... and you'd rather flatten it and extend your response to 30hz. So, maybe you take your existing design, and lengthen the port coming out of the larger chamber - I'd think you could do that. Then they wouldn't be equal, and I think you'd have the tweak you want.
...what I don't know is, would that impact the other chamber? Part of me thinks "yes", because that would fundamentally alter the pressure dynamics inside that chamber. Part of me thinks "no", because that second vent will still function as a vent, and that sub is still the driving pressure actuator in that chamber.
So what if you had the OTHER problem? Maybe you like your low-end response, but you want to change the upper response. This seems very tweaky, like a bandpass box, but whatever - let's say you want to raise or lower your tuning.
I'd be interested in the effects of both the coupling port and the output port, on that secondary chamber.
I'd speculate that you could most easily tune the output port, on the secondary chamber, by shortening/lengthening it.
Changing the length on the coupling port really makes me wonder - especially from my comments on group delay - could you tune the coupling vent higher (shorter length) and then tune the output vent lower (longer) to get the same tune? Could there be a benefit (group delay) or is the combined effect of the shorter-one-place, longer-the-other just going to "net nothing" for you?
Is it as simple as "if you tune the coupling vent higher, without lengthening the output vent, you'll effectively tune that chamber higher?"
I'm definitely curious. Always had luck with that design, but definitely fall short of an expert, despite experience.
2)
Then in the ABC design, with a port between the larger and smaller chambers... most of those call for the same port dimensions in all three sections. How does the lower frequency work? and the port that connects the two chambers seems to not be tuned to 30 Hz?
it seems like if that is for the 30 Hz side, then by the time the 30 Hz tunes come out of the other two ports that they would be 180 out of phase?
And why isn't each port tuned to a specific frequency, along with the two chamber volumes differing?
I used to build a lot of these myself. I always used the "same port dimensions" for all three, but I have speculated that you could probably substitute an effectively same-tune port of different dimensions, if for some reason (maybe depth) you really wanted slot vents to the outside, but needed to use maybe two round ports in the corners, for the coupling port. I never did experiment with that, will be interesting to see what Justin thinks.
I will comment that I always did like this design, specifically because the default tuning of both chambers really seems to line up incredibly well against the typical car interior's cabin gain... So unlike a high-tuned vented box or some poorly designed bandpass boxes that become one-note wonders, you essentially have SPL boost between your two tuning frequencies... it gives you more boost towards the lower frequency, and that slope between the lower and upper tuning frequency lines up really well to pretty much flatten out inside a car.
...which also explains why these things would be total failures in a larger, more open environment like a listening room or a bar or club. The room gains wouldn't be complimentary, like they are in a car.
There's probably too much group delay to call this an "SQL dream", but for someone who likes it loud, and doesn't quite reach the IASCA-SQ-competitor level of audiophile, it can really be a great fit.
Also, Justin-
Could you maybe voice your opinion on what I've always thought was a misnomer, "Aperiodic Bi-Chamber" just doesn't fit - it's all vents. Never seen one done aperiodic. I feel like someone plugged in a word there, for marketing reasons... "Eh, sounds fancy. They'll never know."
That being said, the other acronym "Dual chamber tri-tuned" or DCTT I always felt sort of misnomer-y on also. Yes, dual chamber. Tri-tuned? Ehhhh... I guess kinda sorta but... really there's two vents, and yeah a vent in the middle, but...
Well, maybe that is a good link back to my other questions on the tuning what-ifs.
