Ask an Acoustic Engineer (me)

[Holmz]

There are many variations of 6th and 8th order enclosures, which to kinda answer geolemon's question ... I doubt there is a super scientific method to how the various configurations were named which makes it harder to identify which is which by just using words. To make sure we're talking about the same ones, please share a picture of the one you're interested in. For example, one of these:
https://www.the12volt.com/caraudio/s...ofer-boxes.asp

This one:

So I assume that the lower left chamber would be the 30-Hz.

Then in a 30, 45, and 60-Hz setup, it seems. Like the lower right needs to do the 60?
but then if the upper chamber on top, it at 45-Hz that will effectly choke the higher frequencies form coming out?
or
is lower right 45-Hz, and then some leakage of 60-Hz come out and that gets resonated in the upper chamber tuned at 60-Hz?
or is it some thing like 30 and 60 Hz across the bottom, and then the upper chamber could be at 80-Hz just to allow the unloaded port to pass through everything up to its 80-Hz tuning freq and chockes the higher order distortions?

2) And then once I know the frequencies, then how does one go about selecting the box volumes and chamber port sixes?
can those be done in isolation? Or does the whole thing being tied together cause the deal with the math to change?
(Stumbling off to the link from dégagé)


An there is this: http://dbdynamixaudio.com/off-axis-6...re-calculator/
And this looks better: http://www.carstereo.com/help/Articles.cfm?id=32

Either way I think I got the wrong subs with too high of a QTC.

...

Great question. My only thought is the ported nature allows fresh air exchange with the outside world, allowing the internal box temperature to be cooler than a sealed box. This might be a substantial effect or a tiny one depending on the port non-linearities. Also, if you know the woofer needs to be small excursion then you could optimize the design with a higher BL^2/Re ratio which might make it more energy efficient (1w/1m) to generate less heat in the first place.
...

That cooling question probably probably applies more to this sort of box:

That speaker on the left will not see a lot of fresh air.

...
I have not studied bandpass and higher-order enclosures as much, but the impedance peaks I have seen are due to back emf when the cone has the greatest velocity (near greatest excursion). There is an impedance minimum in a normal ported enclosure at the box tuning frequency there the velocity is minimum (and excursion is minimum), for example.



I think it's the other way around: low impedance at low velocity (low excursion). See question 2 above.
...

Or maybe if we shut off the amp in the middle of some tone that was at a control mode, then the box's response would slowly decay?
And then the back EMF would be the force of the air pressure acting against the cone?
So shutting off any electrical input, and damping... then cone should start moving because the box is resonating?
(dunno...)

...
If you can remove group delay with a FIR filter like you suggest, then latency will suffer so it becomes harder to sync with video such as watching a movie. You would still need to tackle transient response, yes, and also directivity and timbre and all those other things too but purely in terms of a subwoofer enclosure I think that would be most of the problems yes.

Yeah the video sync part is hell for live work.
and often a chin scratcher for home theatre.

I guess we are stuck stuck with impulse response, so it will not be great for thumps at the higher frequencies.

[Holmz]

Seems like you're both talking about the same thing. A couple super-fancy dsp systems like Dirac and the APL1 use a combination of IIR an FIR filters to correct frequency and phase response so that the impulse response becomes more ideal, which results in the group delay being more ideal (flat) too. The same concepts can be used to correct group delay in a high-order subwoofer enclosure at the expense of latency which I mention in my previous post. I've always wanted to try this.

I guess I have require DSP to give it a try it then...

[Cathul]

1) How does the sub stay cool in a bandpass if there is limited excursion at/around each control node?

Easiest answer to that, put the driver in reverse into the box, i.e. the magnet into the vented part of the box.

[Holmz]

Easiest answer to that, put the driver in reverse into the box, i.e. the magnet into the vented part of the box.

Yeah, I go that... but
1) Let's say in this box:

That I wanted it tuned super low and had a passive radiator on the low side (lower right) weighted up for 10-Hz, and then on the lower right side another PR tune to 25-Hz...
(That should really get the curry moving in a scary movie sound track.)


or in this box:

Then the one on the left is totally in an oven, and a PR where the port is makes it on oven, and a PR can be tuned pretty low compared to a port.

[Cathul]

Ok... in these cases you might get a problem.
But isn't the idea of a bandpass that you don't need that much power for the same amount of output?
Shouldn't this lower the thermal stress on the driver?
Maybe i'm wrong, but i always thought that was the main reasons to go with a bandpass.

[Holmz]

Ok... in these cases you might get a problem.
But isn't the idea of a bandpass that you don't need that much power for the same amount of output?
Shouldn't this lower the thermal stress on the driver?
Maybe i'm wrong, but i always thought that was the main reasons to go with a bandpass.

I thought mostly dB drag racers used them for an extra 10 dB AND a 5+ kW amp.

And then Bose also uses bandpasses for more bass with small speakers.

Maybe poeple also use them to actually bandpass as well. (Probably?... and probably before there were passive XOs for subwoofers, and before DSPs.)

[Cathul]

No idea tbh... in Germany a different approach to bandpass subs is quite popular.
Here a lot of people purposely tune the bandpass box to a large peak above 60 Hz and cross them low to flatten the curve to achieve a minimum excursion in the midbass area down to about 50 Hz.

F.e. take a JL 8W3v3 and put it in a 4th order 9 liter for closed, 22 liter for ported and 25cm port length with a diameter of 12cm.
Magnet inside the ported part of the box.

Downside is, you need to know the cabin gain before building/designing the box and you need a DSP to really dial in that sub to the target curve.
Good thing is, above 50Hz in this example you have an excursion well below 3mm at 200Watts of input while with a simple ported box you have at least double that excursion.

Lots of people like these bandpasses in Germany and recommend them if you got a sedan for it's tight and fast midbass (almost no excursion necessary in the midbass area at all).
I myself am wondering for some time now if a bandpass would be good in a S550 Mustang, or if i should build a normal ported box.

[geolemon]

Yeah, I go that... but
1) Let's say in this box:

That I wanted it tuned super low and had a passive radiator on the low side (lower right) weighted up for 10-Hz, and then on the lower right side another PR tune to 25-Hz...
(That should really get the curry moving in a scary movie sound track.)

Here's why I never refer to boxes with any sort of "order" designation... It's confusing and increasingly so with the higher orders that are used rarely-to-never anyway... but we'll get back to that.
4th order... what? Oh, sealed bandpass. OK, that makes sense.
6th order... which? Oh, that's the dual-vented bandpass. Makes sense.
8th order... do you mean ABC box? Tri-chamber tri-vented bandpass? Tri-chamber dual-vented bandpass?
Or this one - dual-chamber tri-vented bandpass?

That's actually the one that I thought you meant when you said "8th order".

The one you pictured above is complex but in that way that makes me ask "Why? What's the goal?"

With even 6th order dual-vented bandpass boxes, I feel like you have enough independent control to design a one-note wonder, or a bandwidth of your choosing, even sloped one way or the other. Yes, there's a point where it becomes an ugly M-shaped split if you try to really widen the bandwidth, but that's often partly a function of the subwoofer you use, if you want to consider that another variable. And the version that I show above is an option that's not causing additional delay or acoustic filtering, per se.

But even then, that's usually the point where people say "Well, if you want the bandwidth wider - why not just go vented?" or even "sealed?" - note again, with those the subwoofer is again a variable, so you can consider ideal candidates for each.
In the one you show - essentially all the output is corralled into that third chamber and only allowed to escape through the vent... Is that possibly for noise filtering? I'd think that indirect approach would have only losses - since the traditional Iron Law tradeoffs of low frequency/efficiency/box size can be achieved without that third chamber.

So do you know, what is the goal of that design you shared? The advantage or the fringe benefit someone would use that for?
And I'm interested also - what's YOUR goal, your design target? What are you looking to get that a traditional design doesn't produce?

PS - I'm not sure if you were differentiating or thinking of them as the same - but an "ABC" or "DCTT" box is actually this - the subwoofer (or subwoofers, in this case) itself fires out, directly into the listening space, of the lower-tuned chamber:

...so it functions more as a standard vented box, essentially having two tuning frequencies, and a sloped (down, towards the lower) response between the two, that happens to usually flatten out nicely when you put it in a car and factor in cabin gain.

I didn't even try to dig out my old Excel workbook, but in searching for a good cutaway pic of an ABC, I found a site where that colored image came from, where they offer one to help plan, although theirs doesn't have a frequency response plot:
http://dbdynamixaudio.com/dual-chamb...re-calculator/
Note on that page, they aren't doing anything magical - in fact they are just using the same simple process, of picking a target tuning frequency using standard single-reflex vented box standards to compute a single port length, which they then use 3x. But still nifty. I have to dig mine out, since it's "niftier", and produces a response plot.

or in this box:

Then the one on the left is totally in an oven, and a PR where the port is makes it on oven, and a PR can be tuned pretty low compared to a port.

I'm not sure how much of a concern this is. Simple sealed boxes have this same issue, which never really seems to be an issue.
I'm thinking that's because like an oven, it heats up, and cools off - you don't need to open the oven door or introduce outside air to cool it. And even if you did, that wouldn't necessarily stop the oven from heating up - I think in both cases the most you could hope to change would be the speed of heating or cooling.

But also a subwoofer voice coil has very high temperature limits. In the oven analogy you could think of it as the heating element, but it's going to fluctuate based on what you are playing - regardless, even in a super-tiny box with a super-huge amp (assuming still within power limits), I think it would take a long time for the interior air temperature to even reach the lower temperature of those fluctuations.

And even with a vented box, that vent suspends a moving column of air, but its motion is similar to cone motion: it moves in and out, but stays in place. So the outside inches may be kissing the outside air, but the same inches inside are kissing the inside hot air. So I don't know how much more temperature transfer you get, other than convection through that air in the column. It isn't really like forced air.
Even if it were, trunks on a hot summer day aren't necessarily "cool" air...

All in all, it's actually probably an interesting discussion of its own, this temperature discussion... including maybe the difference in performance of a hot coil vs a cool coil... hot suspension parts vs cool suspension parts... how much T/S shift you can cause... it's interesting to think about, because really - no one ever does.

[Holmz]

...

The one you pictured above is complex but in that way that makes me ask "Why? What's the goal?"


So do you know, what is the goal of that design you shared? The advantage or the fringe benefit someone would use that for?
And I'm interested also - what's YOUR goal, your design target? What are you looking to get that a traditional design doesn't produce?

PS - I'm not sure if you were differentiating or thinking of them as the same - but an "ABC" or "DCTT" box is actually this - the subwoofer (or subwoofers, in this case) itself fires out, directly into the listening space, of the lower-tuned chamber:

...

In the house the goal is mostly an extension of the bass notes down for "the curry release" when watching movies. The home speakers have descendent enough bass response, but nothing really low, like the subsonic stuff.
but then I also do not want a lot of noise and harmonics buzzing out in the 100-Hz + range.

in "the truck" it is somewhat similar with maybe 25-Hz up to 80/100, and with enough high frequency scraped away to not have it localised. The reason for the third chamber is also tat the internal volume is relatively precious, so that final port could allow thebox to be in the rear and then the port can pump into the back of the cab.
and lastly I have a 300/500W (4/2-ohm) sub, so any help in box gain is great... and the truck has a lot of low frequency noise.

Those boxes with resonant chambers and the cones exposed to the air seem to get the resonant gain, but all the IMD and harmonic distortion can jump off the cone into the room. That defeats the concept of scraping away the higher orders of harmonic distortion.

And then all I usually hear is, "those need to be exact" and little is evident in describing how they work and tools to design them.

Plus, like most people do... I got subs, and only now realise I probably got the wrong sub for the application.
(it is easier to preach, than to practice it)

[geolemon]

In the house the goal is mostly an extension of the bass notes down for "the curry release" when watching movies. The home speakers have descendent enough bass response, but nothing really low, like the subsonic stuff.
but then I also do not want a lot of noise and harmonics buzzing out in the 100-Hz + range.

in "the truck" it is somewhat similar with maybe 25-Hz up to 80/100, and with enough high frequency scraped away to not have it localised. The reason for the third chamber is also tat the internal volume is relatively precious, so that final port could allow thebox to be in the rear and then the port can pump into the back of the cab.
and lastly I have a 300/500W (4/2-ohm) sub, so any help in box gain is great... and the truck has a lot of low frequency noise.

Those boxes with resonant chambers and the cones exposed to the air seem to get the resonant gain, but all the IMD and harmonic distortion can jump off the cone into the room. That defeats the concept of scraping away the higher orders of harmonic distortion.

And then all I usually hear is, "those need to be exact" and little is evident in describing how they work and tools to design them.

Plus, like most people do... I got subs, and only now realise I probably got the wrong sub for the application.
(it is easier to preach, than to practice it)

Car audio wouldn't be fun without challenges!

And now I get what you are doing, and I think for Justin's thread purposes - we're probably at where the purely technical engineering stuff ends - I'd hate to see his thread cluttered up with "what box should I build" threads...

But YOUR two needs here sound like unusually interesting and fun "what box should I build" threads:
1) What box should I build for a real subsonic experience in my house, for music AND movies?
2) What box should I build for my truck - help me save the subs I already bought!
So create them. In those threads you can move off the technical engineering and into the practical install-specific.
...not that Justin wouldn't want to participate in those too, but that will avoid de-railing his thread here.