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Old 11th August 2008, 08:13 PM   #1
pjpoes is offline pjpoes  United States
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Default Bandpass, Alternative method for Low Distoriton Subwoofer

This isn't anything new, so please, I'm just trying to learn a bit more on this subject. While you often hear about ways of reducing bass distortion through linear motors, servos, cleaner amps, better cabinets, etc. I had recently noticed various documents touting bandpass having "potentially lower distortion" within the pass band. Dr. Geddes had also mentioned this, so if he chooses to comment further, that would be great. First thing I'm wondering is, what exactly is responsible for the lower distortion. Is it because the bass is acoustically coupled to the environment instead of mechanically, as with a direct radiator?

I often read that Bandpass designs are very hard to design correctly. However, there are many programs out there which can model bandpass designs. I have found 6th order to be especially hard to design, but 4th order not much more complicated than a normal ported design. my method thus far is to choose a sealed enclosure size which gives me roughly the low end extension I am looking for. Then I choose a tuning for the front chamber that gives me a roughly flat response, and adjust the volume until I have gotten rid of most or all of the ripple. I find that by adjusting these three parameters I can also adjust, roughly, the range it covers. It's efficiency seems to be a tradeoff between how broad a range it covers, and how efficient it is. The wider the range and lower the tuning, the lower the efficiency. Does this all sound right, am I designing them basically correctly (for a 4th order bandpass).

I have two drivers which I'm interested in trying in a bandpass box. The first one is the Dayton Reference 12" HF woofer. I noticed that the HO works better, having more output, greater extension, and works in a smaller box, but the HF is what I have. The one I'm thinking of is really designed for a little bit higher efficiency and a higher overall tuning. It will be optimized for the 40hz-100hz range. It would use a .7 cubic foot rear chamber and a .55 cubic foot front chamber with roughly a 60hz tuning for the front chamber. This means a pretty compact box overall, with decent output ability. I'm guessing a maximum spl, in that range, of roughly 112db's.

Mind you, tuned the same way as above (best that can be done), you can achieve similar results with the HO version in a total package less than 1 cubic foot, with peak output in a slightly broader range of roughly 117db's or so. In fact, the results look so good for the HO version, I'm very tempted to try another one using that driver, since in a slightly larger box, it can be made to have really good low end extension.

Oh yeah, one last question. How do you tune a bandpass design with a passive radiator instead of a port, and what are some important factors to consider with the passive radiator. For instance, is it going to need more excursion than you might expect if it was a typical passive radiator system? It seems like getting a passive radiator that can be tuned that high, 50-60hz, is not an easy task at all. I believe there are some from parts express, which may not have the excursion I need, and otherwise, I think I would need a custom one.
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Old 12th August 2008, 02:24 AM   #2
Pallas is offline Pallas  Pakistan
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Default Re: Bandpass, Alternative method for Low Distoriton Subwoofer

Quote:
Originally posted by pjpoes
This isn't anything new, so please, I'm just trying to learn a bit more on this subject. While you often hear about ways of reducing bass distortion through linear motors, servos, cleaner amps, better cabinets, etc. I had recently noticed various documents touting bandpass having "potentially lower distortion" within the pass band. Dr. Geddes had also mentioned this, so if he chooses to comment further, that would be great. First thing I'm wondering is, what exactly is responsible for the lower distortion. Is it because the bass is acoustically coupled to the environment instead of mechanically, as with a direct radiator?
As I understand it - and my understanding is intuitive rather than based on any formal training, so it could be dead wrong. So if I'm blowing smoke that means we're both going to learn something - a BP lowers distortion because the port by definition acts as a low-pass filter. (HP filter too, hence the name "bandpass," but I think that's less relevant in terms of distortion, simply because one rarely if ever hears of .5 order or .25 order distortion products.) Since the distortion products of a driver are higher multiples of the fundamental, a mechanical low-pass in front of the driver would by definition lower the magnitude of the driver's distortion products.

Quote:
Originally posted by pjpoes
I have two drivers which I'm interested in trying in a bandpass box. The first one is the Dayton Reference 12" HF woofer. I noticed that the HO works better, having more output, greater extension, and works in a smaller box, but the HF is what I have.
On the principle that no question is too stupid to ask out loud, I wonder how important the sealed chamber really is. That is to say, why can't one apply an LT (or just the appropriate EQ) to get the box resonance and Q wherever one wants it? I would assume the shortcomings are the same as in a sealed box with an LT, power compression. But lots of new drivers seem to manage heat better than what we've seen before (JBL's Vertec-derived GTi car subwoofers with their split-opposing coil motor and all that heat-sinking come first to mind) that it might not matter that much.

IIRC, the Dayton HF is a "better" driver than the HO, which is to say it has a longer throw and/or a cleaner motor. So why not just EQ the sealed chamber to do the same thing as the HF would?

Quote:
Originally posted by pjpoes
Oh yeah, one last question. How do you tune a bandpass design with a passive radiator instead of a port, and what are some important factors to consider with the passive radiator. For instance, is it going to need more excursion than you might expect if it was a typical passive radiator system? It seems like getting a passive radiator that can be tuned that high, 50-60hz, is not an easy task at all. I believe there are some from parts express, which may not have the excursion I need, and otherwise, I think I would need a custom one.
I think Mark Seaton is doing a sub like that for AV123. It's a really interesting question. Hopefully somebody who actually knows something will pipe in.
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Old 12th August 2008, 02:43 AM   #3
pjpoes is offline pjpoes  United States
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Ok this will sound really stupid, but for some reason, when I wrote this, I had inserted a highpass and low pass filter to simulate the effects of my "theoretical" bass management idea, and I forgot to turn it off before comparing to the HO. The HF has slightly less peak output, but about the same extension. It does need a slightly larger box to do this, but we aren't talking huge amounts. However, it turns out the HF actually gives the wider less peaky response (neither looks bad though).

I have modeled LT and even ELF in the bandpass to see how it would work. It works fine, and it does appear that the 4th order bandpass really can be treated like a sealed sub as far as that is concerned. However, I really don't like the dayton driver with LT, it runs out of steam really quick. Xmax sky rockets, and when you look at the apparent load power, its well past the pe rating of the woofer at 20hz. Instead a more customized transform circuit could be developed, I modeled one with different poles and Q's than Linkwitz suggests, which gives considerably less boost between 50 and 20hz, and helps reduce the effects of power compression. However, for the Dayton, I wasn't looking to make a sub that could do 20hz at 100dbs, it was designed to do 45 to 120hz at 115db's, and thats it.

My other driver, which is currently in a large ported box, is a one off custom TC3000 subwoofer from TC Acoustics. Now that driver, with its 30+mm of xmax and 2000 watt power handeling, would handle an LT circuit much better. I've been modeling a larg Bandpass design for this sub using a 4.5 cubic foot sealed section and 2-3 cubit foot front ported section, tuned to something like 35hz. I forget the exact specs I came up with, but it looked very nice, with around 120db's max output from 20hz to 50hz or so. Those are all models of course, but that driver matched its model pretty well last time around (The model was actually quite conservative down low).
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Old 12th August 2008, 02:56 AM   #4
Pallas is offline Pallas  Pakistan
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I wouldn't bother with the TC3k in a bandpass box. Actually, I wouldn't bother with it at all. I would try to sell it. With that much inductance, it's really not good for anything that reaches for high fidelity. Lots of people who want their cars to go boom (or who want their home theaters to sound big on explosions but who don't really care about the quality of the upper bass) would pay top dollar for it, though. (TC Sounds' old TC2+/TC1k, TC2k, and LMS were all pretty good, as were their old underhung drivers. Everything else, IMO, not so much.)

I think a bandpass with a reasonable xmax 12 (like the Dayton Reference or Peerless XLS/XXLS) should give plenty of output. Especially if you follow the Geddes placement of one in a corner, and two in randomized spots in the room at staggered heights.

What are you using to model stuff? I pretty much use Unibox through Paralells, though I'm annoyed that there isn't a native box modeling program for good computers (i.e. those running OSX) right now.
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Old 12th August 2008, 03:13 AM   #5
pjpoes is offline pjpoes  United States
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I mostly use WinISD, though I have a specific bandpass modeler, unibox, and various others i will compare with.

The TC3000 I have has much lower inductance than the standard model and was designed for a lot larger box. It actually measures really well, and I enjoy its sound. I don't find it boomy at all. I've never heard the other TC3000, so I can't compare with those, but this one really isn't bad.

Keep in mind too that this sub is being used over a very limited low bass range, I really don't think the driver inductance is much of an issue there. None the less, measured with the WT3 and my ABT PC Pro, I got an inductance reading of around 1.2mh, so well within line with many high end drivers.

PA Sub This would be cool in a bandpass.
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Old 12th August 2008, 03:42 AM   #6
Pallas is offline Pallas  Pakistan
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Is that 1.2mH with all the coils in series (IIRC the TC3k had four 1ohm coils, so ~4ohms.)?But even if that's at 1ohm, it's way, way better than any other 3HP-motor based sub I'd ever heard of.

FWIW, I've found that subjectively (used in direct radiators, not bandpass boxes, as I've not built a BP) subwoofer drivers with a lower Le/Re ratio seem to sound better. Maybe that's just a correlation and not just a causation, but it's worked for me thus far.
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Old 12th August 2008, 03:47 AM   #7
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I have personally found that plots made by winisd, bassbox, and unibox don't seem to match the output I get from 6th order BP, but maybe 4th order BP won't be such a problem. Maybe akabak or something would work better?

Never seen that 'PAsub' before. Has anyone got experience of it as a replacement for the drivers it mentions?

Have you also tried push-pull to reduce distortion?

PS
OSX needs some nice audio programs
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Old 12th August 2008, 02:35 PM   #8
pjpoes is offline pjpoes  United States
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PA Sub is Dan Wiggins new company as I understand it (The old Adire Guy). Its a brand new company and I don't think the drivers are being sold at retail right now. Something tells me that he may not be willing to sell direct to consumer either, but prefer to sell only to manufacturers. Its worth an email I suppose.

Yeah I rechecked the number, I had that off, it was 2.3mh. However, as I said, this is not a normal 3000. First, it only has 2 coils, not 4. The top assembly is a combination of the TC3000 cone and surround, the LMS4000 spider and coil, and the TC3000 upper frame. The Magnet assembly I am less sure of, as the guys weren't clear about that, but I got the impression is was basically a prototype piece they had laying around. What I was told by the TC guys was that this was built to use up parts during the switch to the new company model. The performance would be lower distortion and more linear travel like an LMS, but not as good as an LMS. However, it would have greater output ability more like the standard models. The one drawback was that it ended up needing a lot larger boxes than the normal LMS or TC3000 subs.

The coils are wired in series right now, giving a 4.6 ohm load according to wt3. I don't recall how the coils were wired when I took the inductance measurement, but I think it was series. I need to pull the driver at some point soon, so maybe I will take fresh measurements. None the less, yeah, that is a lot lower than the 6+mh inductance of the normal TC3000 subs.
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Old 12th August 2008, 04:21 PM   #9
sreten is offline sreten  United Kingdom
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Default Re: Re: Bandpass, Alternative method for Low Distoriton Subwoofer

Quote:
Originally posted by Pallas


As I understand it - and my understanding is intuitive rather than based on any formal training, so it could be dead wrong. So if I'm blowing smoke that means we're both going to learn something - a BP lowers distortion because the port by definition acts as a low-pass filter. (HP filter too, hence the name "bandpass," but I think that's less relevant in terms of distortion, simply because one rarely if ever hears of .5 order or .25 order distortion products.) Since the distortion products of a driver are higher multiples of the fundamental, a mechanical low-pass in front of the driver would by definition lower the magnitude of the driver's distortion products.

Hi,

The above is correct, the mechanical low pass attenuates distortion,
the same electrical filter does not attenuate distortion products.

The downside is that varying the low pass is not easy at all.

/sreten.
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Old 12th August 2008, 06:13 PM   #10
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Quote:
Originally posted by aubergine
I have personally found that plots made by winisd, bassbox, and unibox don't seem to match the output I get from 6th order BP, but maybe 4th order BP won't be such a problem. Maybe akabak or something would work better?

Don't know about the others but winISD seemed very close when I last built a 'proof of concept' 6th order isobaric BP.

Drivers were infinity perfect 12.1's.

Rob.

Modelled vs real world (unsmoothed 1/24th oct)
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