Another sub question...

[CJ Paul]

What happens when you put multiple sub drivers in a small sealed box. Can you even do this? What about a push pull arrangement. Would you get increased output? Lower distortion etc.? Those are my general questions, here are the specifics.

As many of you may know, I ordered some drivers for an IB which cant be built until I move. The new house will be done in early July but we could be moving any time because selling our current house if of key importance.

That being said, I have asked about building a dipole sub etc. but after careful consideration, it would probably be to big for my room if I use the right baffle size and could be a waste of time and money if the house sells a week after getting it built.

But, I still would like to play around with the drivers if I could, so I thought about putting two of them in my current box (130L sealed box). What will happen if I do? Is this going to kill my drivers or explode the box? I would put the drivers on opposite walls of the box as the wall opposite the current driver could have a hole cut in it with no problems and no needs for mods to the current bracing design. What happens if I put two drivers in there and wire them electrically in phase? What happens if I put two drivers in there and wire them electrically out of phase but then they'd be in a push pull configuration.

Am I totally crazy, should I just test the drivers and then put them away? What would I gain by putting two of them in the current box?

[Andrew Pratt]

If you wire them up properly they can be used to cut the required volume in half...to model this just divide the VAS in half and pretend you only have one driver. You won't gain output doing this but you can made those 15's fit in a much smaller box. The alternative is to just wire them up so they both work together...this likely isn't going to work for you as its no different then a single driver so you really need 2X the volume as required for a single. Also since size is likely to be an issue you don't technically have to mount the second driver inside the box...you can clam shell them together on the outside. I think ThomasW still has his dual shiva project on his site that did that. It looks odd but works quite well and would save that box for other duties if that's an issue...plus it doesn't take away any internal volume from an already small box.

[CJ Paul]

Yeah, I did some more research since I posted this, and it looks like I could pretty easily do a true Isobarik design. It would give me more output down low. My understanding is that you lose 3dB with an isobarik design but you gain the power handling of the second driver, so you have double the power handling. This of course would affect the sound in more than one way. First of all, I think it would lower distortion. Second, isobarik allows you to use a box that is half the ideal volume, but if I use my current box and do an isobarik, its like doubling the volume of that box, which would impact the output obviously.

[CJ Paul]

Here it is...

http://home.comcast.net/~klone-audio/page28Rava2-1.html

[ThomasW]

These kind of subs sound very good.

The only drawback is that they're very 'tippy' with the one driver cantilevered out in space.

Also they collect a ton of dust unless one makes a special stretchey fabric grill cloth.

As pointed out, there is no gain in output using the additional driver. The power handling is doubled since there are twice the number of voice coils.

[CJ Paul]

OK, hmmm.... still just musing here. I wish I had more time. I could look at this way. If I build a totally new enclosure, the house will sell the next day I'm not sure what I want to do I guess. I guess if I want to play around, I have three options.

1) box stays as is, clamshell a second sub and make a true isobarik. I gain no output but I reduce distortion because two subs are doing the work. Actually, I lose three dB but I double my power handling getting me back to square one.

2) cut a hole in the opposite side of the box and mount a second driver in it (basket out probably). Wire it so that it is mechanically in phase with the first. This would give me a push-pull configuration. I have NO idea what this does at all. Anyone know?

3) cut a hole in the opposite side and mount a woofer (probably basket out) in it mechanically out of phase with the other. This would give me a bipolar sub. I have no idea if this is good or not, and frankly, the box is probably too small for this.

4) cut a hole in the opposite side and mount a woofer (basket in) and then clamshell a woofer to each one, making a dual isobarik dipolar sub in a 130L sealed box and likely exploding it in the process...

[Paul H]

and likely exploding it in the process...

We want video if you're going with option 4

Paul

[CJ Paul]

Hehe. I'll make sure to get behind the blast shield with the camera.

[ThomasW]

Non-isobaric mounting dual woofers means that the box will be overly small. That means a LT circuit and big amp will be needed to get low bass. You could try boosting the LFE with a BFD, but that's not the same as an LT circuit.

Reversing the mounting of the second driver really has little audible impact.

4 driver (dual isobarics) in one box don't get you output equal to a 4 times bigger box.

[CJ Paul]

Hehe, the last one was kind of a joke. I'm floundering here. I want to play but now is a bad time. I appreciate you guys humoring me. I'll post pics if I do anything.

[Brian Bunge]

CJ,

I modeled your box with two of the Stryke IB's last night and the response didn't look good. From what I remember, it was very peaky in the 40-60Hz range and the Qtc was something like .83 or so. Of course, there really wasn't any low bass extension.

The up side is that it should rattle the fillings out of your head!

[CJ Paul]

Yeah, I dont think two subs in that box is going to work. I've modeled it too. It basically scrunches up the low end of the curve, so not only does it remove some of the low end, it also causes a peak right before it drops off. That's probably that ~50hz area you are talking about. I didnt even look at the Qtc yeilded with that arangment. How would an isobarik loading affect the Qtc?

[Jack Gilvey]

Yeah, I dont think two subs in that box is going to work. I've modeled it too. It basically scrunches up the low end of the curve, so not only does it remove some of the low end, it also causes a peak right before it drops off.

You just described a high Qtc.

How would an isobarik loading affect the Qtc?

It'd lower it. Same as doubling the box size.

Here's what each option looks like in your 130L sealed box with some stuffing. Single driver vs. an isobaric pair. I input 100 watts to each driver, so it's 200 watts into the iso pair at 8 ohms:

[CJ Paul]

Well, I modeled it with Unibox yesterday and that's exactly what I got, so its good to know that I'm at least learning something with all of this. Someone pointed out in an isobarik FAQ that I found somewhere that when you use an isobarik load, you decrease efficiency but double power handling so you can get yourself back to the same level of output, but, at that output, the two cones will each be moving twice as far as they would have been to produce that output in a conventional single woofer design. Is that accurate? If so, doesnt that mean that you are going to run out of excursion/headroom sooner with an isobarik?

[Jack Gilvey]

Someone pointed out in an isobarik FAQ that I found somewhere that when you use an isobarik load, you decrease efficiency but double power handling so you can get yourself back to the same level of output,

You decrease efficiency since one driver is there purely to reduce Qtc, it doesn't contribute to output at all. If you're inputting 200 watts total, then 100 watts is being "wasted" in the other "non-heard" driver. You're technically doubling "power handling", but it's meaningless when considering isobaric, or with regard to output.

but, at that output, the two cones will each be moving twice as far as they would have been to produce that output in a conventional single woofer design. Is that accurate?

No. The output will be determined by the displacement of a single woofer, so max output will be the same for a single woofer or for isobaric (in which only the "seen" woofer contributes to output).

If so, doesnt that mean that you are going to run out of excursion/headroom sooner with an isobarik?

You won't run out of excursion any faster as I explained above, but you may run out of amp headroom sooner if wattage is fixed. With an Re of 5.6, a plate amp may be able to handle a pair in parallel, so you might get some gain there.

[CJ Paul]

Thanks. I have an amp that the manufacturer confirmed over email is stable into 2 Ohms but it doesnt double down. Its 250 rms at 4ohms and 400 rms at 2 ohms. So power is technically slightly in favor of the single driver as I dont have enough power to gain back ALL of my lost output. Or, I could drive each driver with the two spare channels of my main amp and feed them each 240 x 2 at 4 ohms but at that point, 80 watts is probably not going to make a huge difference, and the wire is already routed from my monoblock to the sub location.

[RobP]

I have done many isobaric designs in the car audio enviroment, you can mount them face to face, back to back, or the best space saving design is to mount them magnet to cone face. in this application the second woofer is basicly buried inside the cabinet and is not seen, then the other is mounted directly above with a space of basicly 1\3rd the drivers diameter away from the cone of the bottom woofer. Of course every woofer is different so the box size will vary. They do sound really good. Very tight and controlled, but they do take more power and if you are into "boom" and higher SPL dont go this route. good luck