Donut Sub (toroidal)

[skorpioskorpio]

OK, so I just started reading this forum. I started reading because I'm thinking about building a center channel speaker and a subwoofer to suppliment 2 pair of speakers I want to use for a surround system. Space is pretty specific as it's going to sit on a wooden latticework shelf (pretty heavy duty latticework) about 8" off an oak floor. I have 9.5 x 22 x 32in wide to put this and I'd also like to use all shielded drivers which seriously limits my options.

To build a balanced center channel that mimics the reponse of my current speakers (8" ported) I plan on going with a dual 6.5" setup for the center and use the same high end components thet the sides and rears use, I've done the WinISD model and got a config that models really close to the model of the 8s, same volume but stretching the aspect ratio and adjusting the port a bit. Mains and center I plan to roll off around 70-80Hz since they start to drop their flat reponse around there and that's where plate amps want to kick in on the low side anyway. Also 80hz doesn't put human voice range in the middle of a crossover point.

Anyway this leaves me about 9.5 x 14 x 32in wide to play with for the sub. Since I need to use a friends table saw, I'd like to keep trial and error to a minimum so I like the adjustable aspect to a passive radiator system and they seem to model a bit flatter than band pass boxes. I'd really like to hit in the mid 30Hz and be flat to the 70-80Hz range and seems the configuration that gets me that in the models is isobaric.

I'm thinking of building a 2 pair face to face isobaric manifold with passive radiators in a sort of donut shaped cabinet. So centered in line from side to side passive-out of phase woofer-in phase woofer-in phase woofer-out of phase woofer-passive with in phase and out of phase mounted cone to cone and in phase to in phase mounted magent to magnet.

Basically: > [<>] [<>] < if you catch my crude ascii diagram.

The 2 in phase drivers would share a downward facing slot. The space can accomadate another 2 inches on the top of the cabinet but you wouldn't be able to get the box into or out of the space then, so I was also thinking of adding granite slabs on top in that space after the box is put in to kinda force the downward firing aspect a bit more.

In the end you'd have a downward facing manifold port similar to the EV MTL series PA speakers (which are really tight fast and punchy for paper cone 18s) and opposite side facing passive radiators. Anyway it models really well when I model it in WinISD Pro with a parametric 5-6db boost at 37.5Hz which is a stock setting on the Bash plate amp I'd probably use.

So I know there are those on here that are not isobaric fans, but fact is that cabinet dimentions dictate that 8" drivers are what I can use and to get parametric boost I have to go to the bigger amps anyway so the efficiency loss is not that big a deal, really the amps that have the dimentions and the features I want to use will have stupid headroom in this application. The cost of the extra drivers is also not that big a deal in the grand scheme of things and with the shielded requirement there just isn't any big X-max drivers.

The closest thing I've seen to a design like this is in a post someone made that I read while scaning through here (which I can't find again). Although they refered to it as a push-pull-push-push configuration which I didn't get because it eludes to 2 face to face drivers being driven in phase which seems to me would do little more than blow the surrounds out of those 2 drivers. I also don't understand why there seems to be a lot of people that want to do cone to mag in-phase isobarics, is it just for aestetics? Seems like the airspace inbetween the drivers would eat up half of the volume reduction gain an isobaric gives you.

I don't see a lot of drawbacks to the design given the requirements. I've modeled it with 2 different drivers both Dayton. First is the Dayton SD215-88 Shielded DVC Subwoofer:
Dayton SD215-88 8" Shielded DVC Subwoofer | Parts-Express.com
and the other is the Cast Basket Dayton RS225S-8 8" Reference Shielded Woofer 8 Ohm:
Dayton RS225S-8 8" Reference Shielded Woofer 8 Ohm | Parts-Express.com
Both model similarly with the first one needing another db of boost at 37.5 to get where it needs to be. The cabinet works out to be a hair more than 1 cu ft. to put it in perspective. If I lost the slot and fired regular 8s out the back to gain as much volume as possible it'd only be about 1.2-1.3 cu ft and I can't get a model with that config and the drivers I have to choose from to get anywhere near mid 30s without bottoming out the drivers in the model.

This all leads to the actual question: Does this sound like a sane approach to the requirements I've made for myself? Or have I overthought myself into some far away place?

[jbell]

You know where you can find 8" PR's with enough excursion?

Personally I'd just use a quad of mcm 55-2421's in a manifold/ported box, or design a TH.

Kind of like this, but with an 8" driver.
Reed Exodus Anarchy 25hz Tapped Horn - AVS Forum

[skorpioskorpio]

I don't have the room for an 8" loaded horn of any kind of length, cool build though. Problem is that I do want it shielded and I just haven't found any suitable long throw drivers shielded. I thought about making a shield but it's probably more in materials than the driver cost to do it not to mention the welding or lathe work for what amounts to trial and error.

One thing about that horn build though, he used baltic birch plywood which considering the fact that I'm planning on putting an 8" driver in a 9.5in high cabinet may be a smarter move than MDF.

[skorpioskorpio]

I found the previous post I was talking about

Opposed drivers

In this thread Planet10 refers to his idea of a torus sub. This is pretty much what I'm looking to do but with the addition of passive radiators behind each of the enclosed halves of the isobarics.

[Cokewithlime]

A pair of these Tang Band 8" subs in a simple ported 1 ft^3 enclosure will get you a F3 of around 30Hz with a huge amount of output without the need for any EQ (which ultimately causes group delay and sloppy muddy LF) and also without even getting close to exceeding xmax.

[cowanaudio]

Huge output from a pair of ported 8"s??? Our reference must be VERY different!

[planet10]

Originally Posted by skorpioskorpio

I found the previous post I was talking about
Opposed drivers

Here is the picture:



Not shown are the feet to lift it off the ground, and a spaced off top to disguise what it is. All drivers wired acoustically in phase. Isobarik to halve the required volume. A torus to hide the magnets. A push-pull pair of isobarik modules loaded push-push.

Why do you need shielded drivers? Still have a CRT? Keep in mind that in the diagram shown, the middle pair of drivers are self-shielding.

Does your model consider room gain?

dave

[skorpioskorpio]

Originally Posted by Cokewithlime

A pair of these Tang Band 8" subs in a simple ported 1 ft^3 enclosure will get you a F3 of around 30Hz with a huge amount of output without the need for any EQ (which ultimately causes group delay and sloppy muddy LF) and also without even getting close to exceeding xmax.

Um, hmmm actually they do model impressively as a ported pair, I actually looked at those but passed thinking they were not shielded, but they are.... and apparently they like a *really* small enclosure, I could probably get 3 in the box and still be... you know I was all very content with my really cool complex solution to this dilemma and you, you just walk by and say, here use these...

Originally Posted by cowanaudio

Huge output from a pair of ported 8"s??? Our reference must be VERY different!

Actually, compared to my model with the isobarics, the Tang Bang drivers if given the same amount of eq tweeking look like they could get down in the mid to low 20s.

Originally Posted by planet10

Here is the picture:

Not shown are the feet to lift it off the ground, and a spaced off top to disguise what it is. All drivers wired acoustically in phase. Isobarik to halve the required volume. A torus to hide the magnets. A push-pull pair of isobarik modules loaded push-push.

Why do you need shielded drivers? Still have a CRT? Keep in mind that in the diagram shown, the middle pair of drivers are self-shielding.

Does your model consider room gain?

dave

So yea Planet10s design is very cool and the one I was refering to in my original post.

Do I still have a CRT? Come on? Do you really think in this day and age that... Ok ok you got me, yes I know it's the new millennia, dump the tube and get a panel, I know, but I've been busy you see? Yes, ok there is a tube currently in the cabinet above where I want to put this, a big heavy tube that I just had to move a couple months ago, that quite literally my friends chanted "dude, panel! dude, panel! dude, panel!..." the entire way as they helped carry the near 350 lb thing. However, when the panel does get there, the space behind it in the cabinet will probably occupied by a computer system...with disks that I'd rather not have sitting on top of a strong oscillating magnetic field.

Did I figure in room gain? Mentally yes, mathmatically no. The space where this will be put will probably have lots of room gain actually, it'll be 8 inches from a thick plaster wall at the rear, 8 inches above a pretty bouncy oak strip floor layed over gapped diagonal fir sub flooring and a couple inches under the cabinet, which would probably best be described as having more than a little in common with a large concert PA horn loaded base bin. The whole thing is a tower of boom sitting in front of a reflector, so yea there would probably be some room gain, but I have a German Shepherd that lays in front of all that so it'll probably balance out a bit.

[tb46]

Hi skorpioskorpio,

Just wondering, have you tried to calculate how much volume is left in your box after installing four 8" drivers and two 8" passive radiator and planet10's output coupling chamber?

Regards,

[Cokewithlime]

Originally Posted by cowanaudio

Huge output from a pair of ported 8"s??? Our reference must be VERY different!

The neo TB subs are not your typical 8 inchers - model them yourself in a ported box (EBS alignment). With a pair of them, if you have enough power on tap, you can get close to 112db with a F3 of about 30Hz using no EQ and without even getting close to exceeding their xmax.

I've used multiples of the 6.5" version of the same sub many times when dealing with the need for high output LF content in highly space restricted areas.

skorpioskorpio - btw you could use 4x of the TB 6.5" version and achieve the same as the two 8"s