Extremis 6.8 as Subwoofer

Milehighguy · « **on:** 21 Sep 2005, 08:24 pm »

I was thinking of possible options for a compact bedroom-sized subwoofer. I was wondering if two extremis 6.8 woofers mounted on opposite sides of a cabinet would work. Any thoughts on this? The unit would be sealed, and with an outboard amp. What volume would be appropriate? Maybe shooting for at least 25hz in-room response.

Russell Dawkins · « **Reply #1 on:** 21 Sep 2005, 09:41 pm »

I don't know about the Extremis 6.8s in particular, but I do know that the best sounding sub I ever built had two drivers back to back as you describe, although it was not a cube but a longish rectangular baffle. I think the configuration is inherently good sounding and if the drivers are very well matched in terms of free air resonance is, paradoxically, inherently non-resonant, which is what I heard.
There was a description of a cabinet like this 30 years ago in Hi Fi News where there was an impedence plot shown through the entire bass range. It was almost dead flat - no indication at all of the usual peak at resonance, which to me was surprising confirmation of what the designer said and what I heard. Also surprising was the incredibly small cone movement when outputting large quantities of bass. Self damping, indeed!
One major caveat: this only works reliably when playing a true mono signal, so you would need two for stereo. I tried it with one woofer receiving left and the other right and it didn't work as well at all.
Russell

Milehighguy · « **Reply #2 on:** 22 Sep 2005, 06:21 am »

Thank you for your reply, Russel. I think the opposing woofer setup is a good idea to somewhat cancel vibrations from the drive units, and of course as you point out, this would only work as a mono configuration. Another advantage to this setup is changing the impedence from 8 to 4 ohms, and therefore greatly increasing the power output.Also twice the cone area and less distortion vs. a single driver.There are a few commercial models using this kind of configuration. I was hoping that someone with more technical expertise than myself could run the numbers through a software program and help me come up with an appropriate size for a cabinet. And I was wondering if there is any reason why this driver would not be a good idea for this application.
I have my eye on three possible pre-made enclosures. One is .67 cubic feet, one is 1.34 cubic feet, and one is 2.01 cubic feet. Can anyone tell me how any or all of these enclosures would work?
If what I'm asking is too much work to bother with for a casual question, I'll need to do some investigating and see if I can figure it out myself.
In that case, if someone could direct me to where I could find that kind of information, I would appreciate it.

Russell Dawkins · « **Reply #3 on:** 22 Sep 2005, 07:09 am »

Part of what I was implying in my response was that as this configuration, (as long as the speakers are symetrical in relation to each other, not necessarily the box) is inherently non resonant, so that "tuning" doesn't come into the picture. The larger the box, the lower the bass extention. The box I used was "too small" for the drivers, by acoustic suspension or sealed box standards, so the extension was not great, but the quality of the bass - the naturalness of it - made me not care much, and I'm a bit of a bass freak. Resonances in the bass range = mud in my view and hearing bass without them is a treat, plus I found it endlessly amusing to see how little the cones moved in the process of producing hefty outputs. These were ten inch woofers - CTSs. El cheapos.
Russell

Kevin Haskins · « **Reply #4 on:** 22 Sep 2005, 02:07 pm »

Quote from: Milehighguy

I was thinking of possible options for a compact bedroom-sized subwoofer. I was wondering if two extremis 6.8 woofers mounted on opposite sides of a cabinet would work. Any thoughts on this? The unit would be sealed, and with an outboard amp. What volume would be appropriate? Maybe shooting for at least 25hz in-room response.

You could make is small.... say less than 1.0 ft^3 and use a LT circuit to EQ it for whatever extension you want in room. I'd say an 25Hz f3 in-room should be easy to obtain. I'm not sure what SPL you will reach.... will depend on the room. You can use one of the Adire ADA-300 amps which will allow you to do the EQ, have plenty of power and give you a couple bands of PEQ to boot.

That would be my suggested set-up for a small sealed box with a pair of them.

Milehighguy · « **Reply #5 on:** 22 Sep 2005, 06:02 pm »

Thank you for your input, Kevin. I'll mull it over some more. maybe I'd be better off just getting the 61's and not use a sub at all.

kfr01 · « **Reply #6 on:** 22 Sep 2005, 07:03 pm »

I agree 100% with the decision to just get the 61's. For stereo listening, a subwoofer is simply not necessary with the 61's. I am still amazed every time I listen to them.

Kevin Haskins · « **Reply #7 on:** 22 Sep 2005, 07:11 pm »

Yeah... a pair of 61s with the Hypex amplifier provides VERY nice bottom end in everything but the very largest rooms.

ryno · « **Reply #8 on:** 23 Sep 2005, 01:33 am »

A bedroom system was tough for me, small room and no flexibility in speaker placement or listening position. Kit 41's in the recomended ported boxes were way to boomy in my room. With bigger sealed boxes and a small 8" sub, I was able to use the xover point to counteract the biggest peak, and got much more even sound. Maybe the 6.8's with a plate amp and a little eq might be better. But, my room is almost a cube 10x12x9 and I had no placement choices.
Ryan

Milehighguy · « **Reply #9 on:** 23 Sep 2005, 06:14 pm »

The reviews of the 61's focus on their bass and treble, but not much is said about the midrange. Any comments about the mids from owners out there?

Also, I still may at some time make the extremis sub with two drivers. Hopefully one that is reasonably flat without EQ. If anyone can give me feedback as to how that would work in a 2cf. enclosure, I would appreciate it. Or info as to how one extemis in a 1cuf. enclosure would work, and how much damping material to use (then I would just double the amount for a 2cf box). Thanks.

When the design options for the new sub drivers are released, I'll be looking for a sealed design that can be doubled to make a bi-polar woofer for my HT system. I think it's the way to go. The only problem will be that with the low impedence, they may need to be wired in series.

klh · « **Reply #10 on:** 23 Sep 2005, 06:28 pm »

An easy thing to do is get a couple stereo 41s and have a two 6.8 sub placed at a more appropriate location for bass. It's a little more pricy but has more options, particularly with eq and phase. I'm thinking about doing that in the master bedroom.

Milehighguy · « **Reply #11 on:** 23 Sep 2005, 06:49 pm »

That sounds like a great idea. If you come up with a bi-polar 6.8 sub design, please post the details and results.

mcgsxr · « **Reply #12 on:** 23 Sep 2005, 07:11 pm »

I have been looking at those woofers, and thinking about using them with my open baffle Visatons.

The great luck I had building up the KIT41, and with using the FR125S from these guys, builds my confidence that further investments in their drivers would further please me!

I will sit on my budget for a while though, with the new baby coming soon...

Bob Reimer · « **Reply #13 on:** 23 Sep 2005, 07:49 pm »

Dave and Chris in Victoria are working on some TL and bipole designs with the Extremis/Extremii. As soon as they are available they will be posted.

Bob

klh · « **Reply #14 on:** 23 Sep 2005, 08:21 pm »

A TL with the 6.8 would be very interesting. Please report findings and details on enclosure measurments.

Milehighguy · « **Reply #15 on:** 24 Sep 2005, 06:26 am »

Great! That's what I was looking for. Is it possible to combine TL and Bipolar?