Dual SDX15 Project

**Geoff St. Germain** · **07-30-07, 04:46 PM**

I received my two CSS SDX15 from Bob a couple of weeks ago. I think that I've finally settled on a design for them. At first I was strongly contemplating a sealed design, but I think that the additional extension of an LLT better suits what I want out of this setup. The plan is for this to replace the lower section of my TV stand. Hopefully the opposed drivers will sufficiently reduce vibration for this purpose.

The enclosure is about 450 L tuned to 16 Hz, which is a little smaller than the enclosure recommended by CSS (they recommend about 240 L per driver, so 480 L). The external dimensions of the enclosure are 44" L x 34" H x 26.5" D.

Some pictures:

Any input?

Hopefully I'll get started on this sometime next week.

**Exocer** · **07-30-07, 11:05 PM**

Looking awesome! It models really well in that size enclosure.

**Bent** · **07-31-07, 08:22 AM**

Geoff, I think I'd try to tie the front and rear panels together too.
To me, it seems like these are pretty large un-supported panels, they might be subject to a bit of vibration.

Opinions?

**Geoff St. Germain** · **07-31-07, 12:30 PM**

Bent,
My initial thought on that was that the drivers are mounted perpendicular to that axis and those are also two smallest faces.

Anyone have any thoughts on this?

**Geoff St. Germain** · **08-16-07, 09:41 PM**

Alright, I picked up all of my plywood today and have begun building. Here is the finalized design:

It's 420 L tuned to 16 Hz with a 3" x 23" slot port that is 42" long. The external dimensions are 44" (L) x 24.5" (D) x 33.5" (H).

I'll have some progress pictures up tomorrow.

**WillyD** · **08-16-07, 11:05 PM**

I dig it. Very functional as a TV stand while performing very well. Good call on that slot port. Air speed looks to stay low enough and 1st port resonance isn't too low. Sweet. Are you gonna power this one with the Crown XTI amp you already have?

**Warpdrv** · **08-16-07, 11:15 PM**

Sweet.... Im really looking forward to seeing your photos, and build progress....

That box is really close to the size I would like to do... if not a bit over sized.....

**Geoff St. Germain** · **08-17-07, 12:16 AM**

WillyD,
Yeah, I'll be using the XTI-2000 for now. It doesn't really have enough power to drive these things to their limits in this enclosure. From my modeling, it should be able to take 2500 W or so, which is about 1000 W more than I'll get to them out of the XTI-2000.

The slot port provides a lot more ability to fine tune to port area compared with round ports. I also like the way it acts as bracing.

**Geoff St. Germain** · **08-17-07, 04:13 PM**

Well, I have all of the pieces cut, now it's just a matter of piecing it all together.

Here's the start:

**Bent** · **08-17-07, 04:28 PM**

good lookin' roundover there on the bottom of the port :tup

**Exocer** · **08-17-07, 05:46 PM**

Really nice wood! I'm envious/

**Geoff St. Germain** · **08-17-07, 09:46 PM**

Thanks guys. Here's the rest of the work from the day. Built and installed the rear wall of the port. The small cut you can see is a 1/8" recess so that the quarter section of sonotube I'm using to make the curve in the port will sit flush.

**Warpdrv** · **08-18-07, 04:20 PM**

Wow... Looks great... is that the Birch Ply..?
I didn't realize that you were putting both drivers in the same box...
I was looking for 2 seperate boxes, single driver in each...

I was wondering how you were going to make those gradual curves... great idea with the 1/4 sonotube...

keep posting those awesome pics...

**Geoff St. Germain** · **08-18-07, 06:49 PM**

Yeah, dual opposed drivers in the same box. I want to have as little vibration as possible for my TV.

Yeah, it's birch ply, it is called Lion King Birch 3/4" Cabinet grade. I need about 3.5 sheets of plywood for this enclosure, and at about 90 lbs per sheet for MDF there it was going to make for a finished enclosure pushing 400 lbs. I'm not entirely sure how much weight saving there will be, but a sheet of this plywood sure seems a heck of a lot lighter than a sheet of 3/4" MDF. I'd built my last two subwoofers with MDF so I figured I'd try plywood. So far the main thing I'm noticing is none of that MDF dust that gets inside and over everything when you shape MDF. That alone is almost worth the difference in cost.

Here's today's work.

Mock fitting the top of the port:

**WillyD** · **08-18-07, 10:20 PM**

Great work so far, Geoff.That birch-ply looks nice.

**Geoff St. Germain** · **08-19-07, 04:56 PM**

Well, the port roof and front/back are on. Now I have to get all the bracing in before attaching the top.

**Geoff St. Germain** · **08-20-07, 09:13 PM**

Well, the box is all sealed up now. It's just a matter of sanding, painting and installing a few odds and ends (feet, handles, binding posts, drivers

).

**andersonbc** · **08-21-07, 10:20 PM**

Looking killer ... I have been thinking about a very similar design for a few weeks now (box design with slot port). I'm very interested to hear how it sounds!

**brucek** · **08-22-07, 04:43 AM**

Looks like a great job.

You're using magnet supports. Do the drivers touch at the back once they're installed or is there a gap?

Can you tell us a bit of the basic theory why the opposing drivers help to reduce box vibration?

brucek

**Ilkka** · **08-22-07, 05:28 AM**

Quote:

brucek wrote:

Can you tell us a bit of the basic theory why the opposing drivers help to reduce box vibration?

brucek

It's the Newton's third law: To every action there is an equal and opposite reaction.

http://en.wikipedia.org/wiki/Newton's_laws_of_motion

When there are two drivers, net forces are cancelled out, leaving no forces at all. That means the drivers won't transmit any vibrations to the box.

I can say that it works great!

**Geoff St. Germain** · **08-22-07, 08:07 AM**

Quote:

brucek wrote:

Looks like a great job.

You're using magnet supports. Do the drivers touch at the back once they're installed or is there a gap?

Can you tell us a bit of the basic theory why the opposing drivers help to reduce box vibration?

brucek

Thanks Bruce.

There's a 3 7/8" gap between the drivers. That isn't a special number, but there should be at least enough of a gap to allow proper venting of the pole piece vents.

Ilkka covered the other part.

**andersonbc** · **08-22-07, 02:58 PM**

Pardon me if this is a dumb question

:

So, one of the subs isn't wired out of phase for a push/pull configuration? It seems to me that a push/pull would not cancel out forces. If they are not push/pull is there any loss because the drivers push against each other?

Thanks in advance for the explanation.

**Ilkka** · **08-22-07, 04:20 PM**

Quote:

andersonbc wrote:

Pardon me if this is a dumb question

:

So, one of the subs isn't wired out of phase for a push/pull configuration? It seems to me that a push/pull would not cancel out forces. If they are not push/pull is there any loss because the drivers push against each other?

Thanks in advance for the explanation.

They have to be in-phase, push-push configuration. But they are not pushing against each other per se. They are only working in the same air volume.

**andersonbc** · **08-22-07, 09:02 PM**

Thanks Ilkka.

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