Duet10SS - A new build

[Bear]

Folks that know me over at AVS Forums (userid: Bear5K) know me as more of a video guy, rather than an audio guy. That being said, I've had the same pair of B&W monitors for almost 10 years now, and it's time for a change. I look at the space underneath my speakers where the stands are and keep thinking, "I could get a compact 3-way into that same space, and get rid of the sub AND get better sound." I had come across the Duet design in looking at Jed's site, and the more I drew and designed, the more it kept resembling the cabinet structure and driver choices in the Duet. So, a Duet it is.

One thing I wanted to do differently, though, is not to build the same speaker as everyone else. To that end, I was playing with variations on laminated designs using 18mm Baltic Birch and a local CNC shop, so the "twist" in this is going to be a rounded back-end on the cabinet that's a half-inch deeper than Jed's spec. I also wanted to put a bit more bracing into the cabinet because, well, I'm paranoid.

As I see it, this project breaks into five phases:

1. Design/Procure - Where I'm at now.
   
2. Manufacturing - 3+ sheets of 5'x5' 18mm BB ply get turned over to the CNC machine shop to manufacture the basic parts for the cabinet.
   
3. Assembly - Glue-up and wire-up. I guess this is really a "GIY", rather than a "DIY" project.
   
4. Test - Measure and tweak.
   
5. Operate - The fun stuff!

Where I am at today is in having a nearly completed (re-)design for the cabinet. The only tricky thing in the design is where to place the downward-firing 3" Precision Port. If anyone has firm numbers for ODs for the various parts of the port, I'd greatly appreciate it. I can always measure the thing once I get it in-house, but I'd rather get the design over to the CNC shop for a quote sooner, rather than later.



Ignore the dotted line in the post, above. It's an artifact from some spacing I was doing. More later.

Bill

[Clearwave]

You can do 1" facets or an angled chamfer like the above sketch. Note this sketch has a larger facet than what I used for the test baffle. It was 3/4"-1".

Here's where the port should be located in my estimation:

[Bear]

Is the vertex of the vertical facets even with the middle of the woofer? If so, I may ditch the CNC route, and just go get the sliding compound miter saw I've been thinking about. What does the varying facet depth do to diffraction and the as-designed BSC?

Bill

[Clearwave]

Is the vertex of the vertical facets even with the middle of the woofer? If so, I may ditch the CNC route, and just go get the sliding compound miter saw I've been thinking about. What does the varying facet depth do to diffraction and the as-designed BSC?

Bill

Yes, the vertex is even with the middle of the woofer. Looks sharp IMO. The diffraction pattern will be nearly identical with a 1" chamfer. Shown is 1.25" or so. That should be ok as well since the baffle width and driver locations are unchanged. If anything it will smooth the diffraction ripple a bit more. Like .25db or so. The varying section near the woofer will have no effect because the FR most effected by the edge transitions of the cabinet are the mid and tweeter frequencies. Around the mid and tweeter, the cabinet is nearly identical to the test cabinet. Hope this helps.

[Bear]

Is the vertex of the vertical facets even with the middle of the woofer? If so, I may ditch the CNC route, and just go get the sliding compound miter saw I've been thinking about. What does the varying facet depth do to diffraction and the as-designed BSC?

Bill

Yes, the vertex is even with the middle of the woofer. Looks sharp IMO. The diffraction pattern will be nearly identical with a 1" chamfer. Shown is 1.25" or so. That should be ok as well since the baffle width and driver locations are unchanged. If anything it will smooth the diffraction ripple a bit more. Like .25db or so. The varying section near the woofer will have no effect because the FR most effected by the edge transitions of the cabinet are the mid and tweeter frequencies. Around the mid and tweeter, the cabinet is nearly identical to the test cabinet. Hope this helps.

Sounds great. Shoot me some measurements for the facets, and I'll go ahead and add them in to my drawing. Should be fun learning to do kerf cuts on BB ply.

[Admin]

It's a 1" chamfer at the corners that tapers to nothing adjacent to the center of the woofer. 45 degrees.

[Clearwave]

Bill recently asked me about whether he could change the baffle to a very large chamfered/curved baffle and here's my response:

The more one deviates from the contour of the original test baffle, the more the diffraction signature changes, thus disrupting the original FR. Depending on the size, angle, and shape of your front baffle, they all will have an effect. I can't really predict exactly how much the response will change, but if the roundover/chamfer goes from 1 to 2 inches there will be a definite change. The audibility will depend on whether the result creates a dip or a peak in the response. Peaks are more audible, and dips are less so. Larger round overs tend to smooth the difraction ripple, and because I used some contour shaping networks in the crossover, I do not have a clear picture on how the eletrcial side of things in the crossover would interact with the acoustic response variations. In a design that is this precise, I personally wouldn't change much.

To answer your question. Let's take for example the area around the midrange. The effective baffle width is around 10" or so. If you did an avalon facet type enclosure- even though the box width is still 11.5" (depending on how much you bevel away from the front) the effective width will be much smaller than the original and the diffraction will be improved somewhat. But, not like the original response that was shaped by the crossover; thus, it deviates away from the intended response. So what was thought of as an improvement, is now not an improvement and the "upgrade" is essentially not working with the crossover to get the best response. How much change that occurs in the frequency response depends on a great many variables, all which can only be understood with correct measurements.