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PostPosted: Wed May 15, 2013 8:13 am 
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Joined: Mon Oct 22, 2012 2:11 am
Posts: 1
Location: Utah, USA
I'm hoping to get some input on the front end of my mastering room build as I have a few unusual solutions for flush-mount monitors and acoustic treatment.

Below you'll see a few images of my ideas. The monitors themselves are a mild variation of some well-known and respected monitors on the market, and uses the same drivers and volumes. The baffles themselves make up the angled sections of the front wall. Knowing that the baffle itself could become a radiating surface, it will be constructed of 1.75" mdf, will be braced against the box in several points, and the whole enclosure assembly will be bolted to the concrete floor and concrete wall behind with steel members, which will be filled with sand. It's not moving.

The monitors themselves can't be stuffed all the way into the corner as that puts my listening position in the middle of the room. In order to make room for a couple pieces of gear, I'd like to stay away from the usual RFZ approach of continuing the flared wall on the outer edge (1st reflection points will still be treated), so I've returned the wall surface to parallel to the front wall. This is where my knowledge gets a bit fuzzy on implementing this design. Strictly speaking, this isn't the 2π space with further reinforcing angles that we're looking to achieve with flush-mounting. There could be some mild baffle-step on that far side. Does anyone know of research on single-sided baffle step diffraction? Any experience with this in their designs? I've seen Phillip Newell do something like this, but in wider rooms with wider baffle walls.

You'll see that I have two variations for the baffle width, one at 2.5', the other at 3'. Thoughts on how the 6" difference could play into my solution? With the 2.5' baffle, the drivers are offset in the baffle. This won't be an issue until the very low-mids, but given the baffle width and pseudo flush-mount this could pull it into the realm where sound loses its directionality. Again, thoughts? Formulae to calculate the potential affected frequencies and attenuation? I'm considering the narrower baffle because it allows for some rackspace in the wall and I'm hoping to use that as my 'machine room,' as it leaves my gear accessible, surrounded by sound damping, and vented to a small utility room in an adjacent room.

Additionally, the adjoining wall surfaces are intended to be binary-amplitude-type diffusers with heavy bass-trapping behind. You can see a cross-section in the third image. Considering the angle and relative density of the materials (.5 mdf and 3~5lb/Ft2 fiberglass) I'm anticipating that diffraction will keep the low waves from the speakers skipping across the surface instead of getting sucked behind the wall. Again I'd like to hear thoughts and math.

I appreciate your input. I read the Everest book years ago, and a few perusals therein during my planning process over the recent months hasn't refreshed my memory on solutions to apply to this. It doesn't help that baffle-step, SBIR, and horn effects of angled walls remain topics of current research and discussion. I've read articles by Lachot, King, and others, but can't find anything concrete. I'm hoping you have.

Thanks again.


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PostPosted: Wed May 29, 2013 2:39 am 
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Joined: Thu Aug 21, 2008 10:17 am
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Location: Santiago, Chile
Hi there "Corusco", and welcome! :)

From what I see, you are planning to build the entire soffit as a speaker, with the drivers alone embedded in the front panel, which then becomes the entire front baffle of the speaker/soffit. That's an interesting concept, and I do recall that another member here was planning something similar a while back, but not sure if he actually did it or not.

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The baffles themselves make up the angled sections of the front wall. Knowing that the baffle itself could become a radiating surface,
Not "could become", but rather "most definitely will be". There's no doubt about that. For low frequencies, the baffle IS the radiating surface, or at least it is the surface that forces half-space radiation, preventing full space radiation.

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and the whole enclosure assembly will be bolted to the concrete floor and concrete wall behind with steel members, which will be filled with sand. It's not moving.
That is a bit worrying, since it is the opposite of what a normal soffit is supposed to accomplish. With a normal soffit-mounted speaker, the idea is to fully decouple the speaker from the front baffle and form the structure, by isolating the original speaker cabinet, allowing it to "float" independently, such that it cannot transmit vibration into the floor, building structure or baffle: What you propose is the exact opposite: firmly connecting everything. I suspect that you would end up getting "early-early" sound arriving at your ears BEFORE the direct sound from the drivers, due to transmission and re-radiation from surfaces closer to your ears than the drivers are. I would suggest that you re-think this approach, and instead try to de-couple the section of the baffle where the drivers are mounted, in the same way as for a more traditional approach.

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I'd like to stay away from the usual RFZ approach of continuing the flared wall on the outer edge (1st reflection points will still be treated), so I've returned the wall surface to parallel to the front wall.
I¿m not so sure that's a good idea: that discontinuity is going to create edge-diffraction artifacts in the mid-lows, belw whatever the baffle-step frequency turns out to be for the soffit width. You would lose part of the big benefit of soffit mounting like that.

Quote:
I'm anticipating that diffraction will keep the low waves from the speakers skipping across the surface instead of getting sucked behind the wall.
... where they will happily hit the side walls of the room, this creating a second set of edge diffraction artifacts, and also interacting with their own reflections from the resonant "front" section.

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It doesn't help that baffle-step, SBIR, and horn effects of angled walls remain topics of current research and discussion.
Baffle step issues and SBIR are fairly well understood these days, and easily calculated, at least for typical rectangular rooms and also for traditionally soffited speakers. It can also be done for more complex room shapes, but you need to resort to FEM/FEA for that, using a good operator who can set the boundary conditions intelligently, and interpret the results correctly. That can be pretty costly...

- Stuart -

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