Since they WILL end up surrounded with insulation, aside from that back channel column allowing the vertical movement of hot air, it seems like the heat would keep building up inside that semi enclosed box and eventually cook them (even if the back of the unit has fresh air flowing and is kept at good temp). Can I build an oversize box inside that front wall, and have the bevel be tight against the front face of the speakers like regular soffit recommendations? does it still behave like a normal soffit installation? I'm thinking 2x or 3x the size of the speaker...
The way I do it, there's about an inch of airflow space all around the bottom and sides of the speaker cabinet itself, plus about 3 inches behind the rear panel, fed by a huge hole up through the shelf. There's no shortage of airflow. I create a "suspension" system that "floats" the speaker inside the enclosure box using Sorbothane rubber pads for separation. I carefully calculate the tensions, pressures and deflections in the Sorbothane to ensure that the speaker floats down to very low frequencies, well below the cut-off (or cross-over, if I'm using a sub as well). So there's plenty of space for air movement all around the cabinet....
Like I said: "When done correctly...."
The switch to turn them on and off is on the back of the unit. So I'd have to look into making some sort of jig-stick or something to poke into "some hole" (probably not the very slim opening between the the front of the speaker and the bevel (5mm iirc) ... or maybe some RC servo motor finger??? ... or maybe live with switching them on and off from an interrupter in their mains AC power supply (feels like it could damage them??)
There's no need to invent any fancy way of turning them on and off. Just have an external switch for that, probably on your desk, along with all your other gear. Use something like a Furman power controller for that: Turn everything on and off from one point. Or just hard-wire them into the distribution panel, on their own breaker.
Finally I examined them more closely and remembered that they are Ported (pretty big hole) in the back... It feels like that Port is part of their tuning and sticking the speakers inside soffit box would throw off their phase alignment or other important params. Alternatively one could say I could try plugging the Port, but what effect would that have on their sound?!
Take a look at the thread I linked you to above, for Studio Three (previously called "Monstertrax"). Those are Eve Audio SC-408 speakers in there: they are rear ported, just like yours. And they are soffit mounted! And look at the acoustic response graphs for that room... It would be very, very hard to get better response than that!
The bass reflex port on a speaker is NOT part of the tuning of the speaker: It's actually part of the "de-tuning"
The only purpose of a bass reflex port is to extend the lower end of the frequency response, such that the speaker seems to be able to produce frequencies lower than it actually can. It works because it is a tuned port: It is basically a Helmholtz resonator, in reverse. It resonates at a certain frequency, which was set by the designer. So for example, of the speaker by itself without the part was able to get down to 40 Hz (hypothetically), then the designer might choose to set the resonant frequency to the bass reflex port to 34 Hz (hypothetically), thus apparently "extending" the response by a couple of notes. The reason it works is because even though the "cut-off" for the speaker by itself was 40 Hz, it is still putting out substantial energy below that (the frequency response curve "rolls off" slowly), so there is plenty of energy to trigger the resonance at 34 Hz. And when that happens, the resonance "appears" to add even more energy, thus seeming to push up the volume at that frequency: it makes the tones below 40 Hz seem louder, and thus the speaker seems to have a better frequency response than it really does.
But it's all fake! I mean, it works, but at a cost! Since that is a RESONANT device, it takes a short amount of time for the resonance to be established, and it continues for a short extra time after the driver stopped producing that note. So in reality, it "smears" the note over time: It sounds louder, yes, but it loses "tightness" because there's a delay while the resonance builds, and another delay while it dies away again. The times are very short, yes, but still long enough to make a difference. That's why reflex speakers never seem to sound as "tight" in the low end as sealed speakers. In fact, many engineers complain that they can hear the resonance itself happening, and there's even a semi-technically term for that: "chuffing". It can also produce what many people call "one note bass", where all the bass notes seem to play at the exact same frequency; so as your bass player runs up or down the scale, it all sounds like one single note, repeated. And it can also sound "boomy".
So what happens if you damp that port? What happens if you put some insulation in the end of it, such that the resonance is killed? Well, what happens is nothing!
The resonance does not happen, there is no "smearing" of the bass, and the frequency response returns to roughly what it would have been if the bass port was not there at all. So you lose a bit of intensity in the low end... but that's no big deal at all if you soffit mount your speakers! One of the major benefits of soffit mounting is that it restores the power imbalance, increasing the entire low-end response by 6 db! And it does so naturally, without any resonance, smearing, or "chuffing" or "booming". The reason for this is also interesting:...
For any object, tones where the wavelength is significantly smaller than the object will get reflected off it, and notes where the wavelength is significantly larger than the object will be partly reflected and partly wrap around it: For a speaker, that means that notes where the wavelength is smaller than the dimensions of the box will only go forwards, into the room, and notes where the wavelength is larger than the dimensions of the box will go out on all sides. So high frequencies will be focused in one direction, like a spotlight shining at you, and lows will go out in all directions, like a balloon expanding around the speaker. But what does that mean in terms of power? It means that ALL of the power put out by the speaker in highs goes towards your head, but only HALF of the power in the lows goes to your head: the other half goes behind the speaker: it wraps around "backwards". So there is an inherent "power imbalance" between highs and lows. Only half the lows get to you. In decibels, that means that you get 6 dB less in the lows than in the highs (half the power). The point where things change over from "spotlight beam" to "expanding balloon" is governed by only one thing: the smallest dimension of the front panel of the speaker, which is technically referred to as the "baffle". The wavelength that corresponds to the width of the baffle is the mid point: all shorter waves shine forwards, and all longer waves wrap around backwards. So, since there is a jump in the frequency response curve at that point, this whole effect is called the "baffle step response", because it looks like a "step" in the frequency response. Actually, it's not really a "step": just a change in the curve.
To compensate for the "baffle step response" problem, all speaker manufacturers put twice as much power into the lows as the highs, and they have a cross.over circuit that carefully applies just the right amount of power to each driver, following the exact opposite curve of the "baffle step response" curve.
But soffit mounting does that for you! It solves the power imbalance problem naturally! It removes the baffle step completely, since the front panel of your soffit acts like a giant baffle that is bigger than all the wavelengths. Technically, it is called an "infinite baffle". And since it solves the problem naturally, there are no drawbacks, only benefits. And since you no longer need the complicated circuitry and double-extra power inside your speaker, you can turn that off. That's why most good speakers have a control on the back, called something like "room correction" or "bass roll-off" or "bass tilt" or something like that. It allows you to remove the correction that the manufacturer had to put inside, since you don't need it any more. It also means that the speaker runs cooler, since it does not need to put out twice as much power in the low end....
As I mentioned, the Eve SC-407s in that thread I linked you to are not rear-ported, and are not even supposed to be used that way, in soffits. But I spoke to the chief designer at Eve Audio, explained how I intended to do it, and he agreed with me that it would work just fine. He did not object, and even gave me some tips on how to do it more efficiently. A very helpful guy. And fantastic speakers.
So that's the looooooong explanation: You can soffit mount rear-ported speakers, if you do it right. You need to allow for sufficient cooling, but since the power amps only need to run at a lower level due to the power imbalance correction, they will be running cooler anyway. And if the bass reflex port is on the back, you need to damp it, in one way or another (not "plug" it! Just damp it). That's the only difference, as compared to other speakers. The only speakers that you cannot soffit mount, are ones that have extra drivers on the sides, top or bottom.
I used to think that it was impossible to soffit-mount rear ported speakers. Then later I thought it was just a bad idea and really hard to do. But now I know better: it can be done, and it's not that hard, once you understand the issues. The proof that it works extremely well is right there in that thread: Look at the acoustic response graphs, and you'll see how well it works! If you can find a room that has better, flatter, cleaner response than that, then I'd love to see it! And yet those speakers are rear-ported, and meant to be mounted horizontally, not vertically...
If I had a line with the speaker designer/engineer like you did it would be great to consult on that one.
I'm here! John is here! Glenn is here! Andre is here! Others are here!
If the frontwall(+soffit installed speakers) is built, I think the corner bass traps on the front become irrelevant (they're outside the room, technically).
Nope! The bass traps above an below the speaker section are NOT outside the room! The SPEAKER is outside the room, but the bass traps are not. The center section, where the speakers are mounted, constitutes the infinite baffle that puts the speaker outside the room, but the rest is still inside. There's a very large air gap down next to the floor, normally 12" or so high and the full width of the soffit, which exposes the hangers to the room. That's also where the cooling airflow comes from, under the speaker. And there's another large space above the speaker, which can be used for additional bass trapping.
Then the room has only one bass trap, so ... maybe make up a horizontal superchunk to reside on top of the front wall, between a horizontal plane and the inclined roof
It's a small room: it will need LOTS of bass trapping. As much as you can fit inside, in fact. The smaller the room, the more bass trapping you need. It is impossible to put "too much" bass trapping in a room. In Studio Three (which is a fairly large room) there's a huge amount of bass trapping above the soffits, an along the side walls (wall/ceiling corner), and in the vertical corners, and on the rear wall, and under the raised platform where the client couch is.... And even then, it's not enough. I would have liked to put more in, but there's no more space!
Is attaching 3x 5/8" Gypsum boards green glued together
Nope! despite the name, "Green Glue" is not glue. It is not adhesive. You cannot use it to stick things together. That is NOT what it is for! It is an acoustic damping compound. Technically, it is a visco-elastic polymer that acts as a constrained layer damping material between two layers of drywall (or plywood, OSB, MDF, etc). But it is not glue.
directly into the cement above and below with long cement screws
Why do you want to put drywall on your cement walls? For what purpose?
I'm just trying to create solid surfaces for the Absorber treatments.
The cement is not a solid surface? What is that wall surface like now? If it is cement, then that's all you need. Paint it with masonry sealer ("sellante de cal") to seal the porous surface, and you are done.
The rear wall of your room will never be seen again: You will have very large superchunks in the corners, floor to ceiling, and the rest of the wall will be covered with 6" of fiberglass or mineral wool insulation: It does not matter what the wall looks like behind that...
- Stuart -
) it would be kind of a dead maybe lifeless room ... This is why I'm thinking of placing some fine wood paneling or real-brick tile on one of the open surfaces to try to liven things up in there. 
Again, thanks and hopefully some action shots soon!
Thanks! The final results are actually even better than you see on those graphs... We did a bit more work after Rod posted that... I really should update that thread with the final graphs.