Hi again Jason. Glad to see you are still moving forwards!
I plan to do the control room floor with 1" concrete board, possibly use a thin layer of self-leveling concrete to seal it all up, and finish with a ceramic or stone tile
That's probably about as good as you can hope for, given the limitations of the structure. Make that self leveling cement as thick as possible, within reason, and as thick as your structural engineer will allow!
Also, I've hired my HVAC contractor who has done two nice studios in town and knows all the ins and outs which is a big relief.
Great! hopefully he'll be able to do it right, with decent silencer box design, slow speed, and high volume.
I've seen many different designs and methods and dont know which one I should go with.
The basic concept is simple: You need a very solid, rigid heavy, massive structure that cannot be moved easily by the speaker vibrations, and you need a very solid, rigid massive baffle (front panel) that also cannot be moved by vibrations. The main difference in the methods of how to actually build it, is based around weather or not those two "very solid, rigid massive" things are tied together, or kept separate. Both approaches work.
One in particular was the soffit at ClearTrack Studios
Yeah, I've seen plenty of photos and videos of that place floating around the internet. Nice looking place, isn't it?
He incorporated the soffit into the wall design and drywalled the front of the soffits and then went back and did what looked like a layer of OSB and MDF.
Yup. And probably plenty of other stuff that you can't see!
Unlike in the Sayers style where I believe the drywall layers are on the inner wall studs and the soffits are constructed within the leaf from OSB and MDF
It's the same concept, if you look closely...
There's no difference.
In the Lachot design, no ports or bass traps are used because the soffit is actually part of the leaf and can't be penetrated.
It might look that way, but I'm betting that some place in there is plenty of ventilation for those speakers! They need cooling....
What you see in John's soffit designs are NOT acoustic ports: they are ventilation ports, that allow the cooling air to flow up behind the back of the speaker, then return to the room. If you don't want to see those ventilation ports, then there's no problem hiding them inside the soffit some place, provided that you also include suitable silencer boxes if the HVAC ducting from the speakers then goes straight out of the room.
It looks like just the boxes that house the speakers we're built and they seat in the frame and extend back into the insulated air cavity that is between the inner and outer wall frames.
Once again, it's the same concept as with John's design: there is a massive box around the speaker itself that holds it rigidly in place. With both designs the speaker itself is NOT in the air gap between the leaves: that would be silly.
The only difference I can see is that with John's design the speaker box is vented with the soffit back to the room, whereas with the other design the box is vented in some other way that cannot be seen, and that does not come back into the room. If you have the budget to do the extra complexity for that ventilation, then by all means go for it! The end result won't be any different. Or use passive speakers that run very cool and do not need cooling, even when run for long periods of time at high power.
However, you did say you were planning on using BM5A's, which are active, have huge heat sinks on the rear that get hot, and can't be soffit mounted anyway! Or at least they cannot be soffit-mounted not easily, and certainly not the way seen in that video: You have rear-firing bass extension ports on those....
In the Sayers' design, the extra space above and below the speaker can be used for trapping.
John normally only uses the space below for trapping. The space around and above the speaker is filled with insulation, to damp cavity resonance. But there's an awful lot if useful real-estate under a soffited speaker, and it seems such a shame to just waste that, when it could be put to good use for trapping....
Personally, I like the Lachot design because the soffit frame is a part of the wall frame
It is with John's design too. Especially so if you build the front wall inside-out, as John normally does.
the drywall on the fronts seems like it would make it a very massive surface compared to just using OSB
Why? Drywall weighs about 650 kg/m3, give or take (depending on manufacturer, style, etc). OSB weighs about 600 kg/m3, and some types as much as 640 kg/m3. So there's not much in it. OSB is also more rigid than drywall...
But the speakers being in the air cavity between leaves makes me uneasy.
The speakers are NOT in the air cavity! Take a close look at the video again, around 1:35 to 1:48: There are several views that clearly show the box that will later contain the speakers. I'm betting those boxes are massively constructed, with rigid framing around them.
It feels like there should be another layer behind them to contain any backward resonance from hitting the next leaf,
Well, yes, I do have to agree with you there: if the box does resonate for any reason with that design, then that resonance will indeed be inside the wall cavity, which is not a good place for it.... On the other hand, with John's design, that cannot happen...
isn't that the whole point of "front-loading" the speakers anyway? So that all sound is pushed forward, instead of 360 degree resonance?
Well.... sort of! The issue is correction of power imbalance. With any free-standing speaker, the high frequencies are projected straight forwards, like rays of light, while the lows wrap around behind the cabinet and spread out in all directions, like a balloon. So the highs will be 6 dB louder than the lows, since the highs are going out into "half space" (the hemisphere in front of the speaker), while the lows are going out into the full sphere ("full space") all around. so if you pump the exact same power into all frequencies, then the lows would sound 6 dB quieter. The frequency where the change from "half space radiation" to "full space radiation" occurs is know as the Baffle Step Frequency, and is set by the width of the front panel of the speaker itself. If you then extend that front panel infinitely in all directions, then all frequencies are no forced into half-space, and there is no longer any power imbalance. That's what a soffit does: it forces all frequencies to radiate into half-space. Except, of course, with a rear-ported speaker. With rear ported or side ported speakers, things are different since a lot of the low energy is also coming out that port, and being lost inside the soffit....
I guess my question is should I design my soffits to be a part of the leaf or inside of the leaf?
In BOTH cases they are part of the leaf!
Technically, the soffit removes the speaker from the room and thus also removes all the associated artifacts, since the speaker is flush with the inner-leaf of the room.
What you are talking about is different CONSTRUCTION techniques, not different acoustics. Both techniques, (and the Barefoot technique as well...) accomplish the same thing, acoustically: remove the speaker from the room, and hold it rigidly in the wall.
I've also been considering not soffit mounting any monitors since I always work on nearfields.
If you don't soffit-mount, then you re-introduce all those nasties that soffits eliminate, such as SBIR, comb filtering, other forms of phasing issues, reflections off the walls behind the speakers, modal coupling, etc., etc,. etc. ... There's a reason why studio designers flush-mount their speakers in world-class rooms, and that is to ensure that they sound as best they possibly can, the way the designer intended, with as little distortion as possible coming from interactions with the room boundaries.
I've had my eye on Barefoot monitors for a while now, which cant be soffit mounted due to their 360 design.
BM5A's suffer from the same issue: they are rear-ported.
Lets say I we're to use such a configuration. How would be best to handle the walls where the soffits would've been? I want to keep the 30 degree angles of the walls but should I make the surfaces absorptive, maybe broadband or targeted slots, or reflective?
If you don't soffit mount, then the second-best is to move the speakers far enough away from the walls that SBIR is no longer an issue, which means that they need to be at least 1.5 meters away from all walls, and preferably 2 or more meters away. I'm not sure that your room is big enough to be able to do that. The THIRD best thing, is to make the room rectangular and push the speakers right up against the front wall, where SBIR will be forced up the scale to frequencies that aren't so objectionable.
Trying to keep it short although I have a million questions.
Questions are fine! That's what this forum is all about!!!
- Stuart -
