Wow! I've just spent some time reading through all of your thread about the HVAC part, Jennifer, and going over the calculations, and it sure looks like you have done your homework here!
One caveat, which Starlight already brought up: using outside air to cool your room might not do all that you expect. I'm thinking specifically of humidity. This is something that even many studio owners and musicians don't take into account, but it is rather important! So I'll climb on my soapbox again, and vent one more pet peeve....
(Excuse me hijacking your thread again, Jennifer!)
So: Relative Humidity is the subject...
Relative humidity refers to the amount of moisture vapor present in the air. It ranges from 0% (very dry) to 100% (very moist). Here's a rather common HVAC chart that shows how humidity affects your health:
0% humidity is on the left edge, 100% on the right edge. Each red triangle shows the range where a specific health problem occurs: the wider the wedge, the greater the problem. As you can see, there's a zone in there, from about 35% to 55%, where health issues are minimal.... and not surprisingly, that's also the zone that people find to be the most comfortable, regardless of temperature. 50% in a hot climate feels nicer than 25% or 75%, and 50% in a cold climate also feels nicer than 25% or 75%.
So that's the first part: human health and comfort. But what about your gear and musical instruments? For equipment, if the humidity is too low (below about 40%) then there's a good chance of electrostatic discharge happening: often just called "static electricity" or simply "static": What happens when you walk on a wool carpet with synthetic shoes, then touch the doorknob... ZAP! Not good for gear, or people! On the other hand if the humidity is too high (above about 70%) then you can get corrosion of metallic surfaces (including rust), as well condensation, and muck accumulating on cooling fans and grills, rather than just dust. Somewhere around 50% seems to be the "sweet spot" for gear.
And instruments? Well, here's where it gets really interesting for studios. Many musical instruments are made of wood. Wood is a natural fiber that "breathes": it exchanges its own internal humidity with the atmosphere around it. In a damp atmosphere, it absorbs moisture and swells up. In a dry atmosphere it releases moisture and shrinks. Dry for too long? It cracks. Damp for too long? It warps. Joints loosen...
And of course, as the wood swells and shrinks with changes in atmospheric humidity, the tuning changes! Yup. And not by just a little bit. Taylor Guitars have a lot of data about this, and they say that the top panel of an acoustic guitar will change in width by about 1/8" (3mm) when the humidity changes by about 20%. Try turning the tuning pegs on a guitar to make a string 3mm longer or shorter, and you'll see just how much that is! And that's for an acoustic guitar: imagine what the change is like on a very much larger instrument, like a double bass, cello, kick drum shell, floor tom, ...or piano!
So, if you have a room where the humidity is swinging wildly, or even mildly, you'll find the need to re-tune your instruments all the time, to compensate. Either that, or put up with songs starting off in one key, but ending up in another... unintentionally!
So, getting back on track; it is important to keep the humidity in your room fairly constant at all times. So far, I showed that people are good with humidity in the range 35% to 55%, gear is good in the range 40% to 70%... and instruments? Fortunately, it runs out that wood is happiest at about 47%.... which is the exact humidity that Taylor has in their factories, throughout, in all locations.
Conclusion: keep your room at 47% relative humidity, and your instruments will sound great, won't need returning, your gear will be happy, and you will be comfortable and healthy.
Now for that "caveat": The outside humidity might or might not be 47%! Most likely not. The air you bring into the room directly from outdoors, might be a lot more humid than 47%, or a lot less humid. If it just rained out there, or if it is foggy, or if you live close to a large body of water (river, lack, ocean, loch...) then chances are the humidity is way up there, around 100%. If it hasn't rained in days, and it's been warm and sunny... or cold and sunny, chances are that the humidity is much lower: probably under 35%.
Sooooo... I'd suggest that you add in "something" to keep the humidity under control. Depending on your local climate, you might need to dehumidify the incoming air... or to humidify it. A mini-split system will always dehumidify when it is cooling the air, so that might be fine if the air is humid. Or you might need a humidifier. But something a little more sophisticated than this one:
(At least it is cheap to run: plugs into a USB port...).
The best solution here (but not cheap!) is an ERV: "Energy Recovery Ventilator". It is similar to an HRV ("Heat Recovery Ventilator") in that it transfers heat between the incoming and out going air, but the ERV also transfers humidity. So, assuming that you have 47% in your room, an ERV will help to keep that in the same range, because if the incoming air is drier then it will transfer humidity from the outgoing air to the incoming, and the reverse happens if the incoming ir is more moist... except that the difference between indoor and outdoor has to be rather large, to get good transfer. And there's also the issue that... " it transfers heat between the incoming and out going air"! Which is what Jennifer does NOT want! She wants to get the cool air in, and the warm air out, without transferring any heat....
Conundrum!
This is what Starlight was getting at, with his comment on humidity.
To find out how bad this issue is, I'd suggest that you get a good quality, calibrated hydrometer, and measure the indoor and outdoor humidity under various weather conditions, to see what the levels are like. Maybe you don't have a problem at all! And maybe you do....
Once you figure this out, then you can take a look at possible solutions, ... if any are needed. I'm hoping that your location has a mild climate, where humidity is almost always around 50%, naturally.
The other thing I noticed with your plan, Jennifer, is that you are bringing in and removing air from the same end of the room: the end where the glass is. I usually try to bring in air at the rear of the room, and take it out at the front, so the air can move through the entire room. It can be done your way, but ensuring that it circulates well is more problematic: the rear of the room might not get the cooling and "freshening" that you are looking for. I'm wondering if it is possible to run a duct from the inlet silencer to the rear of the room, inside the room, and have your register(s) back there to supply fresh air to the room. I had a case like this a few years back, where the only place to position the silencers was at the front... so this is what I did:
You can see the return registers in the ceiling itself, but there's also a strange wooden box running between the soffits, above the door... that carries one of the supply ducts... there's two of them, and one runs down each side of the room, up against the ceiling:
I had to split the flow into two paths to keep the flow velocity low and also reduce the static pressure drop. The ducts down both sides of the room end in large registers on top, directly behind the air intake for the mini-split.
I am concerned that having the bottom of the silencer in such close proximity to the ceiling below would lead to more sound getting into the room below. Does this fear have any foundation?
That should be fine. If you are worried about resonance in the boxes possibly getting into/out of the rooms through flanking, then put Sorbothane rubber "feet" under the box, to decouple it from the floor below.
This will be soon after phase 1 is finished and working properly. It's a simple method of getting better air circulation and distribution by extending the supply vent closer to the rear of the room. If ever a mini split is required, around this area would be a good place for it I think.
... and I just came across your "phase two"!!! Which takes care of what I was highlighting above, so no need to repeat that! You already thought of it..-
This part will eventually be integrated into the "as yet to be finished" soffit design that will be something like this;
I know that Genelec sells a "soffit mount" box that has the speaker poking half way out the wall, as you are showing in your model, but to be honest I've never really understood WHY they do that! My guess is to get decent air flow around the speaker, for cooling, but there are other ways of doing out. My concerns with it sticking out so far from the wall, include edge diffraction, SBIR, and other potential issues. It makes more sense to have it fully flush... which is hard to do, but I do have a design that I did for soffit-mouting the egg-shaped Genelecs! It was never built (as far as I know), and the guys I did it for never even paid me for it (!)
, so its untested.... but theory says it will work just fine.
Actually, I just remembered that they could NOT have built it as designed, since I left some key parts out of the version I sent them
, when they were supposed to pay... I had a feeling it was going to go south, so I left out those critical details until the payment came through... which it never did!.... Thus, if they tried to build it without that, it would not have worked...
But anyway, the design is there still, some place. I'll see if I can dig it out.
Total static pressure at 186 m3/h (6 changes per hour) is 147 Pa.
Sounds about right. That's roughly 0.5 inches H20.
All in all, it looks like you have all your ducks in a row, Jennifer!
With the above caveats about humidity.
- Stuart -