If those HVAC experts claim to have installed so many mini-splits in studios with satisfied customers, then I'd suggest that you ask them for the names of those satisfied studio owners and contact them yourself! Perhaps even arrange a visit, to hear just how loud (or quiet) they are.
The last guy basically told me that the 72-80db of noise from a ducted unit (which would be parked outside and behind my garage) would be so great that I'd still be shutting it off for quiet takes
Well, I think you can safely forget about that guy; clearly he has no clue what he is talking about, and never has installed anything in a properly isolated studio. Just think about what he said: he's concerned that a 72 dB noise outside is going to be intolerably loud inside: Assuming that the studio is isolated reasonably well, it would be providing about 50-something dB of isolation, so the 72 dB of compressor noise would be down to under 20 dB inside... that's below the "inaudible" level for a typical home studio.
In addition, if he is proposing a system that produces 72 - 80 dB of noise at the compressor, then you'd better be running
far away from what ever unit he is proposing! Fast! That is WAY loud for a typical compressor. Most come in at around 60 or so, or even less for a really good one. It shouldn't be too hard to find a system that will produce no more than 50-something under normal operation. Right now I'm looking at installing a 120,000 BTU system for a church, and the compressor on even that huge thing unit is spec'd at 65 dB, max. (And I'm betting you don't need 120,000 BTU for your garage....
) So if that guy wants to sell you something that would rattle and whine long at 80 dB, you'd have a major problem: Apart from anything else, your neighbors up in arms and the cops would be at your door regularly, since that violates every noise regulation I've ever heard of! Most residential zones have daytime limits in the 45 to 55 dB range, and night time usually goes down to around 35-45. You would never be able to use it, legally.
So I'd say that you can very safely ignore that guy, and whatever it was he was advising you to do.
Regarding the other guys: Ask for those references, call the studio owners, and go visit them if possible, to actually hear for yourself.
I designed a studio for a guy in LA last year, using a mini-split system plus ducts. If you want, I'll ask him if he wouldn't mind you giving him a call, so you can ask about his experience with that. He made one mistake, though: he used cheap registers from Home Depot, instead of proper low-noise registers, so his duct system is a bit noisier than it should be. I'm not sure if he ever changed those, but you could ask him how quiet the room works when he has the duct fans off and only the mini-split running: He doesn't need it to be as silent as you do, but he can still give you his impressions. Let me know if you'd like to talk to him, and I'll see if he is willing.
Now, on the actual question itself: Undoubtedly, a ducted system with an AHU in it WILL be quieter than a mini-split, if installed properly. Mini-splits (even the quite ones) do make noises, and it's not just the fan and air noise. I have a small 9,000 BTU unit in my office, and although it runs pretty quietly most of the time, it "clicks" softly each time the compressor cycles on or off, the vane makes a slight noise as it moves up and down, the water (condensate) trickles out noisily every now and then, and occasionally there are noises in the refrigerant pipes themselves if it has been off for a while. It doesn't bother me, since this is my office, not my studio: I would NOT be happy if it was my studio, and I were trying to record something soft. I would not be able to do that with this unit. I would have to turn it off. On the other hand, if it were an AHU in a ducted system, then it would not matter how much noise it made, since it would be outside the studio and isolated at the other end of a long duct, as well as being behind silencer boxes: I would never hear any of that at all.
Also consider that you WILL need the ducts anyway: there's no way around that. Even with a mini-split system you still need the ducts for fresh air injection and stale air exhaust, so you still need silencer boxes, fans, registers, and all the rest of it. So if the duct system is unavoidable, you might as well just put the actual AHU inside it, far away from the studio, and keep things as quite as possible inside. It will be slightly more expensive, but much quieter. No matter how quiet a mini-split is, it will always be louder than a well-designed ducted system.
Probably the biggest issue with badly designed HVAC systems in studios, is air noise: The air moving through the ducts themselves makes a noise, due to turbulence and too-fast speeds, and even more noise where it moves through the registers. All of that can be eliminated by doing the math, and making sure that the duct sizes, cross-sections, lengths, flow velocity, flow volume, and static pressure are all figure carefully. For example, the flow velocity at the register must be under 300 fps, and preferably even 100 fps if you really need things quiet. So the entire system must be designed to deliver the correct volume of air flow such that the speed at the register does not exceed 300 fps. The duct run just before the register must be long enough to ensure that there is no turbulent flow, the register itself must be a proper low-noise type with large open area and only minimal obstruction, the ducts must be suitable lined, etc., etc., etc.
With a correctly designed ducted system, it is quite possible to get your NC (or NR) down below 20, or 15 even. That should be plenty good enough for most studios. You'd only need it quieter than that if you were doing Foley work, which doesn't seem to be the case.
I'd still be shutting it off for quiet takes if I felt that the 30db of noise from a mini-split in my space would be too loud,
Right. So would I. That's still too loud, in my opinion.
I've always been led to believe that a ducted system with the appropriate duct silencers and baffle boxes in place would be the quietest way of having AC and fresh air,
Absolutely, but with one caveat: if it is properly designed and installed. Most HVAC installers don't understand recording studios, and would not have a clue how to design such a system. They could probably install it if you gave them the design already done, but designing it is likely beyond their capabilities.
insulating and using Resilient Channels with 2 or 3 layers of sheetrock/green glue for the inner leaf, and I am on a fresh 8" slab of concrete.
If you are so concerned about need a very silent space to record soft, quite things, then I'm wondering why you are not going to isolate the studio better? Having a really silent HVAC system down at (say) 20 dB in a room that doesn't isolate down to 20 dB seems a bit pointless... You say that you live in the LA area, so I'm guessing that you are subject to the typical LA loudness all around you. Even residential areas still get noisy at times: Have you considered things like aircraft, and helicopters flying over, fire, police and ambulance sirens, traffic noise, thunder, rain, hail, wind, lawn mower, dogs, motor bikes, neighbors, etc. I would suggest that you might want to consider a more substantial isolation system for your studio itself.
- Stuart -