Thanks Mark! I think....
OK, here is the scoop on the 1/2" and 5/8" (or two other different thickness of drywall used together). The idea is that each thickness has a natural resonance frequency based off of its thickness and mass. In turn, each layer then also blocks different frequencies and so forth. It sounds good doesn't it? There might be some science behind that somewhere that says "sheet thickness 1 blocks A to B frequencies well" and "sheet thickness 2 blocks B to C frequencies well" and so combining them together you should get frequencies A to C blocked well... However, it doesn't work that way exactly. At least not in a measurable way.
However if IS proven that the more mass you have (regardless of thickness of individual sheets) the more mid to lower frequencies you are stopping. So if you can put up two layers of 5/8", then that is better than a layer of 1/2" and another layer of 5/8" which is better than two layers of 1/2" and so forth.
So don't get hung up on the *need* for 2 different thicknesses. The key is just getting as much mass as possible. If that ends up being two 5/8" layers, excellent, if it is a layer of 5/8" and 1/2" I would put up the 5/8" first. This will be quite a bit heavier, and if you are going to use Green Glue, it will be easier to work with the lighter 1/2" layer as the second layer.
As for the clips and channel. My ceiling has two layers of 5/8" drywall with Green Glue in the middle, all hung from the hat channel and clips. Kind of scary, but they are rated for a lot more. I think that each piece of hat channel would have only needed another clip or two maybe tops per 12 foot length (spaced 2 feet apart) to hold an entire 3rd layer with lots of extra wiggle room. With my two 5/8" layers, I am NOT at their rated capacity yet. Also, I am getting no cracks in the drywall, and it is really solid. My biggest issue is that my ceiling height was already only 7 feet 10 inches or something like that. Adding clips and channel was going to drop it a couple of inches (maybe 3.25" or something) plus the two layers of drywall, so I had to make a number of "stringers" to tie my ceiling joists together, but these stringers were actually recessed up from the bottom of the ceiling joists. These stringers are what I connected my clips and then the channel to. I saved some space there. Not sure what your ceiling height is, but mine doesn't feel low at all, even when standing on my 12" riser for the second row.
Here is a picture from The Soundproofing Company that shows the recessed stringers...
As you can see, the channels are spaced 2 feet apart (except for the outside edge) and the clips are every 48" (plus one around the perimeter). If you go this route, we can map out an exact clip count and orientation, or call someone like The Soundproofing Company and they will calculate the number of clips you need.
Now, if you REALLY want to try to do something about the sound in your ceiling, you can do this:
Basically it is the same as above, but two layers of green glue and two layers of 5/8" drywall up attached to bottom of the *floor* above the theater. For me, I found that the joists were never very straight making the cutting of drywall to fit a pain, plus the added cost and effort was beyond my personal desired "bang for my buck."
One thing to notice, in these pictures, you see just regular pink fluffy insulation (the cheap stuff) shown. The stuff I used was yellow, but it was still the cheap stuff. I used John's Manville brand R19 for the walls and I believe the ceiling (or at least that is what I saw in one of my construction videos).
In this video of my theater, at about 2:35 into it, I start showing the clips and channel.
Go to 2:35 for clips and hat channel I did not go with clips/channel on the walls, but built a staggered stud wall which gets me really close the the same effect as using clips/channel. I am getting an estimated 66 STC for my walls and ceiling (because I am using clips/channel up there) which is a good, high rating for sound proofing. That is with cheap R19 insulation, two 5/8" layers of drywall and one layer of Green Glue.
Going without the clips (two layers of 5/8", Green Glue, and R13 insulation) yields about 52 STC
Using something else besides Green Glue (the stuff REALLY is THAT good) like 1/2" Sound Deadening Board between the two layers of 5/8" drywall (and R13 insulation) yields just 44 STC. OK. That is still pretty decent, but still considered just a "moderate" improvement. It isn't until you start using Green Glue that you really start to improve the low frequency soundproofing, and also reduce "flanking" (sound traveling between two or more different surfaces like wall to ceiling, ceiling to the ceiling in the room next door, etc.
Here are some pretty pictures and more details.
Soundproofing Walls - Different Configurations. And here is one with even more pictures and information specifically for ceilings.
Soundproofing a Ceiling - Different Methods I did SPC Ceiling Solution #3.
I reference The Soundproofing Company a lot. I am in no way associated with them. I just had a good experience with them for their customer service in answering my questions, and their great pictures and diagrams that get referenced a lot on the internet (just like I did above).
They explain things very easily on their web site, and it is worth poking around there for a while, even if you buy stuff from someone else or don't buy anything at all.
