Cost. Have a look through Brolic’s build. He used 3 layers and it’s very effective.

 

I would think it's the law of diminishing returns when factoring in sound leaks such as outlets and what not. If you go the 3 layer route let us know how it turns out.

 

I’ve worked on two three layer theaters. During one a couple of the drywall crew mentioned they worked on local high end cineplex that had recently opened. The walls separating rooms had three layers on both sides.

As for clips and channel you simply move from channels 24 OC to 16 OC and keep the clip spacing at 48 inches. Working the math a clip is designed for roughly 30-35 lbs. count the clips in your layout and divide into the weight of the drywall. If you are into the rumble go with three layers.

Design your space so that their are no outlets or lights cut into the shell without backers. Build stages, riser and soffits inside the shell. Beefy door system and sound attenuated duct work.

 

Thanks for your advice!

I've asked Green Glue company for the question. Just now they replied that the Whisper Clips can hold up to 4 layers of 5/8” type X drywall and the spacing does not change.
I also found the spec of Whisper Clips says 3 layers is OK:

Recommended load: 36 lbs ~ 3 drywall layers
Max safe load: 46 lbs ~ 4 drywall layers

Well, I'm not sure the weight of my drywalls and I'll calculate that.

 

my resource for spacing is Ted, the co-developer of Green Glue at Soundproofingcompany.com Pick your expert.

 

Exactly. I'm using three layers and when I purchased my soundproofing materials, Ted said the same thing...16" channel spacing to accommodate for the extra weight of the drywall. I don't think I'd want to chance possibly having my ceiling fall on top of me in the middle of high levels of bass-enjoyment from trying to save a little money by using the standard 24" channel spacing with three layers of drywall.

 

According to the installation guide of RSIC-1, 5 clips per 32 sq. ft (3 sq. m) are required for 2 layers, about 28 lbs loading per clip; and 7 clips are required for 3 layers, about 30 lbs per clip.

In my case, some of the walls are small in dimensions, and the extra clips on the edges contribute a lot. For example, 15 clips are required on a 6 sq. m wall, only 28 lbs per clip for even 3 layers of drywall. (supposing drywall hanging on wall will not deform and the spacing of clips is even)

Just discussing in math or theory, not suggesting anything.

 

So it shows channels 16 OC for three layers with 48 inch clip spacing. What do you do for a ceiling that has joists at 19.2 inch spacing? A real world problem.

 

Challenge accepted! This got me curious enough to figure it out. This would work for either walls or ceilings with 19.2" OC spacing.

For 1 or 2 layers, use 19.2" x 57.6". This gives you 5 clips per 4x8 panel (32 sq ft).

For 3 layers, use 19.2" x 38.4" This gives you 7-1/2 clips per 4x8 panel (32 sq ft).

 

WE HAVE A WINNER, Fields medal nominee for sure!

 

This thread is exactly what I've been looking for. My ceiling joists are 19.2" on center engineered 2x4 trusses, which kind of blows up up the standard 24" x 48" staggered clip spacing — unless I went crazy with 2x4 blocking. Furthermore, due to the atmos backers and 4 big HVAC joist silencers, there are several spots I can't even do blocking to put clips, so when I tried doing a 48" spaced layout in sketchup it looked like total a mess.

From there I did a 24" x 38.4" staggered spacing (because 57.6" seemed like too far) and the layout looks good to me, but seems like it might be overkill on the clip count for 2 layers of drywall at 97 total clips. The ceiling area is 410 square feet, which is about 1800 lbs of 5x8" DD (plus whatever GG weighs). That puts 18.5 lbs on each clip.

This morning I finally stumbled across this thread, and it sounds like for 2 layers that 57.6" staggered spacing could actually work, but the example above has the channel spacing at 19.2" instead of 24". The problem I see with that is then you can't install the two layers of drywall in opposite directions because there would not be a channel at 4' for the long edge of the drywall to land on for the layer running parallel to the channel.

So, I tried a layout at 24" on center, but I don't know if this is enough clips to support the channel. It's definitely the "cleanest" layout I've come up with. It reduces the clip count to 75 total clips, for a 24 lbs per clip load when averaged over the ceiling. That's acceptable overall, but the clip layout is biased to the edges, so there would likely be more load in the middle.

Unsure about the load in the middle of the room, I did another layout putting the channel 16" on center with 57.6" staggered spacing across the joists. Due to the extra channel rows, clip count goes up to 111. Some of that is also due to some extra clips due to having to cut the channel around the atmos backer boxes, but it's also must more clips overall. With the clip count being so high, plus having to break around the atmos boxes, I think the first 24" x 38.4" spacing I drew up is actually better than this.

My goal is the best sound isolation possible, while still appropriately supporting the load so the ceiling stays above my head where it belongs. I don't care about cost.

38.4 x 24" spacing for 97 clips:

57.6" x 24" spacing for 75 clips:

57.6" x 16" spacing for 111 clips:

I'd love some feedback on this!

 

The problem I see with that is then you can't install the two layers of drywall in opposite directions because there would not be a channel at 4' for the long edge of the drywall to land on for the layer running parallel to the channel.

I'd love some feedback on this!

The beauty of a 2 or three layer drywall installation is that the edges don't have to fall on a channel, just screw them to the underlying layer with course thread drywall screws or drywall laminating screws. Also sheets don't need to install sheets in opposite directions, just overlap seams by at least a foot.

Factoid: in a lot of current house construction 12 ft sheets are mounted horizontally perpendicular to the studs, there is nothing for the long edge not even a second layer underneath. From a finishers perspective it is a lot easier taping and mudding a horizontal seam 4 to 4 1/2 ft off the ground around thee room than climbing up and down a ladder.