simple question... not so simple answer..
Take the existing common wall..
1. What is the construction?? Single 5/8 GB on each side??
2. Doubling the mass of the wall will offer approximately 6dB improvement in broadband Sound Transmission Loss... of course no where near that amount at lowest 2 or 3 octaves.
3. Improvement of existing GB walls with sheet block is not something we typically recommend. First, the mass of the sheetblock is typical less than 1 layer of 1/2 or 5/8 GB.. about half! The limp mass effect of the sheetblock is lost when it is added to, or sandwiched between other rigid laminations... so typically we resort to more standard rigid building materials that are more readily available, weight more and are easier to install (think MDF and another layer of GB.. that will add some mass!!).
4. To make a significant improvement over doubling the mass the first time, you have to double it again!! FOUR layers on each side?? Probably not gonna happen.
5. RC channel is great stuff!! It has to be installed and used properly: FIRST, it is meant to be used over the stud bay or airspace to get the best improvement in LF transmission loss.. if used over an existing layer of GB it can actually decrease the LF transmission loss over the addition of a 5/8" sheet directly to the existing wall (is that clear??)..
SECOND, it has a maximum load capacity above which it is no longer "resilient".. typically for 1/2" RC-1 (USG) this is 2 layers of 5/8" Gyp Bd on a wall or ceiling (pressing it on the ceiling).
THIRD, it should be installed with the free leg facing up and run horizontally across the open studs, insulate the wall and add the two layers of 5/8".. This type of installation can actually offer up to a 5dB improvement in LF Transmission loss over the same construction without RC channel..
so what's best for you??
1. What's the exisiting common wall? 1 layer of 5/8 on each side of stud? (Return to top and cycle indefinitely!!)>>>>