Might try adding empty eggshell cardboard cartons in the sag space. Each empty hole in the carton acts as an absorber for the vibration. It basically prevents transfer of one problem area to another.Talked to my dry wall guy and he suggested using this method for helping to sound proof a ceiling:
The method requires attaching one side of the channel to the existing joist,through the existing drywall or sound board.
Then,new drywall is installed,fastened to the metal channel,and allowing it to "sag".
Theory is the points of contact are reduced,isolating new drywall from direct sound/vibration.
Anyone had any experience with this method?Good?Bad?Things to watch for when installing?