Batting insulation is great for thermally separating temperatures on both sides of a common surface. The old school of thought was that stuffing this same material between the studs in a wall or the joists in a common floor/ceiling assembly will also serve to sound insulate one room’s noise from the next. Well, not so. As we teach here on this website (see the Walls, Ceiling, Floor link to the left), your goal with sound insulating a common surface is to line the surface first with weight, and then with disconnection. The weight portion impedes vibration, much like grasping the prongs of a tuning fork to kill the vibrations and the sounds that go with it, while the disconnection portion forces the collapse of the structural vibration much like snipping a string pulled tight between two coffee cans. This combination can trigger up to 90% collapses in sound bleed through a common surface.
But the reality is, fiberglass batting ignores the need for either of these components. Sound waves can bleed straight through fiberglass much like water seeps through a sponge, and stuffing the batting insulation between the framing of your wall or ceiling joists wholly ignores the vibration points that are connecting your two rooms together. Batting insulation might yield a 5% reduction in noise, while the disconnection/density formula can trigger up to a 90% collapse.
If you are in a remodel, new build or retrofit scenario, use batting insulation for thermal protection only. dB-Bloc and a good disconnection technique in your assembly will trigger the reduction in noise you are seeking!