The sound performance of a wall or ceiling is given a number rating called STC, or Sound Transmission Class. The higher the number, the better the sound control.
Another rating, the IIC (Impact Insulation Class), is used to describe the sound control performance of floor/ceilings on impact sounds, such as footfalls or moving furniture.
As an example, standard wall construction containing 3 1/2" of insulation with a single layer of 1/2"-inch gypsum board on each side produces an STC of 39. The wood stud wall cavity has been filled with 3-1/2 inch thick fiber glass insulation, one layer of 1/2 inch gypsum board has been mounted to resilient channel spaced 24 inches on center, and the perimeter edge has been sealed. The STC value is 46. The combined effect of absorptive material in the cavity, using 1/2 inch resilient channel to reduce the structural tie between the gypsum board layer and the wood studs, and air sealing the perimeter edge results in increased system acoustical performance.