Soundproof isolation should feature variable density, increased mass and placement between two separate interior walls with an empty space left in between. Soundproof isolation materials typically alternate between high and low-density fibrous materials, offering acoustic isolation throughout the entire sound frequency spectrum.
One example of a successful soundproof isolation material for interior drywall installations is mineral wool. For maximum effect, mineral wool needs to be combined with a denser outer material like vinyl or rubber, forming a slight barrier between it and the much heavier drywall. Mineral wool is a compressible material, however, and home improvement laborers should take care not to fill the interior wall cavity completely, as tightly compressed materials transmit sound better than porous ones.
Acoustics experts also recommend fiberglass insulation for double-wall installations. Density variance between two walls is optimal when a layer of drywall, hung on resilient clips, leads to a thin sheet of vinyl or rubber before meeting a layer of fiberglass insulation, an empty void of air, another layer of fiberglass, another sheet of vinyl or rubber and then the other sheet of drywall. For even greater sound isolation, use metal studs in the interior wall instead of wooden ones and isolate the walls from the floor by framing them in the vinyl or rubber material.