Mass is one of the three fundamental principles of soundproofing — alongside decoupling and damping. Adding mass to an existing wall increases its resistance to sound transmission, particularly for mid and high-frequency sound. Here is how to do it effectively.
The Mass Law in Simple Terms
The mass law states that doubling the mass of a wall improves its sound isolation by approximately 6 dB — roughly a perceived halving of loudness. This is a significant improvement, but it requires substantial mass addition. Adding a single additional drywall layer is a practical and meaningful improvement, but dramatic results require multiple layers or other techniques combined with mass.
Option 1 — Add a Layer of Drywall
Screwing a second layer of 5/8 inch drywall over an existing wall adds mass and improves STC performance by approximately 3 to 5 points on its own. With Green Glue damping compound between the layers, performance improves by 8 to 12 points. This is the most practical mass-addition approach for owners making permanent improvements.
Use 5/8 inch drywall rather than 1/2 inch — the additional weight contributes meaningfully to performance. Stagger the seams of the new layer so they do not align with the seams of the existing drywall, and seal all edges with acoustic caulk.
Option 2 — Mass Loaded Vinyl
MLV can be attached to existing wall surfaces before adding a drywall layer, or hung directly on a wall as a surface treatment. Within a wall assembly, MLV combined with an additional drywall layer outperforms either product alone. The combination is particularly effective because MLV adds limp mass — mass that damps vibration rather than simply resisting it.
Option 3 — Heavy Furniture
For renters, furnishing shared walls with heavy, dense items is the most accessible mass-addition strategy. Fully loaded bookshelves — particularly those with solid backs — add substantial mass and some absorption along a shared wall. While less effective than structural treatment, this approach requires no installation and no landlord permission.
Option 4 — Plywood Panels
3/4 inch plywood attached to walls adds significant mass — more than drywall per panel, as plywood is denser. Plywood panels can be finished attractively with paint or fabric. The trade-off is cost and the labor required for installation. Plywood is an option where drywall is impractical — in basements, garages converted to living space, or acoustic studio builds.
Combining Mass With Other Techniques
Mass addition works best when combined with decoupling and damping. A wall with doubled mass but no decoupling still transmits vibration through its rigid connections to the structure. Adding mass while also using resilient channels, Green Glue between layers, and mineral wool insulation in the cavity produces results far superior to any single approach.
If budget is limited, prioritize: seal all gaps first, then add damping compound with a second drywall layer, then add insulation if the wall cavity is accessible. This order maximizes performance per dollar spent.