Absorption versus transmission loss

Two ratings describe two different jobs. Sound absorption is summarised by the Noise Reduction Coefficient (NRC), a single averaged figure for how much sound a surface soaks up rather than reflects. Transmission loss is summarised by Sound Transmission Class (STC), describing how well an assembly blocks airborne sound from passing through it. A panel can have a high NRC and contribute almost nothing to STC.

If a room sounds echoey, you have an absorption problem. If you can hear a neighbour's television through the wall, you have a transmission problem. Treating one will not fix the other.

Cavity insulation: mineral wool and glass wool

Inside a stud or joist cavity, fibrous insulation damps the resonance of the air gap and improves the assembly's transmission loss modestly. Stone wool and glass wool are the usual choices. Density matters less than complete, gap-free filling; an under-filled cavity leaves a path for sound.

Insulation alone rarely transforms a wall. Its contribution is real but bounded, and it works best in combination with added mass and decoupling rather than as a standalone fix.

Filling a cavity with insulation typically yields a smaller change in transmission than people expect. The larger gains usually come from adding a second layer of dense board and from breaking the rigid connection between the two faces of the wall.

Added mass: dense boards and mass-loaded vinyl

The mass law describes a basic relationship: heavier, limp barriers block more airborne sound, and each doubling of mass produces a predictable improvement at most frequencies. In practice this means an extra layer of dense gypsum board, or a limp mass-loaded vinyl (MLV) membrane, raises transmission loss.

Mass-loaded vinyl is a heavy, flexible sheet used as an added barrier layer. It is useful where rigid mass is impractical, but it is heavy to install and only helps when sealed continuously and combined with the rest of the assembly.

MaterialPrimary roleCommon rating
Mineral / glass woolCavity dampingContributes to STC; some NRC
Acoustic foam panelReflection controlNRC
Dense gypsum boardAdded mass barrierSTC
Mass-loaded vinylLimp mass barrierSTC
Acoustic sealantSealing gapsProtects assembly rating

The detail that protects everything: sealing

Airborne sound exploits any opening. Non-hardening acoustic sealant at wall-to-floor junctions, around electrical boxes, and at perimeter gaps keeps the assembly performing close to its rated value. A heavy wall with an unsealed perimeter behaves like a much lighter one.

A reasonable order of operations

  1. Decide whether the problem is reflection (absorption) or transmission (isolation).
  2. For transmission, seal existing gaps first — it is the cheapest improvement.
  3. Fill cavities completely with fibrous insulation.
  4. Add limp mass, ideally on a decoupled layer.
  5. Re-seal every new penetration created during the work.
absorption_problem -> NRC materials (panels, fibrous surfaces) transmission_problem -> mass + decoupling + sealing mixed_problem -> treat each path separately

Publicly available references

Continue

Next: Decoupling techniques explains how breaking the rigid path multiplies what added mass can do. For a room-level plan, see soundproofing a room at home.