Sound & fire
Soundproofing a wall: mass, damping, decoupling and the STC numbers
A soundproof wall is built from four ideas: more mass, a damping layer, decoupled sides and sealed edges. The numbers show what each buys.
What STC measures
Sound transmission class (STC) is a single number for how well a wall or floor blocks airborne sound, such as voices, a television or music. It is measured in a laboratory under ASTM E90 and rated under ASTM E413. Higher is better: each 10 points roughly halves the loudness heard on the other side. A common rule-of-thumb scale:
| STC | What you hear through the wall |
|---|---|
| 30–35 | Normal speech, understood |
| 40–45 | Loud speech, heard but hard to understand |
| 50–55 | Loud speech barely heard; music bass comes through |
| 60+ | Most household sound blocked |
The International Building Code (section 1206) requires STC 50 (45 if tested in the field) between dwelling units in multifamily buildings. Nothing in the codes sets a level for walls inside a single home; there, it is a matter of how much quiet you want to pay for.
The four ideas
Mass. Heavier walls vibrate less. Doubling the mass of a layer adds a few STC points. A second layer of 5/8 in board is the cheapest mass there is.
Damping. A viscoelastic compound between two layers of board, such as Green Glue, turns vibration into heat. Its makers’ tests show several points of improvement in the same assembly.
Decoupling. If the two sides of a wall do not touch, vibration has to cross the air in the cavity, which it does poorly. Resilient channel, isolation clips, staggered studs and double stud walls all decouple.
Absorption. Insulation in the cavity absorbs sound bouncing inside it. On its own it adds a few points; with decoupling, it adds much more.
And one thing that undoes all four: leaks. Sound goes through any air gap. An unsealed gap at the floor or a pair of back-to-back outlets can cost 10 points or more.
Common walls and their numbers
A 12 × 8 ft wall in four assemblies, 5/8 in board
- Single 2 × 4 studs, one layer each side, insulation: STC 34–39. 8 sheets, 12 studs, 2 packs of mineral wool.
- Resilient channel and two layers on one side: STC 48–52. 11 sheets, 6 pieces of 12 ft channel.
- Staggered studs on a 2 × 6 plate, two layers of type X each side: STC 50–54, and a 2-hour fire rating (GA-600 WP 3910). 14 sheets, 24 studs.
- Double row of studs 1 in apart, two layers each side: STC 55–59, also 2-hour (GA-600 WP 3810). 14 sheets, 24 studs.
These are laboratory ranges; built walls usually test 3 to 5 points lower. The soundproofing assembly calculator shows every assembly’s range and counts its materials.
The full list, with sources, is on the sound and fire assemblies table.
Resilient channel, and how it fails
Resilient channel is a thin, springy steel strip screwed across the studs; the drywall is screwed only to the channel, so the board floats on it. Done right, it adds a lot. Done wrong, it adds nothing: a single screw that goes through the channel into a stud “shorts” it, connecting the board directly to the framing. So do cabinets, shelves and baseboards fixed through to the studs. Use screws short enough that they cannot reach the studs, mark the stud lines, and hang nothing heavy on a channel wall. The resilient channel calculator counts the channel and screws.
Damping compound
Green Glue is applied as random beads on the back of the second layer of board, which is screwed over the first within the open time on the tube. The maker recommends two tubes per 4 × 8 sheet, three for the heaviest damping, and a tube of its sealant for every six of compound.
A second layer of 6 sheets on one side of a wall
- 6 × 32 = 192 sq ft at 2 tubes per sheet: 12 tubes, one case
- 2 tubes of sealant for the perimeter and boxes
The Green Glue calculator counts tubes, cases or pails.
Insulation
Mineral wool or fibreglass fills the cavity without compressing; mineral wool is denser and easier to fit tight. In a single-stud wall it adds a few points; in a staggered or double-stud wall, where the sides are decoupled, it adds much more. The acoustic insulation calculator counts the packs.
Sealing
- Perimeter: leave a 1/4 in gap where the board meets the floor, ceiling and adjoining walls, and fill it with acoustic sealant, which stays flexible.
- Electrical boxes: offset them by at least one stud bay on either side of the wall, never back to back; wrap them in acoustic putty pads.
- Doors: a solid-core door with seals and a threshold, or the wall is only as good as the door.
- Ducts: a duct through a sound wall is a speaking tube; add bends or lined duct.
Flanking
Sound also goes around walls: through the floor and ceiling structure, through shared ducts and through gaps above a ceiling. A wall that stops at a dropped ceiling lets sound over the top. Treating the ceiling is covered in the guide to soundproofing a ceiling. Steel studs often help too, as the guide to metal vs wood studs explains.
What does not work
- Egg crates and foam panels. They reduce echo in a room, not sound through a wall.
- Mass-loaded vinyl alone on a single-stud wall. It adds mass, but less than a second layer of board in most cases.
- Insulation alone. A few points; the other ideas do the heavy lifting.
- Expensive board with leaks. Seal first.