Sound insulation · Stone wool
ROCKWOOL Safe 'n' Silent Pro
- Thickness
- 25–100 mm
- Density
- 40–100 kg/m³
- Sound absorption (NRC)
- 0.90–1.00
What is sound transmission through walls, and what is flanking?
In short
Sound gets from one room to another in several ways: directly through the separating wall, through leaks such as gaps under doors or outlets, and by flanking — sound that goes around the separating wall via the floor, ceiling, side walls or ducts. What the neighbour hears is the sum of all paths, so the sound insulation achieved on site is often lower than the laboratory STC or Rw of the wall.
How sound travels
- Airborne sound — speech, music and TV travel through the air and set walls or ceilings vibrating
- Structure-borne sound — vibration travels through the structure, such as floors, beams and columns, and is radiated as sound in another room
- Impact sound — footsteps, dropped objects, chairs dragged; structure-borne sound from direct impacts on the floor
Wall insulation mainly helps with airborne sound through the wall. Structure-borne and impact sound also have to be handled at junctions and floors.
The paths sound can take
| Path | Examples |
|---|---|
| --- | --- |
| Directly through the separating wall | The boards vibrate and radiate sound on the other side |
| Leaks | Gaps under doors, around frames, wall-to-floor or wall-to-ceiling joints, outlets cut through the wall |
| Flanking over the ceiling | The wall stops at the suspended ceiling, with an open void above shared by both rooms |
| Flanking through ducts | Ducts or diffusers connecting the two rooms |
| Flanking through the structure | Floors, side walls or ceilings continuous under or beside the separating wall |
An example of how much flanking matters
A National Research Council Canada (NRC) publication reports tests in a wood-frame test building where the separating wall rated STC 52 in the laboratory, yet the whole system between the two rooms measured ASTC 44–45 because sound went around via the floor. In another case, adding gypsum board raised the wall itself to STC 57, but the system only reached ASTC 43, because the dominant flanking path had not been dealt with.
These figures belong to that test construction, not to every building, but they show the key point: upgrading the wall alone may not help if most of the sound is going another way.
Laboratory vs on-site results
| Laboratory | On site | |
|---|---|---|
| --- | --- | --- |
| What is measured | A single wall or floor, with flanking kept to a minimum | Sound between two real rooms by all paths |
| Test standard | ASTM E90 / ISO 10140-2 | ASTM E336 / ISO 16283-1 |
| Rating | STC / Rw | e.g. ASTC, DnT,w, R'w |
On-site results are usually lower than laboratory ones because they include flanking, leaks and workmanship. If a project specifies an on-site result, flanking has to be designed for from the start. More in What are STC and Rw.
Reducing flanking and leaks
- Seal leaks around the wall perimeter, at joints and where services pass through, with acoustical sealant or as the wall system specifies
- Electrical outlets — avoid placing boxes back to back on both sides. NRC research found that the position of metal outlet boxes can affect a wall's sound insulation. Electrical work should be done by a qualified electrician
- Walls and ceilings — for serious sound separation between rooms, the separating wall usually needs to go up to the floor or structure above, or the ceiling void must be treated as the designer specifies
- Ducts — ducts running through several rooms can carry sound between them; duct layout and internal lining are for the HVAC designer
- Structural junctions — continuous floors or side walls should be considered at design stage; fixing them later is usually harder and more expensive
More wall insulation alone may not be enough
If the sound comes via the ceiling, door or floor, adding insulation or boards to the wall helps little. Find the main path first. See Why cavity insulation improves a wall's sound insulation.
See solutions: Noise from next door / neighbours · Machine noise
Sources
- National Research Council Canada — Construction Technology Update No. 66, Airborne Sound Insulation in Multi-Family Buildings (Quirt & Nightingale, 2008)
- National Research Council Canada — Construction Technology Update No. 27, Effect of Electrical Outlet Boxes on Sound Isolation of Gypsum Board Walls (Nightingale, Quirt & Sultan, 1999)
- ASTM E336 — Measurement of Airborne Sound Attenuation between Rooms in Buildings
- ASTM E1414 — Airborne Sound Attenuation Between Rooms Sharing a Common Ceiling Plenum
- ISO 16283-1 — Field measurement of sound insulation in buildings — Part 1: Airborne sound insulation
- ASTM E90 / ISO 10140-2 — laboratory measurement of sound insulation
- USG — Sound Control Manual (perimeter caulking to prevent leaks)
Related products
Categories and example products for this topic
- ROCKWOOL Safe 'n' Silent Pro — Stone wool for wall and ceiling cavities - both are paths sound can take
- SCG Cylence ZoundBlock — Glass wool for the separating wall cavity, improving the wall system's sound insulation
- Micro Duct Liner PFL — Duct path: perforated-foil glass wool liner absorbs noise in ducts; NRC 0.70–0.85 by model
- Aeroflex ACOUSTEX — Duct path: fibre-free open-cell foam duct lining, NRC 0.65 at 25 mm
Sound insulation · Glass wool
SCG Cylence ZoundBlock
- Thickness
- 50–100 mm
- Density
- 32 kg/m³
- Thermal conductivity (λ)
- ≤ 0.033 W/m·K
Duct and HVAC insulation · Glass wool
Microfiber Micro Duct Liner PFL
- Thickness
- 25–50 mm
- Density
- 32–48 kg/m³
- Sound absorption (NRC)
- 0.70–0.85
Duct and HVAC insulation · Acoustic foam
Aeroflex ACOUSTEX
- Thickness
- 25 mm
- Sound absorption (NRC)
- 0.65
- Service temp.
- -20 – 85 °C