Isolation
Doors, windows and flanking paths
The wall is rarely the weak point. The door, the window, the slab, the ceiling void and the duct are — and sound finds whichever is easiest.
7 min read10 sourcesChapter 11 of 24
Schematic
A theatre and the bedroom beside it, in section. The party wall stops the direct path. Sound still reaches the bedroom over the wall through the ceiling void, under it through the shared slab, through the gap at the bottom of the door, and along the air-conditioning duct that serves both rooms.
Animated illustration of the principle. Not measured data.
Ask what a wall achieves and you will be quoted a number for the wall. Ask what the room achieves and the answer is always lower, because sound leaves a room by every path available and the total is set by the weakest one. In an existing building the weakest one is almost never the wall.
Doors
A standard flush door is a thin hollow panel with a gap under it and no seals. It leaks sound as freely as an open window leaks air, and a room with a superb wall and a standard door performs like the door. The upgrade is a solid-core or purpose-built acoustic leaf, heavy enough to matter, hung in a frame with compression seals on the head and jambs and a drop seal or threshold seal at the foot. Where the target is high, two doors in series with a small lobby between them outperform any single door.
Every seal has to close. A door that is heavy but not sealed is a heavy door with a gap.
Windows
Ordinary glazing is light and stiff and sits in an aluminium frame full of gaps. The options in order of effect: remove the window and build the wall through; keep it but add a second, independent glazing unit on the room side with a wide sealed gap; or replace it with a purpose-made acoustic unit of laminated glass in dissimilar thicknesses. Curtains, however heavy, do not change what passes through the glass — they absorb some high frequencies inside the room and that is all.
Where a window must stay for light or for the view, a secondary unit on the inside face, sealed all round and spaced as far from the outer glass as the reveal allows, is the practical answer in most Indian construction.
A rule from the first isolation chapter applies here twice over: seal first, then separate, then add mass. A window frame is mostly seals.
The structure
In a building of RCC slabs and columns, the floor of the theatre is the ceiling of the room below, cast as one piece. Bass from a subwoofer drives the slab directly, and the slab carries it to every room it touches. A partition wall built on that slab and up to the slab above is a bridge between the two. This is structure-borne flanking, and no wall specification addresses it.
The answers are a floating floor — a second slab or deck on resilient mounts, carrying the loudspeakers and the seats — an inner ceiling hung on isolation hangers, and inner wall leaves that stand on the floating floor and stop short of the structure. That is the room-within-a-room, and it is the only way to a high target in a shared building. It also needs the structural engineer’s sign-off for the added load.
Ducts, penetrations and the ceiling void
A false ceiling with a void above it is an open corridor between rooms unless the wall continues up through the void to the slab. Air-conditioning ducts that serve two rooms carry sound between them; the fix is a lined duct, a silencer, or separate runs. Cable trays, conduits, back-to-back electrical boxes and the gap around a pipe are each a hole; each is packed and sealed. It is unglamorous work, and it decides the result.
For integrators and architectsThe numbers, the standards, and what to specify
- Composite rule
- The overall transmission loss of an assembly is governed by its weakest element in proportion to area. A 25 dB door taking a fifth of a 55 dB wall brings the composite to about 32 dB — far nearer the door’s figure than the wall’s. Composite: TL = 10·log(S_total / Σ τᵢSᵢ). Calculate the composite before promising a number.
- Doors
- Specify leaf surface mass, tested performance of the door set (leaf, frame and seals together, e.g. to ISO 10140 / ASTM E90), seal type on all four edges, and a threshold detail. Consider a lobby with two doors for targets above roughly 45–50 dB.
- Door data
- Specify leaf surface mass, tested performance of the door set (leaf, frame and seals together, e.g. to ISO 10140 / ASTM E90), seal type on all four edges and an automatic drop seal at the threshold. Published data: hollow-core door STC 18 against STC 30 for a solid-core door with gaskets and a drop seal (NBS Monograph 77); single acoustic doors are available from about STC 32 to 54 (Steel Door Institute); the US federal door specification accepts an installed result up to 5 points below the laboratory rating. Consider a lobby with two doors for high targets.
- Glazing
- Secondary glazing with 100 mm or more gap and dissimilar pane thicknesses; laminated glass for the coincidence dip; absorbent reveal lining between the panes; seals continuous.
- Glazing data
- Saint-Gobain data: single panes Rw 30 (4 mm), 32 (6 mm), 35 (10 mm); ordinary double-glazed units about Rw 34 whatever their 6–16 mm cavity; acoustic laminate improves on plain glass of the same thickness, and dissimilar pane thicknesses reduce the coincidence dip. For more, secondary glazing with a wide gap, absorbent reveals and continuous seals.
- Structure
- Floating floor on resilient mounts or continuous mat with a stated natural frequency well below the lowest frequency of concern; isolated ceiling on hangers; inner leaves free of the structure. Confirm slab capacity.
- Services
- Duct silencers or lined duct on any run shared between rooms; crosstalk attenuators; every penetration detailed with a sealing method; no back-to-back boxes.
What to write into the brief
- A flanking path schedule listing every route between the room and its neighbours, with the treatment for each, before the wall build-up is priced.
- Door sets and glazing specified as tested assemblies, not as components.
Sources
- 01Herrin, University of Kentucky Vibro-Acoustics Consortium (2021) — design of enclosuresComposite transmission loss: τ_total = Σ τᵢSᵢ / S_total.
- 02Warnock, NRC Canada Building Digest CBD-239 — Factors affecting sound transmission lossMass law, double-leaf walls and coincidence, from Canada’s National Research Council.
- 03ASTM E90-23 — Laboratory measurement of airborne sound transmission loss
- 04ISO 10140-2:2021 — Laboratory measurement of airborne sound insulation
- 05ISO 16283-1:2014 — Field measurement of airborne sound insulationField results include flanking paths.
- 06ASTM E336-25a — Measurement of airborne sound attenuation between rooms in buildings (field)
- 07Berendt & Winzer, NBS Monograph 77 — Sound insulation of wall, floor and door constructions (1964)Door types, gaskets and gaps.
- 08Steel Door Institute — Specifying acoustic doors
- 09UFGS 08 34 73 — Sound control door assemblies (US federal specification)Field performance against laboratory rating.
- 10Saint-Gobain Glass UK — Factors influencing glazing performanceRw of single, laminated and double glazing.
External links open in a new tab. Figures quoted in this chapter are taken from these sources; a Science of Sound measurement is only ever presented as one, with its file, date and instrument.
Keep reading