Acoustic materials: what is used and why

A treated room is built from a short list of materials, each doing one job. Confusion starts when a material is sold for a job it cannot do — foam as a bass trap, a curtain as soundproofing, a slat panel as a diffuser. This is a plain guide to what each one is for.

Absorbers

Mineral wool (rock wool) and glass wool are the standard cores for panels and traps: fibrous, fire-resistant, available in panel-grade densities, and predictable in performance. Polyester fibre is a common alternative where fibre-shedding is a concern; it is a little less absorptive for the same thickness but easier to handle. Open-cell acoustic foam absorbs well at high frequencies but is thin in practice and often flammable; it has a place in small studio applications and very little in a theatre.

All of these need a facing. Acoustically transparent fabric — an open weave you can breathe through — lets sound reach the core. Timber slats or perforated panels over the core change the panel’s character, extending its low-frequency reach and holding back some of the top; that is a design choice, not a flaw.

• Mineral wool, glass wool: broadband absorption, panel and trap cores, 50–100 mm typical for panels, 300 mm+ for corner traps.
• Polyester fibre: similar role, slightly lower absorption per thickness, no shed fibres.
• Acoustic foam: high-frequency absorption only; check fire rating.
• Fabric: must be acoustically transparent; backed or closed-weave fabric reflects.
• Perforated and slotted facings: part-absorber, part-resonator; tune by open area and hole size.

Diffusers

Rigid, sealed wells or blocks in timber, MDF or moulded polymer. The material needs to be hard and heavy enough not to absorb; the geometry does the work. Depth sets how low they act.

Isolation materials

These are the heavy ones. Isolation works by mass, by separation and by sealing, and the materials reflect that. Additional layers of gypsum board add mass. Mass-loaded vinyl is a dense flexible sheet — typically around 5 kg/m² — used to add mass where thickness is limited. Viscoelastic damping compound between two board layers converts vibration in the panel to heat. Resilient channels and isolation clips decouple a board layer from the studs so vibration does not pass straight through. Cavity insulation inside a double wall damps the air space between leaves.

At the openings: solid-core doors with compression seals and drop seals, and acoustic glazing units — laminated glass, dissimilar pane thicknesses, a wide sealed gap — are what turn a room’s weakest points into something close to the walls.

• Gypsum board, cement board: mass. More layers, more attenuation.
• Mass-loaded vinyl: mass in a thin flexible layer.
• Damping compound: converts panel vibration to heat between layers.
• Resilient channel, isolation clips, isolation hangers: decoupling.
• Cavity insulation: damps the double-wall air space; not a blocker on its own.
• Acoustic doors, seals, acoustic glazing: closing the openings.

Things that do less than claimed

Thin foam sold as a bass trap. Egg-crate foam of any kind. Curtains as soundproofing — a heavy curtain absorbs some high frequencies and blocks essentially nothing. Decorative slat walls a few centimetres deep sold as diffusers. “Soundproof paint”. Rubber underlay as an isolation floor without a floating slab. None of these is useless; each does a small fraction of what the name implies.

One test covers most of it. If a material is light, it absorbs at best. If it is heavy and airtight, it blocks. Nothing light blocks sound, and nothing heavy absorbs much of it.

Sources

1. 01Owens Corning, Fiberglas 700 Series Board — Product Data Sheet (ASTM C423, Type A mounting)Measured absorption coefficients for 25, 50, 75 and 100 mm board.
2. 02British Gypsum — Gyproc WallBoard 12.5 mm product data sheetMinimum weight 7.7 kg/m².
3. 03Johns Manville — Mass Loaded Vinyl noise barrier data sheet1 lb/ft² (≈ 4.9 kg/m²), tested to ASTM E90.
4. 04Warnock, NRC Canada Building Digest CBD-239 — Factors affecting sound transmission lossMass law, double-leaf walls and coincidence, from Canada’s National Research Council.
5. 05ASTM C423-23e1 — Sound absorption and sound absorption coefficients by the reverberation room method
6. 06ASTM E90-23 — Laboratory measurement of airborne sound transmission loss

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.