What is the difference between sound absorption and sound insulation?
A room can be hard to talk in because sound lingers inside it, or because sound arrives from outside it. The two problems have different names, and different cures.
The Australian Building Codes Board's handbook on sound transmission defines sound absorption as the ability of a material to absorb sound, converting the sound energy into heat. It defines sound insulation as the ability of a construction or building element to limit sound transmission through it. To soundproof, in Cambridge Dictionary's definition, is to prevent sound from coming into a room or building, or from being heard outside it: that is sound insulation.
The engineering firm NDY puts the difference plainly: concrete has high sound insulation and low absorption, while a lightweight fibreglass acoustic panel absorbs a lot of sound within a room and is not suitable for blocking sound between two spaces.
The two properties differ in what they control, how they are measured and what achieves them.
| Question | Sound absorption | Sound insulation |
|---|---|---|
| What it controls | Echo and reverberation inside the room | Sound passing through a wall, floor or ceiling into another space |
| How it is measured | Coefficients in a reverberation room to AS ISO 354:2006, rated as NRC or αw (decimals) | Rw in a laboratory and DnT,w on site; the higher the number, the better the insulation |
| What achieves it | Absorptive materials such as glass wool, rock wool and polyester, and fabric over a cavity or infill | Added mass, extra skins with a cavity, limp materials, damping, and absorption inside cavities |
| Typical example | A fabric wall panel, ceiling sail or curtain | A concrete wall, or a plasterboard wall with extra layers and an absorptive cavity |
Do acoustic curtains work?
Yes, for what absorption does. A curtain of absorptive fabric adds absorption to a room and shortens its reverberation. At the Health Campus in Bludenz, an attic event space used for meetings, lectures and yoga, Getzner Textil reports that pronounced reverberation had significantly impaired speech intelligibility, and that textile ceiling elements and acoustic curtains reduced the reverberation time.
Getzner makes its made-to-measure acunic curtain in Accentric Crash and Optic Structure, and its datasheets give the absorption of both fabrics measured as gathered curtains. Getzner's datasheet of 14 March 2024 rates Optic Structure, hung as a gathered curtain with 100 per cent extra fabric at a median 150 mm from the wall (Type G-150), at αw 0.90, class A, measured to DIN EN ISO 354 and rated to DIN EN ISO 11654.
How much a curtain absorbs in use depends on the fabric and on how it is hung. Flow resistance belongs to the fabric; absorption in use depends on it and on the construction behind it, which for a curtain means its fullness and its distance from the wall. The acoustic curtains guide covers those choices, and the curtain applications page shows Getzner's curtain systems.
Are soundproof curtains real?
A heavy curtain at a window absorbs sound that reaches it from inside the room. Sealing the window and the wall is the work of the construction itself, and the ABCB handbook notes that the acoustic performance of a sound-rated wall or floor is severely degraded by the presence of gaps.
For traffic or a neighbour, the window, door or wall is where the answer lies, and an acoustic consultant can tell you what it needs.
How do you stop sound passing between rooms?
The ABCB handbook lists the ways to improve the sound insulation of a wall, floor or ceiling: added mass; an extra skin of material, such as another layer of plasterboard with a cavity, where a deeper cavity insulates better; limp materials or materials of low stiffness; damping on thin, stiff elements; and sound absorption inside the cavity of a lightweight element. It also calls for every gap and penetration to be sealed, and for flanking paths, such as ducts and gaps around building elements, to be kept to a minimum.
Absorption helps insulation in one place the handbook names: inside the cavity of a lightweight wall, floor or ceiling.
When do you need both?
The two properties are independent, so many rooms need both. A meeting room needs walls with enough insulation for speech privacy, which the handbook links to DnT,w, and enough absorption inside for speech to stay clear. A home cinema or a music room needs walls that keep the sound in and surfaces that control how it behaves inside. Design them together: the construction does the insulating, and fabric wall panels, sails and curtains do the absorbing.
How should you read claims on product pages?
Product pages describe absorption and insulation in many ways, and three checks sort the claims:
- Is the figure a decimal coefficient, an NRC, an αw or Sabins per object? Then it describes absorption inside a room, and its mounting and standard should be stated beside it.
- Is there an Rw or DnT,w from a test of a whole wall, floor, ceiling, door or window? That is the figure that describes insulation.
- Is a percentage given? Absorption ratings are decimals, so ask what the percentage was calculated from.
How to read NRC and αw explains the absorption ratings in detail.
Acunic fabrics absorb sound. Cubic Pico on a 50 mm frame with 20 mm fibreglass infill measured NRC 0.85 to AS ISO 354:2006 (CATS report 2026-316.1): a figure for absorption in the room, while the wall behind the panel has an insulation rating of its own. The absorption values for each fabric are on its page in the collection.
