Treatment
Diffusion
Scattering sound instead of absorbing it — what diffusers do, where they belong, and where they are a waste of money.
6 min read5 sourcesChapter 6 of 24
Schematic
A flat surface returns the wave as one strong reflection. A diffuser of the same size returns it as a spray of weaker reflections across a wide angle, each arriving at a slightly different time.
Animated illustration of the principle. Not measured data.
Absorption removes energy. Diffusion keeps the energy in the room but breaks up its direction, so a single strong reflection becomes many weak ones spread over time and angle. Done well it makes a room feel larger and more natural without making it dead. Done badly it is an expensive piece of wall sculpture.
How a diffuser works
The classic design is the quadratic residue diffuser: a row of wells of different depths in a sequence derived from number theory. Sound entering different wells travels different distances before returning, so the reflections leave with different phases and scatter across a wide angle. A two-dimensional version — a grid of blocks of varying height, often called a skyline — scatters in both directions.
The well depth sets the lowest frequency the diffuser affects; the well width sets the highest. A diffuser with 100 mm wells is doing useful work from roughly the upper hundreds of hertz upwards. Below that it is a slightly textured flat wall.
Where diffusion belongs
The rear wall of a listening room or control room is the classic position: it removes the slap-back that would otherwise arrive at the seat from directly behind, without absorbing so much that the room feels closed. Side walls behind the listening position, and the ceiling in a larger room, are the other common candidates.
In a home theatre with a full surround array the case is weaker — the decoded channels already provide envelopment, and most designs lean towards absorption with some diffusion at the rear for naturalness. In an auditorium, diffusion on side walls and ceiling is fundamental to how the hall sounds.
Diffusers need distance. Sit close to one and you hear the individual wells as a flutter. They belong on walls a good few metres from the nearest seat.
Where it is wasted
On first reflection points, where the goal is to prevent a reflection reaching the seat rather than to spread it. In small rooms where no seat is far enough from the surface. And at low frequencies, where a diffuser shallow enough to fit a room does nothing at all — bass problems are solved with placement and trapping, never diffusion.
Decorative “diffusers” with a few centimetres of relief affect only the top octave. They may look the part; acoustically they are close to a flat wall.
For integrators and architectsThe numbers, the standards, and what to specify
- Design frequency
- Lowest effective frequency ≈ c / (2 × max well depth) for a QRD; a 150 mm deep unit is effective from roughly 1.1 kHz (with useful scattering somewhat below). Highest frequency is set by well width, f ≈ c / (2 × width).
- Distance
- Minimum listener distance is commonly taken as around three wavelengths at the design frequency, with practitioners recommending 2–3 m for typical units.
- Scattering coefficient
- ISO 17497 defines scattering and diffusion coefficients; ask manufacturers for measured data rather than diagrams.
- Materials
- Wells must be rigid and sealed. Timber, MDF or moulded units; soft or open wells absorb rather than diffuse, which is a different device.
- Hybrid surfaces
- Slotted or perforated panels over absorber combine some scattering with absorption and are often the practical answer in a domestic room where a full QRD is too deep.
What to write into the brief
- Diffuser design range and minimum listener distance stated; location limited to rear wall and rear side walls unless a hall design says otherwise.
Sources
- 01ISO 17497-1:2004 — Random-incidence scattering coefficient
- 02ISO 17497-2:2012 — Directional diffusion coefficient
- 03Pilch & Kamisiński, Archives of Acoustics 36(4), 2011 — Schroeder diffuser design (open access)Well depth sets the lower limit, well width the upper; listeners at least 3 wavelengths away.
- 04Cox, Angus & D’Antonio, Proc. Institute of Acoustics 25(5), 2003 — performance of Schroeder diffusers
- 05Cox & D’Antonio, Acoustic Absorbers and Diffusers, 3rd ed. (CRC Press, 2017)The reference text on absorber and diffuser design.
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.
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