Sound diffusion is the scattering of reflected sound energy in multiple directions rather than a single specular direction. A diffusive surface takes an incoming sound wave and redistributes it broadly across the room, reducing the intensity of any single reflection while preserving the overall acoustic energy in the space.
Diffusion is the middle path between absorption and reflection. Where absorption removes energy from the room and reflection sends it back in one direction, diffusion keeps the energy alive but spreads it out — creating a sense of spaciousness, warmth, and envelopment without the deadness that comes from over-absorbing.
Real-World Analogy
Imagine shining a flashlight at a mirror. The light bounces off in one concentrated beam — that is specular reflection. Now shine the same flashlight at a sheet of crumpled aluminium foil. The light scatters in every direction, creating a soft, even glow rather than a single bright spot. The foil has not absorbed the light; it has diffused it.
Sound diffusion works on the same principle. A flat, hard wall reflects sound like a mirror reflects light — in one direction. A surface with irregular depth variations — wells, ridges, curved sections — scatters sound in many directions simultaneously. The total reflected energy is nearly the same, but no single reflection dominates.
Technical Definition
Acoustic diffusion is quantified by the diffusion coefficient (d), a value between 0 and 1 defined by ISO 17497-2:2012 and measured by AES-4id-2001:
- d = 0 means all reflected energy goes in one direction (perfect specular reflection)
- d = 1 means reflected energy is scattered uniformly in all directions (perfect diffusion)
How Diffusers Work
The most common engineered diffusers use wells of varying depths based on mathematical sequences:
Quadratic Residue Diffusers (QRD), invented by Manfred Schroeder in 1975, use well depths determined by the quadratic residue sequence modulo a prime number. Each well acts as a waveguide that delays the reflected sound by a different amount, causing the reflections from adjacent wells to interfere constructively in many different directions.
Primitive Root Diffusers (PRD) use a different number-theoretic sequence that produces a similar scattering effect with slightly different frequency characteristics.
Curved diffusers — convex cylinders, barrel vaults, polycylindrical panels — scatter sound geometrically rather than through interference. Their effectiveness depends on the radius of curvature relative to the wavelength.
The operating bandwidth of a diffuser is bounded by its physical dimensions. The lowest effective frequency is determined by the maximum well depth (approximately lambda/4), and the highest by the well width (approximately lambda/2). A typical QRD with 50 mm maximum well depth and 15 mm well width operates effectively from about 1700 Hz to 11,000 Hz.
Why It Matters for Design
Diffusion solves problems that absorption alone cannot:
Liveness without harshness. Recording studios and performance spaces need enough reflected energy to sound alive and natural, but specular reflections cause comb filtering, flutter echoes, and harsh coloration. Diffusers preserve energy while eliminating these artefacts.
Even sound distribution. In lecture halls and theatres, diffusive rear walls prevent strong late reflections (echoes) from bouncing back to the stage while keeping the rear seats energised. Absorbing the rear wall would solve the echo but leave the back rows feeling distant and disconnected.
Small room treatment. In control rooms and home studios under 80 cubic metres, heavy absorption is necessary to control RT60 but can make the room feel claustrophobic and fatiguing. Placing diffusers on the rear wall and ceiling maintains short RT60 at the listening position while preserving a sense of space.
Flutter echo elimination. Parallel walls create flutter echoes. The traditional solution is to absorb one wall, but this removes energy. An alternative is to diffuse one wall — the flutter echo disappears because the reflected energy is scattered, but the overall room energy is maintained.
How AcousPlan Uses This
AcousPlan's material database includes diffusive surfaces alongside absorptive and reflective ones. When you select a diffuser material, the calculator accounts for its absorption characteristics — because even diffusers absorb some energy — and incorporates this into the RT60 calculation.
The acoustic treatment recommendations generated by the AI Prescription Engine consider diffusion as a design option, particularly for performance spaces and recording environments where preserving acoustic energy matters. When the auto-solve algorithm detects that a room's RT60 is already near target but flutter echo risk is high (parallel reflective walls), it may recommend diffusive treatment rather than additional absorption.
Related Concepts
- What is Sound Reflection? — The phenomenon diffusion modifies
- What is Sound Absorption? — The alternative to diffusion for controlling reflections
- What is Echo? — A problem diffusion can solve without absorption
- What is Reverberation? — The overall decay that diffusion preserves
- What is RT60? — The metric diffusion influences less than absorption
Calculate Now
Ready to explore how diffusive surfaces affect your room? Use the AcousPlan Room Calculator to experiment with material combinations that balance absorption and diffusion for your target acoustic environment.