Diffusion — what it does and what it doesn't do

What diffusion is

A specular reflector — a flat, rigid surface — reflects sound at an angle equal and opposite to the angle of incidence, like light from a mirror. A specular reflection from a nearby surface arrives at the listening position as a distinct, coherent wavefront shortly after the direct sound.

A diffuser scatters incident sound energy across a wide range of angles rather than returning it as a coherent reflection. The energy is redistributed spatially and temporally: the reflected wavefront is broken into many lower-amplitude arrivals spread over time and direction. The total energy is conserved — diffusion does not absorb; it redirects.

The practical effect is that a surface which would otherwise produce a strong, discrete early reflection instead contributes a diffuse wash of lower-amplitude, spatially distributed energy. This tends to preserve a sense of spaciousness and liveliness in the room without the comb filtering and imaging artefacts associated with strong specular reflections.

What diffusion does not do

Several misconceptions about diffusion are widespread:

Diffusion does not reduce RT60. Reverberation time depends on the total absorption in the room, not on how reflections are distributed. Replacing an absorptive panel with a diffuser will increase RT60, not reduce it. If the acoustic goal is to shorten decay time, use absorption.

Diffusion does not treat low-frequency modal problems. Room modes arise from standing waves between parallel surfaces. A diffuser on a wall will scatter mid and high-frequency energy but has no significant effect on the modal structure below the diffuser's design frequency. Bass traps — not diffusers — address modal problems.

Diffusion does not replace absorption. The two serve different purposes and should be selected accordingly. A room treated entirely with diffusion will be lively and reverberant but lack the controlled decay and reduced early reflection density that accurate monitoring requires.

How diffusers work

Flat surfaces are perfect specular reflectors at frequencies where the wavelength is small relative to the surface dimensions. As wavelength increases and becomes comparable to the surface dimensions, diffraction causes the reflection to spread — all surfaces diffuse to some extent at sufficiently low frequencies.

Purpose-designed diffusers create predictable, wide-angle scattering through surface geometry. The two most common types are:

Quadratic residue diffusers (QRD). A QRD consists of a series of wells of equal width but different depths, arranged according to a quadratic residue sequence. For a design frequency f₀ and well width w, the depth of well n is:

dₙ = (n² mod p) × λ₀ / (2p)

where p is a prime number (the sequence period), n is the well index, and λ₀ = c/f₀. Each well introduces a different phase shift to the reflected sound; the combination of these phase shifts causes destructive interference in the specular direction and redistributes energy to other angles.

The QRD is effective between its design frequency f₀ and approximately f₀ × p/2. Below f₀, the well depths are too shallow relative to the wavelength to scatter effectively. A QRD designed for f₀ = 500 Hz with well width w = 34 mm (half-wavelength at 5 kHz, setting the upper frequency limit) and p = 7 covers approximately 500 Hz to 1.75 kHz.

Maximum length sequence diffusers (MLS). Based on a binary sequence, MLS diffusers alternate between two surface heights — raised or recessed — according to a pseudorandom sequence. They are simpler to manufacture than QRDs and provide good scattering, particularly in two dimensions.

Curved surfaces. Convex curved surfaces scatter sound without the frequency dependence of well-based designs. They are less precisely controllable than QRDs but effective at a broad range of frequencies and easier to integrate architecturally.

The scattering coefficient

Diffuser performance is characterised by the scattering coefficient (s), defined in ISO 17497:

s = 1 − (specular reflected power / total reflected power)

s = 0 denotes a perfectly specular surface; s = 1 denotes perfectly diffuse scattering with no specular component. Real surfaces fall between these limits, with the value depending on frequency and angle of incidence.

The scattering coefficient is used in geometric room acoustic simulation software to model the diffuse component of room reflections. Without accurate scattering coefficients, simulations tend to overestimate late reverberation clarity.

When to use diffusion

Diffusion is appropriate when the goal is to:

• Reduce the strength of a discrete early reflection without shortening RT60 — i.e. preserve liveliness while avoiding comb filtering
• Create a more spatially uniform reverberant field in a room that otherwise has strong specular reflections from parallel surfaces
• Treat the rear wall of a control room or listening room, where absorption would create a dead zone behind the listening position

Diffusion is not appropriate as a substitute for:

• Bass trapping in rooms with modal problems
• Absorption in rooms that need RT60 reduction
• First-reflection point treatment where the goal is to reduce early reflection level

A typical listening room design uses absorption at first-reflection points (side walls, ceiling), diffusion on the rear wall, and a combination of bass absorption and broadband panels elsewhere. The balance between absorption and diffusion is a design decision, not a formula, and should be informed by measurement.