Acoustic Diffusion
Definition
Acoustic Diffusion
Acoustic diffusion is the scattering of sound energy in multiple directions by an irregular surface, creating a more even spatial distribution of sound. Unlike absorption which removes energy, diffusion redistributes it, maintaining the room energy level while reducing discrete reflections, flutter echoes, and standing waves. Diffusers are key elements in performance spaces, studios, and critical listening rooms.
Diffusion is the third fundamental mechanism for controlling room acoustics, alongside absorption and reflection. While absorption reduces the total energy in a room (lowering RT60), diffusion redistributes energy in time and space without reducing it. This makes diffusion particularly valuable in spaces where reverberation is desired but problematic reflections are not — concert halls, recording studios, and home theaters.
Mathematical diffusers, based on number theory sequences, are the most effective. Schroeder diffusers use quadratic residue sequences (QRD) or primitive root sequences (PRD) to determine the depths of wells in a surface. When sound strikes a QRD diffuser, the different well depths cause different phase shifts in the reflected sound, spreading the energy uniformly across a wide angular range. The design frequency is set by the deepest well depth: f₀ = c/(2d_max).
Diffusion effectiveness depends on frequency. A diffuser works above its design frequency (determined by the well depth) and below the frequency where the well width equals the wavelength (above which specular reflection dominates). A typical QRD diffuser with 50mm well depth and 10mm well width works from about 3.4 kHz down to 400 Hz. Larger diffusers or deeper wells extend the low-frequency range.
Common diffusion strategies: rear wall of recording studios (prevents slap-back echo while preserving room energy), ceiling of performance spaces (distributes early reflections evenly), side walls of mastering studios (reduces lateral flutter echo). Bookcase-style irregular surfaces provide broadband diffusion at low cost.
The diffusion coefficient (defined in ISO 17497-2) measures how uniformly a surface scatters sound, ranging from 0 (specular reflection) to 1 (perfectly uniform scattering). This is distinct from the scattering coefficient (ISO 17497-1), which measures the ratio of non-specularly reflected energy to total reflected energy.
SonaVyx RT60 measurements indirectly reveal diffusion quality — a smooth, linear decay curve indicates good diffusion; irregular decays suggest uneven sound field distribution.
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