QRD Diffuser

A Quadratic Residue Diffuser (QRD) is a sound-scattering device consisting of a series of wells of different depths, calculated using quadratic residue number theory. Invented by Manfred Schroeder, QRD diffusers scatter incident sound energy uniformly across a wide range of angles, reducing specular reflections without absorbing energy. The design frequency and bandwidth are determined by the well depths, widths, and the prime number used in the sequence. One-dimensional QRD diffusers scatter in one plane; two-dimensional designs scatter in both planes. QRDs are widely used in recording studios, control rooms, concert halls, and worship spaces to provide spatial uniformity, reduce flutter echoes, and maintain liveliness. Effective diffusion occurs above a design frequency related to the deepest well depth and below a frequency related to well width. ISO 17497-2 defines the measurement of diffusion coefficients.