About this tool
The QRD and Skyline Diffuser calculator computes cell depths for two classical types of acoustic diffuser. QRD (Quadratic Residue Diffuser) is a one-dimensional design that scatters sound in the horizontal or vertical plane. Skyline is a two-dimensional design that scatters in both planes at once. The calculator takes the lower and upper working frequency, panel dimensions, and the maximum available construction depth, then outputs a depth table built from the Schroeder quadratic-residue sequence (1975) and a 2D cell layout.
Why it matters: at mid and high frequencies, after first reflections are absorbed, a room often sounds too "dead" — the lack of diffusion strips the sound of spaciousness and natural feel. A diffuser preserves acoustic energy in the room while breaking it into many directional reflections, turning a coherent "mirror" response into an even diffuse field. This is especially valuable on the rear wall of a control room, on the ceiling above the listener, and in performance spaces.
How to read results: the "Lower working frequency" is determined by the maximum cell depth — the deeper the cells, the lower the diffuser operates. The "Upper working frequency" is determined by cell width — narrower cells operate higher. The calculator warns if the chosen cell width exceeds λ/2 at the upper frequency — in that case high-frequency diffusion degrades and you should reduce the cell pitch.
Common mistakes: placing a diffuser too close to the listener — closer than 3 meters, individual cells are perceived as separate reflectors instead of forming a uniform diffuse field; trying to replace an absorber at the first-reflection point with a diffuser — for delays of 1–5 ms you specifically need absorption, not scattering; building cells with arbitrary aspect ratios instead of relying on primes — breaking the Schroeder sequence reintroduces specular lobes.
Next steps: use the First Reflections calculator to determine where in the room a diffuser makes sense and where absorption remains the priority. To assess the overall balance of treatment, open the Treatment Planner and compare RT60 before and after installing scattering elements.
