Fresnelcut

Choosing

QRD vs PRD vs skyline: which diffuser do you need?

Schroeder diffusers come in a few flavours. They're all built from a number sequence that spreads reflections, but they differ in how many axes they scatter and what they do to the straight-back reflection.

QRD — the default

The quadratic-residue diffuser uses depths from `n² mod N`. It scatters in one axis, is the best-documented design, and is the right starting point for side-wall and rear-wall treatment. If you're not sure, pick this.

PRD — kills the straight-back echo

The primitive-root diffuser uses `g^n mod N` instead. Its special property is a notch in the specular reflection — the sound heading straight back at the design frequency is suppressed. Reach for it when a direct, front-to-back slap echo is the specific problem.

2-D QRD and skyline — both axes

A 2D QRD (`(i² + j²) mod N` over a grid) and a skyline diffuser (a primitive-root height map) both scatter horizontally and vertically. Use them on ceilings and large rear walls where reflections arrive from more than one plane. The skyline is the block-city look and suppresses the straight-back reflection on both axes; the 2-D QRD is the grid-of-wells look.

Quick guide

Related reading

What is a quadratic-residue diffuser?A plain-English explanation of the quadratic-residue diffuser (QRD): what the wells do, why the depths follow n² mod N, and when to use one.Diffuser vs absorber: what's the difference?Diffusers scatter reflections; absorbers remove energy. Learn which one fixes echo, which fixes boom, and when to use both.Where to place diffusers in a roomFirst reflection points, the rear wall, and ceiling clouds — a practical guide to where diffusers help most, and where to use absorption instead.

Design one now

1D Quadratic-Residue Diffuser (QRD)The classic well-and-fin panel that scatters horizontally.1D Primitive-Root Diffuser (PRD)A QRD cousin that suppresses the straight-back reflection.2D Quadratic-Residue DiffuserA grid of square wells that scatters in both directions.2D Skyline DiffuserThe block-city look — a 2-D primitive-root height map.

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