How it works
The well depths follow a quadratic-residue sequence — for a prime number of wells N, the depth of well n is set by `n² mod N`. That specific pattern is what spreads reflected energy evenly across a wide fan of angles instead of beaming it back as a single echo. Fresnelcut uses the Schroeder grating relation, not the hobbyist quarter-wave shortcut, so the depths match published references.
Every well shares one width W and the fins share one thickness. Pick a lower design frequency and Fresnelcut solves the depths; the panel keeps working up to roughly `c / (2·W)` before the wells get wide compared to the wavelength.
When to reach for a QRD
A 1-D QRD scatters in the plane perpendicular to its fins. Mount the fins vertically and it spreads sound left-to-right — ideal behind a listening position or on side walls at the first reflection points. For scatter in both axes, step up to a 2D QRD or a skyline diffuser.
s_n = n² mod N, d_n = s_n · λ₀ / (2N)N = prime well count, λ₀ = wavelength at the design frequency f₀. Deeper wells come from larger residues.
Best for
- First-reflection points on side walls
- Rear-wall treatment behind the listener
- Rooms that sound harsh or echoey in the mids and highs
Not the tool for
- Low-frequency boom or boominess (use an absorber instead)
- Situations needing even scatter in both directions (use 2-D)
Start from a common size
N is the number of wells (a prime). More wells scatter more smoothly but make a wider, deeper panel. Pick one to open the designer pre-set: