Types of Acoustic Diffusers: QRD vs Skyline vs BAD

The main types of acoustic diffusers compared here are one-dimensional QRD wells, two-dimensional skyline blocks and binary amplitude panels. Their surface patterns control reflected sound in different ways: QRD and skyline designs vary reflection phase, while a binary amplitude panel combines reflective and absorptive cells.

Choose by scattering direction, available depth and whether additional absorption suits the room. The comparison below distinguishes calculated geometry from performance that needs a measured curve.

Two numbers, and they are not the same number

Before comparing devices it is worth separating the two published metrics, because vendors quote whichever flatters the product.

The scattering coefficient is defined in ISO 17497-1 and measured in a reverberation room. It answers a single question: what fraction of the reflected energy leaves the specular direction? A surface can score highly and still send all of that energy into one strong off-axis lobe.

The diffusion coefficient is defined in ISO 17497-2 and in the AES information document on surface scattering uniformity, and it is measured from a polar response in a free field. It answers a different question: how uniform is the scattered energy across angle? This is the number that corresponds to what a diffuser is supposed to do.

A device with a high scattering coefficient and a mediocre diffusion coefficient is redirecting energy rather than dispersing it. Ask which method was used and compare results at the same frequencies and incident angles.

One-dimensional QRD

The classical reflection phase grating is a row of wells of different depths separated by thin fins, with depths drawn from the quadratic residue sequence. It scatters in the plane perpendicular to the fins, typically horizontally, and behaves close to a flat reflector in the other plane.

Vertical fins primarily distribute reflected sound horizontally. Rotating the panel rotates that principal scattering plane. Edge diffraction and finite panel height mean a real device is more complicated than a strictly two-dimensional model.

Depth follows the sequence and the design frequency. An N7 device designed at 500 Hz needs a deepest well of 19.6 cm; the full derivation and the tables for other primes are in QRD diffuser well depths.

Two-dimensional skyline

The skyline is a grid of square blocks at different heights, with the height of each block set by a two-dimensional residue sequence, commonly s(m, n) = (m squared + n squared) modulo N. It varies the reflecting surface in both horizontal and vertical directions. That permits scattering in both planes without guaranteeing a uniform hemisphere.

The cost of the extra plane is depth, and by more than intuition suggests. Combining two sequences produces residue values the one-dimensional sequence never reaches. For N = 7, the one-dimensional sequence tops out at 4 of 7, while the two-dimensional version reaches 6 of 7. Designed at the same 500 Hz, the one-dimensional panel is 19.6 cm deep and the block array is 29.4 cm, half again as much, before any consideration of the frame.

There is also a mass problem. A one-dimensional panel is mostly air behind a thin face. A block array is a solid grid of timber, and a full-size 500 Hz skyline is heavy enough that the fixing method stops being an afterthought.

A skyline is a candidate when scattering in both planes is useful and the mounting surface can support the calculated geometry. Ceiling and rear-wall installations still need suitable listener clearance and a fixing design for the actual mass.

Binary amplitude panel

The binary amplitude diffuser takes a different route entirely. Instead of varying depth to vary phase, it varies reflectivity across a flat surface: a perforated or slotted mask sits over a porous absorber, and the pattern of open and closed cells follows a binary sequence with a flat power spectrum, of the maximum length sequence type discussed in the cited literature. Reflective cells return energy, open cells pass it into the absorber, and the alternating pattern breaks up the wavefront without deep phase wells.

The cited patent for the planar binary amplitude diffusor describes exactly this construction, and states the trade-off plainly: the panel both absorbs and diffuses, the energy that is not absorbed is what gets scattered, and the device sits between a reflective panel and an absorptive one of the same size. A maximum length sequence is close to half ones and half zeros, so an open area near 50 percent is the design intent rather than a side effect. Treat it as a hybrid, not as a diffuser with a bonus. The patent publishes no absorption figure, and a real panel’s coefficient depends on the depth and density of the backing, so ask the vendor for a measured curve rather than assuming.

A flat face does not mean a zero-thickness treatment. The mask, absorber and any cavity occupy space. The binary pattern does not by itself determine an absorption coefficient or a certified operating band; those depend on the complete assembly and should be supported by measurements.

Side by side

Design question	1D QRD	2D skyline	Binary amplitude panel
What varies?	Air depth inside wells	Height of reflecting blocks	Reflective versus open/absorptive cells
Sequence	n² mod N	(row² + column²) mod N	Binary maximum-length sequence
Principal scattering directions	Perpendicular to fins	Both surface axes	Depends on the mask arrangement
Elements in an N7 example	7 wells per period	49 blocks per period	MLS lengths are 2ᵐ − 1, not QRD primes
Calculated relief at 500 Hz	N7 maximum well depth: 19.6 cm	N7 maximum block height: 29.4 cm	No QRD depth equation; backing thickness is separate
Face width at 3.8 cm cells	31.4 cm with eight 6 mm fins/cheeks	26.6 cm square for abutting blocks, before a frame	Sequence length and cell size determine mask dimensions
Frequency constraints	Depth scale, well aperture and phase repetition	Height scale, cell size and finite panel dimensions	Cell pattern, overall dimensions and backing response
Absorption	Depends on cavity losses and construction	Depends on materials and construction	Intentional through open cells over an absorber
Material and mounting	Hollow wells, backing and dividers	Many blocks; mass depends on material and height	Mask plus absorber and support
Construction requirement	Rigid, sealed wells with correct clear widths	Correct block order, stable backing and secure fixing	Preserve bit order and specify the absorber separately
Evidence to request	Polar response and diffusion curve for the assembled panel	Response in both planes across frequency	Diffusion and absorption curves for the complete assembly

The geometric examples are calculated with sound speed 343 m/s. Skyline heights use s × c / (2Nf₀); the N7 two-dimensional grid reaches residue 6. The QRD example uses two outer cheeks equal in thickness to its fins. These dimensions exclude additional casing or mounting hardware.

Generate a well sequence in 1D QRD mode, a block-height grid in 2D skyline mode, or an open/reflective pattern in BAD sequence mode.

Choosing between them

Start with the treatment’s job. A one-dimensional QRD suits a design that needs redistribution mainly in one plane. A skyline adds surface variation in the second plane at the cost of more pieces and, for the N7 example, greater maximum relief.

If projection into the room is tightly limited, a binary amplitude panel can avoid deep phase wells. It also introduces intentional absorption. Compare the depth and acoustic data of the full assembly before assuming it is the shallowest or most suitable option.

Placement cannot be selected from the acronym alone. Surface size, listening distance, existing decay and the reflection being treated all affect the choice. The same panel can be useful in one position and contribute little in another.

None of these geometry calculations designs a bass trap or corrects monitor positioning. For the distinction between removing reflected energy and redistributing it, see how acoustic diffusers work.

After the choice

For QRD and skyline designs, choose the prime, design frequency and cell width. For a binary amplitude pattern, choose the MLS length and cell width and specify the absorber separately. The diffuser calculator generates well depths, skyline block heights or a binary sequence, the depth tables for each prime are in QRD diffuser well depths, and the workshop side is covered in building a QRD diffuser.