TL;DR
Acoustic treatment falls into two fundamental categories: absorption (which removes sound energy from the room) and diffusion (which redistributes it). The distinction matters enormously for design decisions, yet the two are frequently confused or conflated — sometimes by manufacturers who market reflective products as "diffusers" when they are merely decorative. Absorption reduces reverberation time, lowers the reverberant sound level, and creates a controlled, "dry" acoustic. Diffusion preserves reverberant energy but scatters it uniformly, eliminating specular reflections, flutter echoes, and comb filtering while maintaining the room's natural liveliness. The best rooms use both: absorption to bring RT60 to the target value, and diffusion to ensure that the remaining reverberant energy is smooth and evenly distributed. This article explains both mechanisms, compares product types, and provides decision rules for when to use each.
The Concert Hall That Sounded Dead
A 450-seat community concert hall in the English Midlands was designed with acoustic treatment to achieve an RT60 of 1.6 seconds — appropriate for orchestral music. The architect, working without a specialist consultant, specified absorptive panels on the rear wall and side walls to control the RT60. The panels were 50 mm polyester fibre with NRC 0.90, covering 35% of wall area.
The RT60 target was achieved. The hall measured 1.55 seconds at 1 kHz — within 3% of the target. But musicians and audience members complained that the hall sounded "dead," "clinical," and "like playing into a pillow." A visiting acoustician diagnosed the problem immediately: the hall had insufficient diffusion.
The rear wall absorption was removing energy that should have returned to the stage as diffuse reflections, providing the "bloom" and "support" that performers rely on. The side wall absorption was killing lateral reflections — the primary contributor to the spatial impression and "envelopment" that makes concert halls feel immersive (per ISO 3382-1:2009, quantified as the Lateral Fraction parameter LF).
The solution: replace 60% of the rear wall absorptive panels with QRD (Quadratic Residue Diffuser) panels, and replace alternate side wall absorptive panels with polycylindrical diffusers. RT60 increased slightly to 1.7 seconds (acceptable for the repertoire), but subjective quality improved dramatically. The musicians' term was "the hall woke up."
Absorption: Removing Energy
Absorbers work by converting sound energy into heat through viscous friction (in porous materials), mechanical damping (in membrane absorbers), or resonant dissipation (in Helmholtz resonators). The sound absorption coefficient α ranges from 0 (perfectly reflective) to 1.0 (perfectly absorptive).
Porous Absorbers
Fibrous and open-cell materials (mineral wool, polyester fibre, melamine foam, fibreglass) absorb sound through friction as air molecules oscillate within the material's interstices. Performance depends on thickness relative to wavelength.
| Material Thickness | Effective Absorption Range |
|---|---|
| 25 mm | > 1000 Hz |
| 50 mm | > 500 Hz |
| 100 mm | > 250 Hz |
| 200 mm | > 125 Hz |
| 300 mm + air gap | > 80 Hz |
Resonant Absorbers
Membrane (panel) absorbers and Helmholtz resonators absorb at specific frequencies determined by their physical construction. They are efficient at low frequencies where porous absorbers would need impractical thickness.
The Problem with Over-Absorption
Rooms treated exclusively with absorption can achieve excellent RT60 values and high STI scores while sounding subjectively unpleasant. The reason is that absorption removes all reflected energy indiscriminately — including the beneficial early reflections that provide spatial impression, envelopment, and tonal warmth. A room with RT60 = 0.3 seconds and no diffuse energy feels oppressively "dead" for anything other than critical monitoring.
Diffusion: Redistributing Energy
Diffusers scatter incident sound energy in many directions rather than reflecting it specularly (like a mirror) or absorbing it. The acoustic effect is analogous to the visual difference between a mirror (specular reflection), a matte white wall (diffuse reflection), and a black wall (absorption).
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Scattering Coefficient
ISO 17497-1:2004 defines the scattering coefficient s, ranging from 0 (perfectly specular) to 1 (perfectly diffuse). Most architectural surfaces have scattering coefficients between 0.1 and 0.3. A well-designed QRD diffuser achieves s > 0.7 within its effective frequency range.
| Surface Type | Scattering Coefficient (s) | Character |
|---|---|---|
| Smooth plaster | 0.02-0.05 | Specular — causes flutter echo, strong reflections |
| Brickwork (exposed) | 0.10-0.20 | Mild scattering |
| Bookshelves (filled) | 0.30-0.50 | Moderate scattering — good general diffusion |
| Timber battens (varied spacing) | 0.30-0.60 | Moderate-to-good diffusion |
| QRD diffuser (within design range) | 0.70-0.95 | Excellent diffusion |
| Polycylindrical surface | 0.40-0.70 | Good broadband diffusion |
QRD Diffusers (Schroeder Diffusers)
Manfred Schroeder (again) developed the mathematical basis for diffuser design in 1975. A QRD consists of wells of varying depth, where depths follow a quadratic residue sequence modulo a prime number N. The well widths must be less than half a wavelength at the upper design frequency, and the deepest well determines the lower frequency limit.
1D QRD: Wells vary in one direction only — diffuses sound in a semicircle perpendicular to the wells.
2D QRD: Wells vary in both directions (skyline diffuser) — diffuses sound in a hemisphere.
Diffuser Design Parameters
| Parameter | Formula | Example (N=7, f_low=500 Hz) |
|---|---|---|
| Number of wells | Prime number N | 7 |
| Well width | w < c / (2 × f_high) | < 43 mm for f_high = 4000 Hz |
| Max well depth | d_max = c / (2 × f_low) | 343 mm for f_low = 500 Hz |
| Design frequency | f_design = c / (2 × w × N) | 350 Hz |
| Panel width | N × w | 7 × 43 = 301 mm |
When to Absorb, When to Diffuse: The Decision Matrix
| Scenario | Absorb? | Diffuse? | Reasoning |
|---|---|---|---|
| RT60 is above target | Yes | Maybe | Absorption is the only way to reduce RT60 |
| RT60 is at target but flutter echo present | No | Yes | Absorption would over-damp; diffusion eliminates flutter |
| Recording studio rear wall | Maybe | Yes | Diffusion prevents focused rear reflection while maintaining room energy |
| Open-plan office | Yes | No | Speech privacy requires absorption to reduce D₂,S; diffusion would spread speech further |
| Concert hall side walls | No | Yes | Lateral reflections needed for spaciousness; diffuse them, don't remove them |
| Conference room | Yes (dominant) | Some | RT60 control is priority; diffusion on rear wall prevents late reflections to presenter |
| Restaurant | Yes | Yes | Absorption reduces overall noise; diffusion prevents "table-to-table" speech paths |
Combining Absorption and Diffusion
The most effective acoustic treatments combine both strategies:
The 60/40 Rule (Recording Studios): Treat approximately 60% of the room's surfaces with absorption (focusing on the front half of the room around the monitoring position) and 40% with diffusion (focusing on the rear wall and upper side walls). This achieves a short RT60 at the mix position while maintaining enough room energy for natural-sounding recordings.
The Reflection-Free Zone (Mixing Rooms): The area around the monitoring position should be free of strong specular reflections (first reflection points treated with absorption), while the remainder of the room uses diffusion to maintain energy. This approach, developed by Peter D'Antonio and John Konnert, separates the direct sound zone from the diffuse field zone.
Absorb Low, Diffuse High: In rooms where both low-frequency control and high-frequency liveliness are needed, use thick porous absorbers in corners (bass traps) and diffusers on walls at ear height. The absorbers handle the frequencies where diffusers would need to be impractically deep, while diffusers handle the frequencies where thin absorbers are already effective.
Common Mistakes
Mistake 1: Treating "diffusers" as decorative panels. Many wall features sold as "acoustic diffusers" are too shallow to diffuse at meaningful frequencies. A 20 mm deep pattern on a wall panel provides no diffusion below approximately 8500 Hz. If the product does not specify a scattering coefficient with a frequency range, it is decoration, not acoustics.
Mistake 2: Over-absorbing a performance space. The instinct when a room sounds bad is to "add panels." If the room is already near its RT60 target and the problem is discrete reflections or flutter echo, the answer is diffusion, not more absorption.
Mistake 3: Placing diffusers too close to the listener. Diffusers need a minimum distance from the listener (typically 3-5 metres for full-range diffusers) for the scattered wavefronts to re-integrate into a smooth field. Placed too close, they produce a "phasey" or "comb-filtered" sound.
Summary
Absorption and diffusion are complementary tools, not alternatives. Absorption reduces reverberation time and removes energy. Diffusion redistributes energy and eliminates specular reflection artefacts. The concert hall that sounded dead had achieved its RT60 target through absorption alone — but an RT60 number cannot capture the subjective quality of the reverberant energy that remains. Diffusion ensures that the reverberant energy is smooth, uniform, and pleasing. Specify both in your acoustic treatment design, and your rooms will sound as good as they measure.
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