Acoustics
Acoustic treatment
Treatment changes absorption and diffusion inside the room. A porous absorber is weak at low frequency if it is thin or hard against the wall. Coverage on a drawing is not an RT60, and this page gives no absorption coefficient.
Absorption turns energy into heat. Diffusion scatters it in direction and time. They are not the same shape. A thin porous panel on a hard wall works where it is thick relative to the wavelength, which is usually the mid and high bands, not the bass.
Definition
Acoustic treatment is the design of the internal surfaces: absorber, reflector, and diffuser. A porous absorber — fibre, open foam, or PET felt — lets air through and dissipates energy by friction. At 100 Hz the wavelength is about 3.4 m if the speed of sound is taken as 343 m/s, and a quarter wavelength is about 0.85 m. A 25 mm layer on a hard wall is not a tool for that frequency unless it is spaced off the surface or made thicker, and even then the verdict is a report, not the quarter-wave rule.
The absorption coefficient α is measured for a named mounting to ISO 354 and is shown with frequency. A single number without a report is not a calculation input. Hatched area on a drawing is drawing coverage, not an RT60. Reverberation time is the result of volume, absorption, and distribution, and in performance spaces it is measured to ISO 3382-1. This page publishes neither a product coefficient nor a hall time.
Common mistakes
Coverage equals RT60 — Hatching the walls is not a time in seconds. The time is the result of a calculation or a measurement in bands.
PET treated as bass control — Thin felt does not treat 100 Hz, because it is thin relative to the wavelength. The name is not a thickness.
An invented coefficient — There is no α on this page. A figure with no frequency and no report invents product data.
Absorber where a reflector belongs — Killing the early lateral reflection makes the room dry and distant even if an average time looks acceptable.
When needed
- An intelligible late echo — A rear wall or a ceiling returns the syllable after a gap the listener hears as echo. Absorber or tilt goes on that path, not on every surface.
- A speech room that is too live — When the tail stacks syllables. The remedy is absorption distributed in the band that matters for speech, not a blanket on every surface.
- Music that needs a tail, not a dead spot — Too much high-frequency absorption kills brightness and leaves the bass. Diffusion and an early reflector may be the tool, not more felt.
- A small room and strong colouration — Low modes are not solved by thin felt. Thickness, spacing off the wall, or a calculated resonant absorber, then measurement.
Where used
- Spoken theatre — A rear wall and limited side absorption so the early reflection that supports speech is not deleted.
- Lecture halls — Ceiling and rear walls where the path is long, keeping frontal support if the talker is unamplified.
- Rehearsal rooms — Enough absorption that level does not rise with playing, without deleting all the return a player needs.
- Small meeting rooms — PET felt or an absorber thick enough for the room volume. Full coverage in thin sheet is a look, not a design.
Parts
- Porous absorber — Fibre, open foam, or PET. It works by thickness and spacing off the hard surface. Thin material is weak at low frequency.
- Perforated face — Wood or metal with an open area and a hole diameter. Without a cavity and an absorber behind, perforation is a texture.
- Diffuser — A surface that scatters according to depth and design frequency. Not every wooden box is a diffuser, and the depth is set by the frequency band, not by the shape.
- Reflector — A hard, dense surface for an early reflection. Solid wood in the speech range is usually a reflector. It is not counted as an absorber because the photograph looks warm.
- Resonant absorber — A membrane or a cavity tuned to a low band. It needs a calculation and a place. It is not a general substitute for porous thickness.
Engineering
- Wavelength — A speed near 343 m/s puts 100 Hz near 3.4 m. A porous absorber hard against the wall weakens when its thickness is a small fraction of a quarter wavelength.
- Distribution, not only area — Absorption only in the ceiling leaves lateral decay and flutter. Sabine already assumes distribution. Concentration breaks the assumption.
- Coverage is not RT60 — A percentage of wall with no frequency coefficients and no volume does not produce a second. The calculation consumes A and frequency, then measurement decides.
- The audience is part of A — Occupied seats absorb. A design tuned empty and handed over full changes RT60. The condition is named in the criterion.
- Do not mix with isolation — Absorber closes neither flanking nor a door. If the complaint is transmission, the question belongs on the isolation page.
Materials
- PET and fibre — PET felt is a common porous face. α comes from an ISO 354 report for its thickness and mounting, not from the word PET. This page states no coefficient.
- Fabric and the face — A closed or heavy fabric can seal the pores and kill the absorption. The tested face is the face that gets installed.
- Fire and impact — The class comes from the report of the build-up, not from the felt’s name. The lower wall in an aisle needs a layer that takes impact, from the product specification.
Related systems
- RT60 — Treatment is one input to the time. The verdict is an ISO 3382-1 measurement in performance spaces, not the area of panels.
- The sound system — Aiming the loudspeaker off a late hard surface reduces what the absorber must hide. Equipment and room together.
- Lighting and services — A spotlight and a pipe sit in the cavity or puncture it. Treatment is drawn with the services route.
- Cladding and the ceiling — A perforated wall, a ceiling cloud, and a mineral tile are ways to build treatment. Each one with its tested build-up.
Questions
Why is there no coefficient table?
Because the coefficient belongs to an ISO 354 report for one build-up. A generic table would invent product data, which this page does not do.
Is diffusion weak absorption?
No. A diffuser scatters. Its absorption may be low. Use it when you want to keep the energy without a specular echo.
Should every wall be covered?
Rarely. Distribution and the early reflector matter more than a blanket. Full coverage in thin material kills the highs and leaves the lows.
Does treatment isolate the neighbour?
No. That is isolation: mass, door, HVAC, and flanking. The absorber stays inside the room.
How do I know the treatment worked?
By measuring the room to ISO 3382-1 in performance spaces, against the brief’s criterion in each band, not by how the panels look.