Acoustics · SERVICE · REV 2026-10-06
Acoustic treatment
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.
When is this service 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 it is 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 of the solution
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 considerations
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.
Design sequence
- 01
Name the complaint: a late echo, a long tail in one band, a dead spot, or bass colour. Each one puts the material in a different place.
- 02
Calculate volume first. A larger hall needs more A for the same time. Panels do not fully compensate for a wrong volume.
- 03
Ask for ISO 354 data for the actual build-up: thickness, cavity, and face. Reject a coefficient with no frequency.
- 04
Distribute absorber, reflector, and diffuser on the section. Do not cover every surface equally because the schedule is easier.
- 05
Leave a measurement criterion: bands, audience condition, and ISO 3382-1 where it applies. Coverage is a drawing target, not acceptance.
Delivery sequence
- 01
The cavity is built as drawn. Felt stuck flat after it was designed on a spacing is a different product.
- 02
The face in the report is the face installed. Paint that seals the pores, or a different fabric, changes α.
- 03
The absorber stays vented if the design needs it, and closed if isolation requires that. The two are not mixed in one cavity without a decision.
- 04
The diffuser is installed at its full depth. Cutting the box to pass a pipe deletes the design frequency.
- 05
Panels are not covered by chairs or a heavy curtain if their face is the absorber. Operational covering is part of the effective area.
Testing
Room measurement
ISO 3382-1 for performance spaces: times in bands, not a phone application in one seat.
Comparison with the prediction
A gap between a Sabine calculation and the measurement is expected when absorption is high or concentrated. The measurement is not forced to match the formula.
No coefficient from site for this page
Inferring α from one room measurement is a rough estimate. The product evidence remains ISO 354 for its mounting.
Commissioning
Room condition
Empty, half full, or with the curtains as the brief named. The figure is recorded with the condition.
Services running
HVAC running, because noise hides the decay and spoils T20 or T30.
Visible defects
Cavities stuck shut, panels removed, or curtains covering the absorber. Correct them before accepting the figure.
Maintenance
Face replacement
A new fabric can seal the pores. Replacement matches the tested face or reopens the acoustic question.
Paint
Paint on felt or fibre kills absorption. Clean only by the method the manufacturer allows.
Casual additions
Boards and signs on the absorber reduce the area. Repeated change is calculated, not ignored.
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.
Short comparison
The comparison is a way to choose a path. It is not a product recommendation and it is not a code.
| Path | Fits when | Weakens when |
|---|---|---|
| Porous absorber | The mid and high bands must come down, with thickness or a cavity matched to the lowest frequency that matters. | When the problem is bass in a small room and the panel is thin and stuck flat. The tool is wrong for that band. |
| Diffuser | The energy is useful if it is scattered, and the room cannot take more absorption. | When a strong specular echo comes from a nearby surface and the diffuser depth is not available. An absorber or a tilt may be required. |
| Early reflector | Speech or unamplified music needs nearby lateral or ceiling support. | When the reflection is late and is heard as an echo. A reflector here makes the path worse. |
Discussion ranges
Any number here is a preliminary discussion range from practice or a named reference. Replace it with the project criterion, laboratory data, and on-site measurement.
| Item | What is discussed | Limit of this page |
|---|---|---|
| Quarter wavelength at 100 Hz | With a wavelength near 3.4 m, a quarter of it is near 0.85 m, as a picture of porous thickness or spacing off a hard wall. | Discussion geometry, not an absorption coefficient. The speed of sound changes with temperature, and ISO 354 for the mounting is the evidence. No α is derived from this figure. |
| α to ISO 354 | A frequency coefficient for a named mounting: type A, type E, or another mounting the standard defines. | This page publishes no coefficient for any product. A figure without a frequency and a report does not enter a specification. |
| Coverage on the drawing | A hatched area ratio. It describes the drawing. It does not describe the room’s energy. | It is not an RT60. The time needs a volume and a frequency-dependent A, then a measurement. There is no conversion from a percentage to a second on this page. |
| Single-number NRC | A coarse laboratory average of several mid bands under a stated mounting, when a product sheet uses it. | It is not an α curve and not a theatre RT60. This page quotes no NRC for a product. Design needs the bands. |
What this page does not claim
No fire classification, absorption coefficient, sound pressure level, or legal capacity on this page may be copied into a quotation. Those values come from a product test report, a measurement, or the local code adopted for the project.
Questions designers and owners ask
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.
Technical sources
- ISO 354
Measurement of absorption coefficient in a reverberation room. This page does not replace the report and does not copy an α.
- ISO 3382-1
Measurement of room parameters after treatment. Coverage on the drawing is not that measurement.
- ISO 11654 where it is used
A single-number αw derived from ISO 354 data under stated rules. It is not a substitute for the curve, and this page does not calculate it for a product.
Last editorial review: 2026-10-06. Review means the text was checked, not that a project was measured.
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