Room Acoustics & Defects
What goes wrong in a room — and how to fix it.
When a room sounds wrong, it is usually one of a handful of named defects — echo, flutter, focusing, dead spots, boom or excess reverberation — each with a cause in geometry or material and a cure. Then acoustic correction: the three absorber types (porous for highs, panel for lows, Helmholtz for a tuned band), diffusers and room shape. Diagnose the defects with the explorer below.
Learning objectives
By the end of this lesson, you will be able to — mapped to the course outcomes for Acoustics in Architecture:
Diagnose the acoustic defects — echo, flutter, focusing, dead spots, resonance.
Prescribe the cure for each defect.
Distinguish the three absorber types and the frequencies they target.
Use diffusers and room shape for acoustic correction.
The defects
Echo, flutter, focusing, dead spots and resonance each have a geometric or material cause — and a cure. Pick one in the explorer.[2]
Acoustic defects · pick one to diagnose
Cause
A strong single reflection delayed enough to hear separately — a delay over ~50 ms, i.e. a path difference over ~17 m (often the rear wall).
Cure
Absorb or diffuse the offending surface, or angle it so the reflection misses the audience.
Reflections out of time
An ECHO is a strong single reflection delayed enough to hear as a separate event — a delay over about 50 ms, i.e. a path difference over ~17 m, usually off a hard rear wall. A FLUTTER ECHO is a rapid 'buzz' of sound bouncing between two parallel hard walls. The cures: for echo, absorb, diffuse or angle the offending surface; for flutter, splay one wall a few degrees or treat one of the two parallel faces. Try the explorer below for the full set.[2]
Acoustic correction
Match the absorber to the frequency, use diffusers to scatter without deadening, and shape the room — splay walls, avoid domes — first.[2, 4]
Each targets a band
Acoustic correction uses three absorber types, each best at a different frequency. POROUS absorbers (mineral wool, foam, carpet, curtains) lose energy to friction in their pores — best at MID-HIGH frequencies (and need real thickness to catch lows). PANEL/MEMBRANE absorbers (a thin sheet over an air gap) flex in resonance — best at LOW frequencies. HELMHOLTZ resonators (a cavity with a neck or perforations) absorb a SPECIFIC tuned frequency. Match the absorber to the problem frequency.[2, 4]
At a glance
| Aspect | One | The other |
|---|---|---|
| Echo vs flutter | Echo: one delayed reflection (rear wall) | Flutter: rapid buzz between parallel walls |
| Curved surfaces | Concave: focuses (bad — hot/dead spots) | Convex/diffusing: scatters (good) |
| Absorber by band | Porous: mid-high · panel: low | Helmholtz: a specific tuned frequency |
| Absorb vs diffuse | Absorb: removes energy (quietens) | Diffuse: scatters energy (keeps it alive) |
| First move | Shape: splay walls, avoid domes | Then treatment: absorb / diffuse |
Key terms
A delayed single reflection (>~50 ms, >~17 m path) heard as a separate event.
A rapid buzzing repeat of sound between two parallel hard walls.
A concave surface concentrating reflections into hot spots (and dead spots elsewhere).
Standing waves (room modes) at low frequencies set by the room's dimensions.
Mineral wool, foam, carpet — best at mid-high frequencies.
A flexing sheet over an air gap — best at low frequencies.
A cavity with a neck — absorbs a specific tuned frequency.
A convex or sculpted surface that scatters sound without removing it.
Studio task
Take a hard-walled rectangular hall and list its likely defects (flutter, echo, focusing); then prescribe a correction for each — naming the absorber type or shape change — using the defect explorer above.
Self-assessment
1. A rapid 'buzzing' repeat of sound between two parallel hard walls is —
2. A concave dome over an audience tends to cause —
3. To absorb a low-frequency 'boom', the most suitable absorber is a —
Recap
References & further reading
- [2]M. David Egan, Architectural Acoustics. McGraw-Hill / J. Ross Publishing.
- [4]F. Alton Everest & Ken Pohlmann, Master Handbook of Acoustics. McGraw-Hill.
Further reading
- M. David Egan, Architectural Acoustics. J. Ross Publishing.
- F. Alton Everest, Master Handbook of Acoustics. McGraw-Hill.
- Leslie Doelle, Environmental Acoustics. McGraw-Hill.
Sources gathered and fact-checked June 2026. Published values vary by source, sample and method — treat as indicative and confirm against the cited standard before structural use.