Treatment
Over-treating and under-treating a room
The two ways a treated room goes wrong — dead and boomy, or live and smeared — and what balance sounds like.
6 min read4 sourcesChapter 8 of 24
Schematic
Reverberation time across the frequency range for three rooms. Under-treated: long everywhere. Over-treated with thin material: short at the top, still long in the bass — the dull-but-boomy room. Balanced: short and roughly even, with the bass under control.
Animated illustration of the principle. Not measured data.
Most rooms that have been “done” are wrong in one of two directions. Either too little was done and the problems the treatment was meant to fix are still there, or a great deal was done — all of it thin — and the room now has a new set of problems that are harder to hear as problems because the room seems quiet.
The under-treated room
Hard floor, painted walls, glass, a ceiling of plaster. Dialogue smears, the stereo image wanders, and a hand-clap produces a metallic flutter between the parallel walls. Bass is uneven from seat to seat. People turn the volume up to hear the words, which makes everything else louder and worse.
This room is easy to diagnose and easy to improve; the first panels in the right places make a large difference. The danger is stopping there, or fixing it by throwing thin material at every wall.
The over-treated room
Foam or thin panels on most of the wall area, often a carpet and heavy curtains as well. The top of the range is gone: the room sounds muffled, voices lose their air, and it feels oppressive to sit in — people describe it as their ears feeling stuffed. Meanwhile nothing has touched the bass, so the low end still booms and rings. The result is a room that is simultaneously dead and boomy, and because the obvious flutter is gone, the owner often believes it is finished.
This is the harder failure to undo, because it means removing material that was paid for.
The tell: clap your hands. A balanced room gives a short, clean, slightly warm response. An over-treated room gives almost nothing at all — a thud with no life in it.
What balance sounds like
Reverberation roughly even across the range and appropriate to the room’s use. First reflections controlled at the seats. Bass that is the same note in every chair. And enough life left in the room that speaking in it feels natural — a conversation in a well-designed theatre does not sound like a conversation in a padded cell.
Getting there is a matter of proportion: thick absorption where it counts, deep trapping in the corners, some surfaces left reflective or made diffusive, and a measurement to confirm the curve is flat rather than tilted.
How much is right
There is no percentage-of-wall-area rule that survives contact with a real room, because thickness, position and the room’s own surfaces all matter more than area. A useful way to think about it: treat the problems the measurement shows, in order of size, and stop when the target curve is met. A room that reaches its RT60 target with 30 % of the walls treated is finished; adding more makes it worse.
For integrators and architectsThe numbers, the standards, and what to specify
- Diagnostic
- Plot RT60 per octave band. A curve falling steeply above 1 kHz while remaining high at 125–250 Hz is the signature of over-treatment with thin material; the fix is to remove thin coverage and add depth, not to add more.
- Design curve
- Aim for the low bands within roughly 1.45× the mid-band value (the DIN 18041 tolerance at 125 Hz) and the top bands close to it. A tilt that rises gently towards the bass is natural; a cliff is not.
- Reflective surfaces
- Plan which surfaces stay hard or become diffusive. Total absorption on all six surfaces is a specialist condition (anechoic-style measurement rooms), not a listening room.
- Sabine check
- Estimate total absorption from the proposed treatment before installing it. If the calculated mid-band RT60 falls well below target, the scheme is over-specified.
- Sequence
- Measure → place loudspeakers and seats → treat bass → treat reflections → measure → adjust. Treatment ordered the other way round is what produces both failure modes.
What to write into the brief
- A measured baseline and a target curve, with the treatment scheme sized to reach the target rather than to cover a percentage of the room.
Sources
- 01DIN 18041:2016 target formulas, as quoted in Germany’s federal BNB rating system (criterion 3.1.4)RT targets by room volume and use, and the tolerance band across frequency.
- 02UK Department for Education — BB93: Acoustic design of schools, performance standards (2015)Mid-frequency RT limits for lecture rooms, drama studios and halls (Table 6).
- 03EBU Tech 3276 — Listening conditions for the assessment of sound programme materialControl-room RT: Tm = 0.25 (V/100)^⅓ s, 0.2–0.4 s.
- 04ISO 3382-2:2008 — Measurement of room acoustic parameters, Part 2: Reverberation time in ordinary roomsInternational standard for measuring RT (T20 / T30).
External links open in a new tab. Figures quoted in this chapter are taken from these sources; a Science of Sound measurement is only ever presented as one, with its file, date and instrument.
Keep reading