How rooms behave · Start here
How sound behaves in a room
Direct sound, reflections, reverberation and resonance — the four things a room adds to every loudspeaker.
6 min read4 sourcesChapter 1 of 24
Schematic
One impulse from a loudspeaker: the direct sound arrives first, early reflections follow within a few milliseconds, and the dense tail is reverberation. At low frequencies the room’s dimensions set which notes ring on.
Animated illustration of the principle. Not measured data.
A loudspeaker in a field sounds the way it was designed to. Put the same loudspeaker in a room and you hear the loudspeaker plus the room — and in most rooms the room contributes more of what reaches your ears than the loudspeaker does.
Everything in acoustic design comes back to four things the room does to sound. Understanding them is enough to make sense of every other chapter here.
1. Direct sound
The straight-line path from loudspeaker to ear. It arrives first, and the brain uses it to decide where the sound is coming from. Everything acoustic design does is in service of keeping the direct sound clear and letting the room support it rather than bury it.
2. Early reflections
Within roughly the first 20 milliseconds, sound also arrives after bouncing once off the nearest surfaces — the side walls, floor, ceiling and the wall behind the loudspeaker. Because they arrive so soon after the direct sound, the ear folds them into it. They change the perceived tone, widen or smear the image, and at low frequencies they cancel parts of the response outright.
Managing early reflections is most of what people mean by “acoustic treatment” in a small room.
The mirror trick: sit in the listening seat, have someone slide a mirror along the side wall. Wherever you can see a loudspeaker in the mirror is a first reflection point.
3. Reverberation
After the early reflections comes a dense, decaying tail made of thousands of later reflections. How long that tail lasts is the room’s reverberation time. A cathedral rings for seconds; a well-designed control room decays in a fraction of one.
Too much reverberation blurs dialogue and turns music into mush. Too little and a room feels oppressive and unnatural. The right amount depends entirely on what the room is for.
4. Room modes
At low frequencies the room stops behaving like a hall of mirrors and starts behaving like an organ pipe. Between any two parallel surfaces there are frequencies whose wavelengths fit exactly, and at those frequencies the room resonates: certain bass notes boom in some seats and vanish in others.
Modes are set by the room’s dimensions, so they are the one thing treatment can tame but never remove. They are also why a room’s proportions matter before a single panel goes on a wall.
Why this is a system, not a shopping list
Each of the four interacts with the others. Absorb too much of the early reflections with thin panels and you shorten the reverberation at high frequencies only, leaving the bass to ring — a room that sounds both dead and boomy at once. Move a loudspeaker to fix a reflection and you change which modes it excites.
That is why a room is designed rather than decorated: the loudspeaker positions, the seat positions, the room’s proportions and the treatment are decided together, and then checked with a measurement.
For integrators and architectsThe numbers, the standards, and what to specify
The vocabulary an acoustician will use with you, and what each term is actually measuring.
- Impulse response
- The room’s complete answer to a single click. Every parameter below is derived from it. Measured with a swept sine and a calibrated microphone at the listening positions.
- Early / late boundary
- Reflections inside the first 50–80 ms (speech vs. music) are counted as early energy, which generally reinforces the direct sound; later energy is counted as late. These are measurement boundaries, not a hard line between useful and harmful sound. Clarity metrics such as C50 and C80 compare the two as a logarithmic ratio in decibels: 10 log10 (early energy ÷ late energy), split at 50 ms or 80 ms.
- RT60, T20, T30
- The time for sound to decay 60 dB. In practice measured over a 20 or 30 dB drop and extrapolated, per ISO 3382-2, because the noise floor rarely allows a clean 60 dB.
- Schroeder frequency
- Below roughly 2000 × √(RT60 / V) individual modes increasingly dominate and statistical acoustics become less reliable — a gradual transition, not a sharp line. For a 145 m³ theatre with RT60 around 0.4 s that is about 105 Hz — below it, the bass is increasingly a modal problem rather than a reverberation problem.
- Critical distance
- Where direct and reverberant sound are equal in level. Listening beyond it means the room dominates; small treated rooms push it well past the seats.
What to write into the brief
- Ask for the design target as a set: RT60 by octave band, early reflection control at named seats, and modal treatment — not a single “acoustic panels” line item.
- Insist on a before-and-after impulse response measurement at the primary seats. Without it there is no evidence the room changed.
Sources
- 01ISO 3382-2:2008 — Measurement of room acoustic parameters, Part 2: Reverberation time in ordinary roomsInternational standard for measuring RT (T20 / T30).
- 02ISO 3382-1:2009 — Measurement of room acoustic parameters, Part 1: Performance spacesDefines EDT, clarity (C50, C80), definition and strength for halls.
- 03Schroeder & Kuttruff, “On frequency response curves in rooms”, JASA 34(1), 1962Origin of the 2000·√(T/V) transition frequency.
- 04Kuttruff, Room Acoustics, 6th ed. (CRC Press, 2016)Standard textbook on room acoustics theory.
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.
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