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phonometry

Room acoustics

Almost everything about the sound field inside a room follows from two quantities: its impulse response, which can be measured, and its sound absorption, which can be designed. The pages of this section cover the measurement chain built on the first, the prediction chain built on the second, and the rating of the background noise that occupies the room in between.

One boundary runs through all of it: the Schroeder frequency. Above it a room has so many overlapping modes that a statistical description is the honest one, and every reverberation formula and decay parameter on these pages lives there. Below it the modes are discrete and separable, and no statistical model applies — which is why the measurement and prediction pages alike carry validity caveats at their lowest bands. A reader chasing a low-frequency problem should start from the modal treatment rather than from the decay parameters.

The measurement chain starts in Measuring the Room Impulse Response: the deterministic excitation signals of ISO 18233, the sweep deconvolution that turns a recording into an impulse response, and the MLS alternative. Room Acoustics then derives the ISO 3382 parameters from it (reverberation time, EDT, clarity, definition and centre time), and Open-Plan Office Acoustics (ISO 3382-3) answers the question a single closed room does not raise: how far speech stays intelligible across an open floor, through the spatial decay rate and the distraction and privacy distances. Image sources and the steady-state room field approaches the same room deterministically, building its impulse response from mirrored sources, its steady-state level from the room constant, and, below the Schroeder frequency where both of those give out, the discrete normal modes of the shoebox itself.

Before the two prediction pages, one page answers a different question about the same room. Room-noise criteria (NC / RC Mark II) asks whether its steady background noise (ventilation, distant traffic) is acceptable for its use, rated against the ANSI/ASA S12.2 criterion curves, with the RC Mark II rumble/hiss tag diagnosing why a spectrum fails.

Prediction gets two pages because two traditions coexist. Both are diffuse-field statistical models fed by the same laboratory absorption coefficients, so they are not rival physics; they differ in what they are admissible for. Reverberation-time prediction (Sabine, Eyring, Arau) covers the classical statistical formulae (Sabine, Eyring, Millington-Sette, Fitzroy and Arau-Puchades), including the models that handle a non-uniform absorption distribution. Sound absorption in enclosed spaces (EN 12354-6) covers the normative European version of the same physics: the total equivalent absorption area assembled from surfaces, objects and air, and the reverberation time that follows from it, as a standard a design report can cite.

Which one? Cite EN 12354-6 when the deliverable is a design report under a European building-acoustics framework, when the room is an ordinary building space inside the clause 4.6 validity limits, and when the receiving-room absorption has to feed an EN 12354 insulation prediction. Use the classical family when the room falls outside that scope — a hall, a theatre, an industrial space, or a room whose absorption is concentrated on one axis so that an axial model is needed — or when a band of predictions rather than a single normative value is what the situation deserves. Both share one failure mode, the loss of diffusivity, and they fail in the same direction: the measured reverberation time comes out longer than predicted, by up to a factor of two in the low-diffusivity rooms the standard’s own accuracy clause records. And neither replaces a measurement — the measured counterpart is Room Acoustics.

Related pages elsewhere: the absorption coefficient the prediction chain consumes is measured in Sound Absorption Measurement and Rating, insulation between rooms continues in Sound insulation, and the speech intelligibility a room affords is quantified by the Speech Transmission Index.

Pages in this section

Section titled “Pages in this section”

What this section does not cover

Section titled “What this section does not cover”

  • Nothing here is a wave solver. The image-source model is specular only: it carries no diffraction, no scattering off a diffuser and no finite-impedance boundary, and it stops when the reflection order runs out rather than when the sound does. Below the Schroeder frequency, where the statistical models give out, what this section offers is the mode positions of a rigid rectangular box — not the field of a real room at low frequency. For that, the wave simulation section runs an FDTD solver on the actual geometry.

  • No auralisation, no ray tracer, no room model. There is no geometry importer, no material database and no renderer: the pages take dimensions, coefficients and impulse responses as inputs, and give back parameters. The absorption coefficients themselves come from Materials and surfaces, and the model errs optimistically when the room is not diffuse — outside the EN 12354-6 clause 4.6 limits (no dimension more than five times another, opposite surfaces within a factor of three in absorption, object fraction below 0.2) the measured reverberation time can reach twice the predicted one.

  • Two coverage boundaries follow the standards. Only the normative clause 4 model of EN 12354-6 is implemented, not its informative Annex D method for irregular spaces. And nothing in this section measures insulation between rooms: that is Sound insulation.

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