Absorbers

An absorber can be characterised at three scales, and this subsection walks them from the finished product inwards: the mounted product in a reverberation room, the raw material in a flow rig, and the small sample in an impedance tube — the flow rig before the tube because the resistivity it measures is the single parameter every model the tube is fitted against starts from. Then come the prediction models that tie the three together, and the metamaterial designs that push them past the classical thickness rules.

Sound Absorption Measurement and Rating is the product scale: the ISO 354 reverberation-room measurement of the random-incidence coefficient α_s, the ISO 11654 weighted rating α_w with its letter class that absorber datasheets quote, and the ISO 12999-2 measurement uncertainty of both. It also answers the recurring question of when a reverberation-room number and a tube number can, and cannot, be compared.

Airflow Resistance is the material scale: the ISO 9053-1 static and ISO 9053-2 alternating determination of the airflow resistance, specific resistance and resistivity σ, the parameter that governs a porous absorber’s low-frequency behaviour and anchors every porous model downstream.

Impedance Tube is the sample scale: the complex reflection factor, surface impedance and absorption at normal incidence, by the ISO 10534-1 standing-wave-ratio and ISO 10534-2 transfer-function methods, plus the ASTM E2611 four-microphone transmission loss, and the virtual FDTD tube that cross-checks the wave solver against the same reduction chains.

Porous and Multilayer Absorbers closes the loop with prediction: the Delany-Bazley, Miki and Johnson-Champoux-Allard equivalent-fluid models turn the measured resistivity into characteristic impedance and wavenumber, and the transfer-matrix multilayer solver predicts the absorption of a whole construction, at any incidence and in a diffuse field, before anything is built.

Metamaterial Absorbers is where the prediction models leave the classical rules behind: slow-sound slit panels loaded by Helmholtz resonators reach perfect absorption at critical coupling from panels a fortieth of a wavelength deep, with the transfer-matrix model, the design solver and the FDTD cross-check of the meshed cell.

Pages in this section

• Sound Absorption Measurement and Rating: the ISO 354 reverberation-room measurement, the ISO 11654 weighted rating and class, and the ISO 12999-2 measurement uncertainty.
• Airflow Resistance: the ISO 9053 static and alternating methods for the airflow resistance and resistivity.
• Impedance Tube: normal-incidence absorption, surface impedance and ASTM E2611 transmission loss, plus the virtual FDTD tube.
• Porous and Multilayer Absorbers: the equivalent-fluid models and the transfer-matrix multilayer solver with perforated, microperforated and membrane layers.
• Metamaterial Absorbers: the critical-coupling condition and the slow-sound slit panel with its design solver.

What this section does not cover

• None of these pages qualifies a laboratory. ISO 354’s room requirements of Annex A — the number of loudspeaker and microphone positions, the diffusing elements — are not checked; the functions convert an already-measured decay pair and only warn when the room volume or sample area falls outside the clause 6 limits. Two editions are cited but not implemented: the code follows the 1998/2001 transfer-function method of ISO 10534-2, not the 2023 edition, and ASTM E2611-19, not E2611-24. The probe-traverse refinements of ISO 10534-1 (extrapolating the minima to the sample face, probe-body corrections) are described but not automated. The prediction models are forward-only: they turn a resistivity into an impedance, and no inverse solver recovers the material parameters from a measured curve. And no measurement standard governs the metamaterial designs at all — a built panel is verified in the impedance tube or the reverberation room like any other absorber, which is why that page states a prediction and not a rating.