Materials and surfaces

Every room prediction and many an insulation model end up consuming a coefficient that describes what a material or a surface does to sound. This section covers where those coefficients come from: the laboratory instruments that measure them, the single-number ratings that summarise them, and the in-situ methods that recover them outside the laboratory.

Acoustic Materials covers the sample-scale instruments. The reverberation room yields random-incidence absorption coefficients that ISO 11654 collapses into the weighted rating α_w with its letter class, the figure absorber datasheets quote. The flow rig measures airflow resistance and resistivity per ISO 9053-1/-2, the parameter that governs a porous absorber’s low-frequency behaviour and anchors most material models. The impedance tube recovers, at normal incidence, the complex surface impedance, reflection factor and absorption coefficient of a small sample (ISO 10534-1/-2) and, with four microphones, its transmission loss (ASTM E2611).

Porous and Multilayer Absorbers turns the measured flow resistivity into predictions: the Delany-Bazley, Miki and Johnson-Champoux-Allard equivalent-fluid models give a porous material’s characteristic impedance and wavenumber, and a transfer-matrix stack of porous, air, perforated, microperforated (Maa) and membrane layers predicts the absorption of a whole construction, at any incidence angle and in a diffuse field, before anything is built.

Surface Scattering, Diffusion and In-situ Absorption moves from samples to surfaces, asking not how much energy a surface absorbs but where it sends what it reflects. The random-incidence scattering coefficient (ISO 17497-1) quantifies how much energy leaves the specular direction; the diffusion coefficient (ISO 17497-2) quantifies how uniformly the reflected energy spreads; and the ISO 13472 methods measure the absorption of a road surface in situ, by the extended-surface subtraction technique (Part 1) or the spot tube (Part 2).

The consumers of these numbers are spread across the site: absorption coefficients feed the reverberation predictions in Room acoustics, dynamic stiffness (measured by a related load-plate method) feeds the floating-floor model in Sound insulation, and the road-surface methods connect to the outdoor-noise interest of the Environment and transport section.

Pages in this section

• Acoustic Materials: the ISO 11654 weighted absorption rating and class, ISO 9053-1/-2 airflow resistance, and impedance-tube absorption, impedance and transmission loss.
• Porous and Multilayer Absorbers: the Delany-Bazley, Miki and JCA porous models, the transfer-matrix multilayer solver with perforated, microperforated and membrane layers, and the random-incidence Paris integral.
• Surface Scattering, Diffusion and In-situ Absorption: ISO 17497-1/2 scattering and diffusion coefficients, and ISO 13472-1/-2 in-situ road-surface absorption.