Sintered porous panels from recycled lead-free crystal glass cullet: Pore structure, acoustic transport parameters and governing role of installation geometry

ABSTRACT Sintering crushed lead-free crystal glass cullet yields a rigid, binder-free porous panel that absorbs sound, yet the mechanism governing its acoustic response has not been documented. The Johnson–Champoux–Allard transport parameters of the sintered structure are estimated by inverse characterisation from normal-incidence impedance-tube data at five air-gap depths, cross-checked against independent determinations of porosity and airflow resistivity, and tested against configurations that did not enter the fit. The inverted airflow resistivity of 18–21 kPa·s·m⁻² agrees in magnitude with air permeametry on the same specimens. With the porosity fixed at the water-accessible pore fraction of 0.14, the deviation of the model from the data increases three- to fourfold; the best fit requires a porosity at least as large as the total porosity (0.25–0.30, depending on the reference volume), so pore volume inaccessible to water appears to take part in the acoustic response. The viscous characteristic length of 180–220 µm is an order of magnitude smaller than the millimetre-scale voids, which suggests that dissipation is controlled by narrower constrictions between the fragments. With a specific airflow resistance of 0.56–0.76 of the characteristic impedance of air, the panel acts as a near-impedance-matched resistive layer: the absorption maximum is set by the rear termination and moves from about 1.1 kHz to 430 Hz as the air gap alone increases from 10 to 70 mm, and is predicted at about 320 Hz for 120 mm. Characterisation of the parent glass, residual stress and flexural strength completes the description.