Assessing airflow performance of ventilated acoustic metamaterials: a methodology to foster comparability of results

One of the main challenges in characterizing ventilated acoustic metamaterials is the absence of a shared and clearly defined approach for assessing their airflow permeability. This paper proposes a laboratory-scale method for evaluating the ventilation performance of metamaterials that, being based on existing standardized procedures, allows for using equipment and measuring tools typically available in acoustic laboratories. The method relies on measuring pressure drops across the samples as a function of airflow velocity, which is not new in the literature, but uses this information in order to derive figures of merit that can be easily compared across different studies. Specific airflow resistance and the still-air discharge coefficient were selected as reference indicators. To demonstrate their practical use, measured and simulated data were compared with analytical results. Six case studies, involving different geometrical configurations representative of different typologies of metamaterials, and perforation rates, were analyzed. The results show that the proposed methodology effectively describes the airflow permeability of the tested samples and, if adopted, could enable simple and consistent comparisons of the ventilative performance of acoustic metamaterials.