Acoustical siphon effect for reducing the thickness in membrane-type metamaterials with low-frequency broadband absorption

Acoustical siphon effect in membrane-type metamaterials for low-frequency broadband absorption is proposed, whose physical mechanism is further investigated by the theoretical analysis and finite element (FE) simulation. This kind of membrane-type metamaterials consists of multiple detuned units, each of which is composed of two aluminum platelets fasten on a piece of silicone membrane above an air cavity. For the multi-unit metamaterial illuminated by an incident plane wave with certain frequency, there exists a certain unit at resonance with maximum acoustic absorption, and meanwhile other units nearly keep static due to the narrow absorption peak of the resonant unit. Therefore, almost the whole incident energy can be forced to flow to this resonant unit resulting in a much enhanced vibration and a reduced acoustic impedance that could be more matchable to air medium, and thus much more acoustic absorption appears without increasing the unit thickness, which is called acoustical siphon effect of the unit. On this basis, by precisely designing the acoustical siphon effect of each unit, the broadband absorption can be obtained by a subwavelength six-unit sample in the low-frequency range of 400–650 Hz with the maximum absorption coefficient of almost 100% and the average absorption coefficient of about 80%, which is then verified by the corresponding experiment. The results presented here would offer a new approach for the metamaterials design for low-frequency broadband sound absorption and could have potential applications in controlling vibration and noise.