Absorption of Broadband Low‐Frequency Sound Beyond the Thermodynamics Limit: An Acoustic Resonator With Magnetic Bearing

This study reports the first experimental realization of magnetic negative stiffness to achieve a significant net gain in broadband, low-frequency sound absorption, beyond the thermodynamic limit posed by the relative incompressibility of cavity fluid at low frequencies that otherwise necessitates impractically large absorbers. Analysis based on experimental data demonstrates a fivefold amplification of the absorption bandwidth within the lowest two octaves of human hearing; theoretically, this could be further enhanced with improved structural design and fabrication accuracy. The device employs a magnetic bearing structure with its moving shaft constrained by a mechanical ball bearing, resolving the rotational magnetic instability that otherwise causes significant parasitic stiffness, an issue that has prevented previous designs from succeeding. From an energy perspective, the device creates a potential well where a small incident wave activates a large kinetic response for viscothermal absorption. The proposed magnetic device offers a highly practical and scalable solution for engineering applications.