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.