The membrane sound absorber (MSA) with a compact magnet has exhibited excellent tunable properties for low-frequency sound absorption. To further clarify its acoustic properties, this paper presents a theoretical model based on a multi-mechanism coupling impedance method. The model predicts the absorption coefficients and resonant frequencies of the MSA at different tuning magnetic states for three cavity configurations. These parameters are then experimentally measured using an impedance tube for model validation, demonstrating good agreement between the measured and predicted values. Subsequent analysis reveals the iron-platelet-magnet resonance mechanism introduced by the tuned magnetic field is the main factor behind the appearance and shift of absorption peaks in the low-frequency region, which are mostly independent of the back cavity. In other words, the MSA with a back cavity of any size can achieve sound absorption in the low-frequency region. This demonstrates the potential of the structure in achieving an ultra-thin, low-frequency, tunable sound-absorber design that can be adapted to different noise sources.
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