A novel tuning strategy for broadband sound absorption using an inhomogeneous micro-perforated panel backed three cavity system

Multi-cavity micro-perforated panels (MPPs) have been widely investigated for broadband sound absorption; however, a systematic methodology for determining the resonant frequencies of individual sub-MPPs is still lacking, particularly for compact systems with limited cavity depth. This study proposes a half-absorption-bandwidth-based resonance-placement strategy to systematically determine the resonant frequencies of sub-MPPs in a three-cavity inhomogeneous micro-perforated panel (iMPP) system, using the resonant frequency and the left and right half-absorption bandwidths of a homogeneous MPP as reference parameters. Three resonance-placement configurations, namely both-lower (LL), lower–higher (LH), and both-higher (HH), are investigated. An electro-acoustic model based on Maa’s formulation is used to examine the effects of perforation ratio, pore diameter, and surface-area ratio while maintaining a 30 mm cavity depth and 3 mm panel thickness. The strategy is first studied for a target resonant frequency of 400 Hz and subsequently validated for a lower target resonant frequency of 300 Hz, demonstrating the applicability of the same tuning approach at lower frequencies. Among the investigated configurations, the LH arrangement with a 50:25:25 surface-area distribution provides the most favorable broadband response. For the theoretical 400-Hz design, the half-absorption bandwidth ( \(\alpha \ge 0.50\) ) increases from 193 to 290 Hz (50.3%), while the corresponding absorption area increases from 54.51 to 75.30 (38.1%) relative to the homogeneous MPP. Reducing the pore diameter of the second and third sub-MPPs increases viscous resistance, flattening the absorption peaks and broadening the effective bandwidth. Thermoviscous finite-element simulations and impedance-tube measurements support the theoretical predictions, demonstrating the potential of the proposed strategy for compact broadband absorbers.