Broadband low-to-mid-range frequency sound absorption using a micro-perforated panel with variable-depth parallel U-shaped cavities

This study proposes a microperforated panel (MPP) supported by four U-shaped cavities with different depths arranged in parallel, referred to as MPPU. A theoretical model was developed using the electro-acoustic analogy to calculate the normalized surface acoustic impedance expressions and the normal incidence sound absorption coefficient (SAC). The broadband sound absorption mechanism is explained using finite element analysis simulations. The performance of the MPPU was compared with both simulation and theoretical results to that of a microperforated panel supported by a straight cavity (MPPS) with similar geometric parameters. The MPPU outperformed the MPPS, achieving an average sound absorption coefficient (SAC) of 20% higher across the 300-1200 Hz frequency range. Theoretical and numerical modeling was successfully validated after printing the sample using additive manufacturing and testing the MPPU sample in an impedance tube for normal incidence sound absorption. The experimental results demonstrated that 69% of the incident sound energy was absorbed in the 300-1200 Hz frequency range and 89% in the 600-1200 Hz octave band. The tunability of the resonant frequency of the proposed MPPU structure was analyzed by changing the boundary condition, such as placing an impervious solid plate on one side of each U-shaped cavity and modifying the geometrical parameters on one side while keeping them fixed on the other. By doing this, the proposed structure can attenuate sound in the broadband spectrum while the overall sample thickness remains the same.