Wideband reduction of in-duct noise using acoustic metamaterial with serially connected resonators made with MPP and cavities

For the design of duct silencers, one should satisfy the essential constraints on the sound attenuation band, additional volume, and backpressure. For wideband sound attenuation, various acoustic metamaterials (AMM) using multiple resonators have been proposed. However, they often do not satisfy the spatial constraint, and the blocking of the conduit makes them impractical. This study proposes a compact silencing AMM unit for wideband sound reduction without deteriorating the mechanical performance. Previous works on the stacked micro-perforated panels (MPP) with different backing air gaps provide the basic idea of this work, which reveals the benefit of multiple resonators in adjusting the bandwidths to attain a wideband attenuation characteristic. The resistive element is also exploited in the MPP for suppressing the acoustic transparency of the detuned resonators. The formulated theoretical design method is tested by using a resonant unit cell configured with a serial connection of quadruple MPP layers, each air gap with a length of 30mm and a uniform sectional area of 8x8mm(2). For minimizing the occupied volume, each cell surrounds the outer periphery of the main duct by folding, and the cell entry is flush-mounted on the duct wall. The test is conducted with the main duct of 30x30mm(2), and the attached 50 cells are arranged periodically with a 10-mm interval. The additional width of the duct is less than 1% of the wavelength. The measured power transmission coefficient is less than 0.2 for the range of 0.4-4.05kHz, which agrees well with the prediction.