Microperforated Panel and deep subwavelength Archimedean-inspired spiral cavities for multi-tonal and broadband sound absorption

Abstract

In recent years, metamaterial-structures and Microperforated panel (MPP) absorbers have been proposed as a valid alternative to porous materials for sound absorption in the low frequency range. However, high and broadband absorption cannot be achieved in the low frequency range as required in some engineering applications where large thicknesses of the absorbers are unsuitable. In this work, a deep subwavelength hybrid parallel-arranged MPP and Archimedean-inspired spiral (AIS) absorber is proposed to obtain broadband sound absorption at low frequency (i.e. 400–2000 Hz). The absorption properties are investigated using an equivalent electro-acoustic model and a parametric analysis is performed to optimise the geometric parameters of the device for the desired frequency range. Two parallel arranged MPPs and AIS structures were designed to achieve sound absorption in frequency ranges between 550–1650 Hz and 380–1250 Hz, with a total thickness less than 1/28 wavelength (24.3 mm). In addition, a parallel arrangement of an AIS and a double layered MPP were also considered to achieve absorption in a wider frequency range (i.e. 480–2800 Hz). The prototypes were then fabricated and tested with an impedance tube to evaluate the normal absorption coefficient via the Transfer Function method (TFM). Experimental results show a good correlation with the analytical models, with a absorption coefficient above 60% over the respective frequency ranges. Moreover, absorption peaks occur at the resonance frequencies and higher harmonics of the structures, with measured values above 95%. The low frequency broadband absorption shown by the proposed subwavelength hybrid structures makes the device suitable for many acoustic engineering applications.