Acoustic metaporous layer with extended neck structures for enhancing low frequency absorption performance

Porous materials with rigid backing are widely used as sound absorbers with good absorption performances particularly at mid and high frequencies, but poor performances at low frequencies. Such poor performances are related to the absorber thickness that is required to be about a quarter sound wavelength for effective absorption. Hence, the materials become bulky for reducing noise at low frequencies. It is necessary to enhance the porous material performance at low frequencies under the same thickness by which a broadband sound absorption characteristic is also present. In this study, the resonator system with extended neck was added to porous material to form metaporous layer. The neck of the resonator was extended by coiling it up to allow longer neck to reside in a limited air cavity thickness of 30 mm. The behaviors of the proposed structures are investigated by varying the length and the width of the extended neck of resonators. For this, the numerical models and simulation are developed using Finite Element Method (FEM) to predict the absorption behavior. The results show that the metaporous absorption increases at low frequency compared to the porous materials only. Moreover, the sound absorption bandwidth of the metaporous increases with extended neck structure width increase. These characteristics are also found from the experimental results. The results suggest that the extended neck structure can be used as a design parameter to enhance the porous material absorption performance. Moreover, the proposed structures can extend the absorption bandwidth further ranging from low to high frequencies.