Acoustic attenuation and effective properties of single and periodic Helmholtz resonators having porous core

An array of Helmholtz resonators is a potential solution to attain the tuned acoustic attenuation in low frequency regime where the higher bandwidth over individual Helmholtz resonator is of interest. It has been shown earlier that the calculated transmission loss (TL) and effective acoustic properties for different configurations of Helmholtz resonators agree to the experimental measurements (Applied Acoustics 155(1), 371–392), however, frequency regimes of interest are of narrowband. In this paper, significant improvement in bandwidth of interest has been demonstrated via installing porous material in the cavity of periodic Helmholtz resonators. The key discovery of the research is that on installing the porous material inside the cavity of a Helmholtz resonator, the peak amplitude of TL at resonance frequency regime is getting damped and makes it impractical. Whereas, with an array of resonators having porous core, the observed TL is having desired amplitude and elevated bandwidth which makes it practical. The influence of filling fractions of porous core, airflow resistivity of porous material, and size of array elements are presented. Next, the effective properties such as effective bulk modulus and effective density, which are negative in broadband, are retrieved from four pole parameters and have been shown agreeing to the experimental measurements.