Geometric shape effects of cavity and neck on acoustic absorption performance of Helmholtz resonator with extended neck

This study investigates the acoustic characteristics of Helmholtz resonator with an extended neck backed by honeycomb cavity. The honeycomb cavities can be arranged closely to save space and light the design. What's more, hexagonal geometry can effectively improve the structural strength. To capture the characteristic of acoustic wave propagation in the extended neck and the backing cavity, the equivalent density and bulk modulus within the neck and cavity are calculated. These equivalent parameters consider the effects of viscosity and thermal conductivity by solving the wave equation over the cross section of the neck and cavity with finite element approach. By combining the equivalent parameters, the transfer matrix method is used to predict the absorption properties of the resonator. The predicted method is validated by experiments. Four different types of cavity including circular, rectangular, triangular and hexagon types configurations are considered to study the effect of cavity shape on the acoustic properties of resonator. Results show that the cavity shape has little influence on the absorption coefficient of resonator under the same cross-sectional area. In addition, the effect of neck geometry is also evaluated. It is found that both the location and the magnitude of the absorption peak of resonator are affected by the shape of neck when the perforation rate is same.