This study investigates the acoustic characteristics of Helmholtz resonator with an extended neck (HREN) with different shapes of backing cavity and neck. To produce the characteristics of extended neck and the backing cavity, their equivalent densities and bulk moduli within the neck and cavity are calculated by solving the wave equation over the cross section of the neck and cavity with finite element approach taking the effects of viscosity and thermal conductivity into account. Based on the obtained equivalent parameters, the transfer matrix method is used to predict the sound absorption properties of the resonators. The approach outline above is validated by simulations and experiments. Cavities with three different cross-section shapes, which are circular, square and hexagonal, are considered. Results show that the cross-section shape of cavity has little influence on the absorption coefficient of resonator with the same cross-sectional area. In addition, the effect of neck geometry is also evaluated and it is found that both the location and the magnitude of the absorption peak of the resonator are significantly affected. Lastly, a non-uniform broadband absorber consisting of inhomogeneous HRENs with hexagonal cavities is designed.
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