Resistance in Helmholtz Resonators: Exploring the potential of sound absorption created by additive manufacturing

This research aims at improving the performance of Helmholtz resonators by adding resistance using additive manufacturing. Current use of Helmholtz resonators is focused on tonal applications and modal issues, because of the frequency specific behaviour. A normal Helmholtz resonator curve has a decreasing bandwidth when the peak increases. The research has been conducted by creating different samples, using the printing technique of stereolithography (SLA) and testing these in the impedance tube. Simultaneously the samples are modelled and analysed in Comsol. Starting from a theoretical research, the focus has been two-sided. First a possibility of adjusting the impedance of the absorbing mechanism was investigated, resulting in samples with larger squared geometrical adjustments. The results showed no significant improvement and very low velocity within the voids of the Helmholtz resonator's neck. Changes in the absorption coefficient were related to the reduction of the orifice radius, as was also predicted by analytical solutions. The second focus was the option of increasing the boundary layer thickness. This has been investigated by squared and triangular geometries with similar thickness. Results showed that there was no increase in boundary layer thickness and absorption coefficient. From these results it was possible to conclude that resistance is effective in the travelling direction of the plane wave, leading to a different iteration in the process. Surface area increase effects were researched by adding finned structures into the neck of the Helmholtz resonator and by using curved orifice patterns. Additional research on extending the fins within the volume proved not to be better performing. The results for the curved orifice showed an increase in absorption coefficient without adjusting the resonating frequency. By using the simulations in Comsol Multiphysics it was possible to predict the trend of the absorption coefficient curve, but additional research needs to be done to be able to rely on these results. From the research it can be concluded that there is a high potential for resistance based performance improvement by increasing the surface area of the Helmholtz resonator neck. The height of the absorption coefficient curve can be increased without decreasing (and thus enlarging) the bandwidth, allowing for an easier combination of resonators to create broadband absorption.Civil Engineering and GeosciencesStructural Engineerin