Impact of leakage on sound properties of 3D printed samples at normal and grazing incidence.

The impact of leakage on sound properties of open porosity 3D printed samples with a periodic microstructure is investigated at normal and grazing incidence. For that, direct numerical simulations (DNS) accounting for leakage are performed. In addition, an extension of the model proposed by Cummings [(1991). J. Sound Vib. 151, 63-75] is developed to predict the surface impedance of a sample surrounded by an air space at normal impedance accounting for dissipation in the leak. Experiments in a Kundt tube are performed for three series of 3D printed samples with different external diameter. Overall, leakage is responsible for a shift of the absorption peak toward higher frequencies and to an increase in its amplitude. Comparison of the measurements with the DNS and the extended Cummings model shows that both approaches predict satisfactorily the impact of leakage on the absorption coefficient. In addition, a duct wall configuration is studied for three geometries of 3D printed samples. DNS results reveal that the impact of leakage on transmission loss varies significantly depending on the 3D printed sample unit cell. Finally, discrepancies between the measured and predicted transmission loss are shown to be attributable to leakage for two of the three geometries.