Based on the results presented in the device optimisation, we opted for a design with wall thickness T = 5 µm and air chamber length L = 500 µm and performed more specific numerical analyses to gain further insights regarding the underlying mechanisms and the acoustic phenomena. Figure a) shows the average wall displacement and the average acoustic energy density for frequencies from 40 to 90 kHz with 100 Hz steps. The maximal energy density correlates with the highest displacement of the wall and reveals two strong resonances (41.89 kHz and 69.03 kHz). Experimental investigations showed that frequencies around 69 kHz lead to much stronger particle attraction and streaming velocities than frequencies around 40 kHz. The deviation from observed and simulated resonance frequency can be attributed to idealised material parameters and the limited significance of the displacement boundary condition, e.g., through lower amplitudes of the piezoelectric element at this frequency range. However, the aim of the numerical investigations is not to attain exact amplitudes but to show trends and get insight into the physical phenomena of the device.