The free ink surface levels (13) of acoustic ink printers (11) are controlled by cap structures (31) that have substantially non-retroreflective aperture configurations. The non-retroreflective configurations of the apertures (32,42) of these cap structures (31) cause diffusive scattering or directional deflection of the reflected surface ripple waves, thereby significantly reducing the time that is required for the oscillatory perturbations, which are caused by reflection of the surface ripple waves that are generated during the droplet ejection process, to dissipate to a negligibly low amplitude in the critical local areas of the ejection sites. This, in turn, increases the droplet ejection rates at which printers (11) having such cap structures (31) can be operated asynchronously.