A droplet ejector (10) for an acoustic printer has an acoustically thin capping structure (12) that permits accurate location of the free surface of a liquid ink (30) to enable acoustically induced ink droplet ejection, and that prevents the ink from spilling from its well. Acoustically thin implies that the capping structure thickness is a small fraction of the wavelength of the applied acoustic energy. One capping structure is a thin wafer of porous silicon placed over the aperture of an ink filled ink well (28). Acoustic radiation pressure pushes liquid ink from the well through the pores (36) so that a thin ink film forms over the capping structure. Another capping structure is a solid membrane placed over the ink well aperture. An ink deposition means (60,90) deposits a thin film of ink over the capping structure. With either structure, applied acoustic energy from a transducer (20) can pass through the capping structure to cause droplet ejection from the free surface of the ink film.