The present invention provides a method and device for the acoustic ejection of fluid droplets having a constant drop size from fluid-containing reservoirs having varying fluid heights contained therein without the need for repositioning the acoustic ejector in the z direction by adjusting the RF frequency and amplitude. In one embodiment, the device is comprised of: a plurality of reservoirs each adapted to contain a fluid; an ejector comprising a means for generating acoustic radiation, means for controlling the RF frequency and amplitude used to generate the acoustic radiation, means for focusing the acoustic radiation at a focal point near the fluid surface in each of the reservoirs; and a means for positioning the ejector in acoustically coupled relationship to each of the reservoirs. The invention is useful in a number of contexts, particularly in the preparation of biomolecular arrays.