A method of ejecting droplets from a plurality of fluid reservoirs, the method comprising: moving the plurality of fluid reservoirs with respect to a plurality of acoustic ejectors or moving the plurality of acoustic ejectors with respect to the plurality of fluid reservoirs , such that a plurality of focal point positions corresponding to the acoustic ejectors move relative to a plurality of fluid surfaces corresponding to the fluid reservoirs in a direction perpendicular to the fluid surfaces; sending interrogation pulses from the acoustic ejectors through a coupling medium towards the fluid surfaces of corresponding fluid reservoirs of the plurality of fluid reservoirs ; receiving echoes corresponding to the interrogation pulses; determining, based on the echoes, at least one of: a plurality of distances between each one of the fluid surfaces and the corresponding ejector of the plurality of acoustic ejectors; and a plurality of predicted ejection times at which each one of the fluid surfaces will be within a respective threshold distance of a respective focal length or multiple of the focal length of the corresponding ejector of the plurality of acoustic ejectors ; and ejecting at least one droplet from at least one fluid reservoir with at least one ejectors, while moving (i) the plurality of fluid reservoirs with respect to the plurality of acoustic ejectors; or (ii) the plurality of acoustic ejectors with respect to the plurality of fluid reservoirs. moving the plurality of fluid reservoirs with respect to a plurality of acoustic ejectors or moving the plurality of acoustic ejectors with respect to the plurality of fluid reservoirs , such that a plurality of focal point positions corresponding to the acoustic ejectors move relative to a plurality of fluid surfaces corresponding to the fluid reservoirs in a direction perpendicular to the fluid surfaces; sending interrogation pulses from the acoustic ejectors through a coupling medium towards the fluid surfaces of corresponding fluid reservoirs of the plurality of fluid reservoirs ; receiving echoes corresponding to the interrogation pulses; determining, based on the echoes, at least one of: a plurality of distances between each one of the fluid surfaces and the corresponding ejector of the plurality of acoustic ejectors; and a plurality of predicted ejection times at which each one of the fluid surfaces will be within a respective threshold distance of a respective focal length or multiple of the focal length of the corresponding ejector of the plurality of acoustic ejectors ; and ejecting at least one droplet from at least one fluid reservoir with at least one ejectors, while moving (i) the plurality of fluid reservoirs with respect to the plurality of acoustic ejectors; or (ii) the plurality of acoustic ejectors with respect to the plurality of fluid reservoirs. a plurality of distances between each one of the fluid surfaces and the corresponding ejector of the plurality of acoustic ejectors; and a plurality of predicted ejection times at which each one of the fluid surfaces will be within a respective threshold distance of a respective focal length or multiple of the focal length of the corresponding ejector of the plurality of acoustic ejectors ; and ejecting at least one droplet from at least one fluid reservoir with at least one ejectors, while moving (i) the plurality of fluid reservoirs with respect to the plurality of acoustic ejectors; or (ii) the plurality of acoustic ejectors with respect to the plurality of fluid reservoirs. (i) the plurality of fluid reservoirs with respect to the plurality of acoustic ejectors; or (ii) the plurality of acoustic ejectors with respect to the plurality of fluid reservoirs. <EOS>