The atomizer described is a novel twin-fluid design that allows independent control of mean droplet size and mean spray density Designed to handle unpressurized fluids at low flow rates, the prefilming double-annular air-blast atomizer (PFDAAA) distributes fuel in a liquid film over a hollow, tapered cylindrical centerbody. Two independently controlled atomizing gas streams shear the inner and outer surfaces of the cylindrical fluid film at the injector tip, resulting in continuously variable atomization. Data taken during a series of cold-flow experiments include both mean droplet size and spray density measures. The data demonstrate that controlling the relative flow rates of the two atomizing gas streams produces droplet dispersions whose mean droplet size and spray density vary independently of each other. Measured values of Sauter mean diameter (SMD) for isopropyl alcohol sprays range from 250 to 3100 mum, and values of normalized interdroplet spacing range from 0.8 to 3.6 mean droplet diameters. When presented collectively, the mean droplet size and spray density data form a performance map indicating the operating range of the PFDAAA for the conditions tested. Parametrically variable sprays such as those generated by the PFDAAA constitute a previously unavailable experimental platform for spray combustion research. Investigations using laser diagnostics to probe a controlled variable spray could provide insights into the physicochemical processes within a spray that ultimately affect combustion efficiency and pollutant formation.