Reduction in the scintillation index of multi-Gaussian Schell-model beams propagating in turbulent air is demonstrated as a function of two source parameters: the r.m.s. coherence width and the summation index. The beams were generated with the help of a nematic phase-only, reflective spatial light modulator at a cycling rate of 333 frames per second and recorded after propagating through a weakly turbulent air channel over a distance of 70 m. Experimental results are in good agreement with theory.