In continuous inkjet devices, an applied perturbation, such as acoustic vibration or thermal modulation, is used to drive the Rayleigh-Plateau instability to cause regular breakup of liquid microjets into droplets of controlled size. Background fluctuations and other processes contribute to subtle variations in droplet velocity within the droplet stream. These velocity variations—droplet velocity jitter—can be observed by stroboscopy or by laser detection. The fluctuations in droplet position or period are influenced by the details of the jet breakup process, and measurement of the droplet fluctuations as a function of distance along the droplet stream provide estimates of the fundamental noise on the jet.