Response of electrohydrodynamic convection to external noise

We report noise-induced threshold shifts in ac-driven electrohydrodynamic convection (EHC) in nematic liquid crystals. By changing the intensity V_N and cutoff frequency f_c of noise, we investigate EHC-threshold shifts. There exists the characteristic cutoff frequency f_c* for the influence of noise on EHC. For cutoff frequencies f_c >; f_c*, noise contributes to stabilizing the onset of EHC (i.e., it shifts the onset threshold upward), whereas for f_c <; f_c*, noise contributes to destabilizing the onset of EHC (i.e., it shifts the onset threshold downward). For f_c = f_c*, noise makes no contribution to the threshold shift (i.e., it is neutral with respect to the onset of EHC). However, the pattern structure of EHC is changed by V_N, independent of f_c.