Noise enhances signal detection in a bistable system. A subthreshold periodic signal becomes detectable at an optimal noise level — not too little, not too much. SNR peaks at the resonance noise strength.
Stochastic resonance (Benzi et al. 1981): subthreshold signal + noise → detectable output. System: dx/dt = x - x³ + A·cos(ωt) + √(2D)·η(t). Optimum noise D_opt = ΔU/ln(T_K/T_signal) where ΔU is the barrier height and T_K the Kramers time. Found in neurons, sensory systems, climate models (Pleistocene ice ages). SNR peak defines the resonance.