We report an experimental study on the effect of an external multiplicative noise on a subcritical bifurcation leading to the parametric amplification of surface waves. We show that the probability density function of the wave amplitude in the presence of noise has two maxima that do not correspond to any of the deterministic states. When the deterministic forcing is varied in the presence of noise, these most probable values give two new branches in the bifurcation diagram that involve a much larger difference in oscillation amplitude. The bistable region is also strongly enlarged. This noise induced bistability can be understood in the general framework of noise induced transitions.

We report a study on the effect of external multiplicative noise on parametric instabilities using two different experimental systems: an electronic RLC circuit, parametrically pumped with a voltage-variable capacitor, and surface waves generated by vertically vibrating a layer of fluid (the Faraday instability). Both systems are forced by the superposition of a sinusoidal and a noisy component. We study the statistical properties of the response of both systems to noisy parametric forcing and compare them with theoretical predictions. When the detuning from parametric resonance is such that the bifurcation in the absence of noise is supercritical, both systems behave in the same way under the influence of noise. We find that the effect of noise is twofold: on one hand, it triggers the instability before its deterministic onset under the form of oscillatory bursts; on the other hand, it inhibits the nonlinearly saturated oscillatory response above the deterministic onset. When the detuning is such that the bifurcation is subcritical, we find that the two systems behave differently. In the case of the electronic oscillator, noise mostly triggers random transitions between the two states of the bistable region that exists in the absence of noise, whereas in the surface wave experiment new states are created by noise and the bistable region is strongly enlarged.

We show how a simple electronic parametric oscillator can be used to exhibit both supercritical and subcritical bifurcations to a subharmonic oscillatory state as the pump frequency is varied, and study the scaling behavior of the oscillation amplitude in the vicinity of the tricritical point.