We have studied the saturation and stimulated decay of an 8 GHz focused acoustic beam in superfluid helium near 50 mK. Because the dispersion is a function of pressure, there are various regimes where different nonlinear processes dominate. At low pressures (upward dispersion) parametric self-enhancement of the noise causes rapid signal depletion. At intermediate pressures (linear dispersion) harmonic formation leads to saturation (a 舠shock wave舡). At still higher pressures (downward dispersion) the power in the main focus far exceeds the shock onset threshold before depleting rapidly in what appears to be a four-phonon parametric enhancement of the spontaneous decay. We believe this to be the first experimental evidence of four-phonon parametric mixing in acoustics. © 1987 The Japan Society of Applied Physics.