  We investigate dissipative dynamical systems under the influence of an
external driving with two or more frequencies. Our main quantities of interest
are long-time averages of expectation values which turn out to exhibit
universal features. In particular, resonance peaks in the vicinity of
commensurable frequencies possess a generic enveloping function whose width is
inversely proportional to the averaging time. While the universal features can
be derived analytically, the transition from the specific short-time behavior
to the long-time limit is illustrated for the examples of a classical random
walk and a dissipative two-level system both with biharmonic driving. In these
models, the dependence of the time-averaged response on the relative phase
between the two driving frequencies changes with increasing integration time.
For short times, it exhibits the $2\pi$ periodicity of the dynamic equations,
while in the long-time limit, the period becomes a fraction of this value.

Mesoscale and Nanoscale Physics