  Consider a measurement in which the current coming out of a mesoscopic sample
is filtered around a given frequency, amplified, measured and squared. Then
this process is repeated many times and the results are averaged. Often, two
such measurements are performed on the same system in and out of equilibrium
(the nonequilibrium state can be obtained by a variety of methods, e.g., by
applying a DC voltage or electromagnetic radiation to the sample). The excess
noise is defined as the difference in the noise between these two measurements.
We find that this excess noise is given by the excess of the non-symmetrized
power-spectrum of the current-noise. This result holds for a rather general
class of experimental setups.
