  We derive an integral expression for the plane-wave expansion of the
time-varying (nonstationary) random field inside a mode-stirred reverberation
chamber. It is shown that this expansion is a so-called oscillatory process,
whose kernel can be expressed explicitly in closed form. The effect of
nonstationarity is a modulation of the spectral field on a time scale that is a
function of the cavity relaxation time. It is also shown how the contribution
by a nonzero initial value of the field can be incorporated into the expansion.
The results are extended to a special class of second-order processes, relevant
to the perception of a mode-stirred reverberation field by a device under test
with a first-order (relaxation-type) frequency response.
