We establish valid Edgeworth expansions for the distribution
of smoothed nonparametric spectral estimates, and of studentized
versions of linear statistics such as the sample mean,
where the studentization employs such a nonparametric spectral
estimate. Particular attention is paid to the spectral
estimate at zero frequency and, correspondingly, the studentized
sample mean, to reflect econometric interest in autocorrelation-consistent
or long-run variance estimation. Our main focus is on stationary
Gaussian series, though we discuss relaxation of the Gaussianity
assumption. Only smoothness conditions on the spectral
density that are local to the frequency of interest are
imposed. We deduce empirical expansions from our Edgeworth
expansions designed to improve on the normal approximation
in practice and also deduce a feasible rule of bandwidth
choice.