  We demonstrate that accurate computation of the spectral proper orthogonal
decomposition (SPOD) critically depends on the choice of frequency resolution.
Using both artificially generated data and large-eddy simulation data of a
turbulent subsonic jet, we show that the optimal choice depends on how rapidly
the SPOD modes change in space at adjacent frequencies. Previously employed
values are found to be too high, resulting in unnecessarily biased results at
physically important frequencies. A physics-informed adaptive
frequency-resolution SPOD algorithm is developed that provides substantially
less biased SPOD modes than the standard constant resolution method.
