High-pass filter cut-off frequency

The shear probe does not respond to a constant cross-axis velocity. It typically responds to fluctuations of cross-axis velocity with a frequency of 0.1 Hz and higher. Additional high-pass filtering should be applied to minimize the spectral content of the data at frequencies lower than the frequency resolution of the spectrum, which equals the inverse of the duration of the fft-segments. Thus, if ${\tau }_f$ is the duration of an fft-segment, then a good choice is a first-order Butterworth high-pass filter with a cutoff frequency of $0.5\mathrm{-}\mathrm{-}1\mathrm{H}\mathrm{z}$. The spectra should be corrected for this high-pass filter

smaller wavenumbers smaller than the spectral resolution, which is the inverse of the length of the fft-segments. For cases where there is vehicular motions with frequencies at or slightly above lowest wavenumber of spectral resolution, the cut-off frequency of the high pass filter could be increased to suppress the shear-probe signals induced by these motions. However, the final spectrum of shear must be adjusted upwards to account for the high-pass filtering.