The high-pass filter

The high-pass filter correction is obtained by multiplying the spectrum by $1+{\left(\frac{f_c}{f}\right)}^2$ where $f_c$ is the high-pass cut-off frequency and $f$ is the frequency. The high-pass filter is assumed to be of single-pole, or first order, and has a transfer function of $\frac{jf/f_c}{1+jf/f_c}$ where $j^2=-1$.