= Relative attenuation by a linear filter
{title2=$\eta=|H(\omega_n)|/|H(\omega_s)|$}
For a <linear time-invariant system>, a sinusoidal wanted <signal> at $\omega_s$ and an unwanted sinusoidal component at $\omega_n$ acquire their separate <frequency response> factors. The ratio of their output <amplitudes>, divided by their input amplitude ratio, is $\eta=|H(\omega_n)|/|H(\omega_s)|$. An amplitude attenuation factor of $F=1/\eta$ corresponds to a factor $F^2$ in squared-amplitude <power> ratios under a common impedance convention. The unwanted component's own absolute attenuation is $|H(\omega_n)|$, which need not equal its attenuation relative to the wanted signal.
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