Filter gain ($G_a(\lambda )=|a(\lambda )|$)

For an absolutely summable linear filter of a stationary time series, the filter gain is the modulus of its frequency response $a(\lambda )={\sum }_r{a}_r{e}^{ir\lambda }$. It multiplies a sinusoidal amplitude, while its square multiplies the spectral density of a stationary process. A zero removes the corresponding frequency; the filter gain of a composition is the product of the two filter gains.