The main property of a filter: its attenuation
Noise suppression filtering in electronic circuits

An electromagnetic interference filter is characterized by its attenuation (as a passive component is by its impedance). The attenuation of a filter is expressed in decibels and is measurable through its parameter S₂₁. This parameter corresponds to the transmission coefficient between the filter's input and output. The parameter matrix S can be described as the ratio of incident a_i (equation (1)) and reflected b_i (equation (2)) power waves, as written in equation (3) with k ≠ j.