In some applications, such as equalization, the goal isn't to pass signals of certain frequencies while stopping others altogether, but to make controllable adjustments, boosting or attenuating a signal, over a frequency range, by a desired gain. Two filter types are useful for this. First, a shelving filter (Figure 8.5) is used for selectively boosting or reducing either the low or high end of the frequency range. Below a selectable crossover frequency, the filter tends toward a low-frequency gain, and above it it tends toward a different, high-frequency one. The crossover frequency, low-frequency gain, and high-frequency gain can all be adjusted independently.
Second, a peaking filter (Figure 8.6) is capable of boosting or attenuating signals within a range of frequencies. The center frequency and bandwidth (which together control the range of frequencies affected), and the in-band and out-of-band gains are separately adjustible.
Parametric equalizers often employ two shelving filters (one each to adjust the low and high ends of the spectrum) and two or three peaking filters to adjust bands in between.