Some types of ac circuits are designed to respond to ac frequencies. Circuits that are used to pass some frequencies and block others are called frequency-sensitive circuits. Two types of frequency-sensitive circuits are filter circuits and resonant circuits. Each type of circuit uses reactive devices to respond to different ac frequencies. These circuits have frequency response curves. Frequency is graphed on the horizontal axis and voltage output on the vertical axis. Sample frequency response curves for each type of filter and resonant circuit are shown in the examples that follow. Decibels as they relate to ac circuits are also discussed in this unit.