A graphic equalizer is a way to control the dynamics of a sound wave. Most sounds aren't just simple tones consisting of a single frequency. Instead, they have many different frequencies playing at the same time. A graphic equalizer allows the listener to selectively adjust certain frequencies without changing the values of others. It can be used to eliminate hiss from a recording, to make a recording sound fuller or warmer, to crank the bass up, and for many other purposes.
Each slider in a graphic equalizer has a filter which only lets certain sounds pass through. The bass and treble sliders only pass frequencies below and above a certain level, respectively. The sliders in the middle all have a range, passing signals through between a certain lower limit and a certain upper one. The sliders are all arranged in a row on the control panel, with the bass ones being all the way at the left and the treble, all the way at the right.
Old style, analog graphic equalizers work by phase shifting the signal a certain amount. Phase shifting means delaying the signal slightly so that it is out of phase with the original signal. When signals are completely out of phase, they cancel each other out, resulting in no sound being produced. When signals are only partly out of phase, they cancel each other out partially. By changing how much of the signal is canceled out, the graphic equalizer changes the volume of that signal.
Modern, digital graphic equalizers work differently. They create a digital image of the signal and change the amplitude of the signal digitally. This produces a cleaner signal with less distortion than the old style of graphic equalizer.
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