All equalizers have the ability to either cut or boost frequencies, and in some cases do both. The Eqs that only cut frequencies are called “passive” EQs. For instance, troche some guitar amplifiers have passive EQ controls. All they do is arrange for certain frequencies to “bleed off” to ground. Other EQ’s can add amplification to frequencies, these are called “active” EQs.
Both graphic and Parametric equalizers can be both active and/or passive.
Graphic EQs: Most people have seen graphic EQ’s in their car stereo or home sound system. A graphic EQ uses a series of controls to adjust the volume of a frequency at a predetermined frequency. An individual can move the controls to manipulate said frequency. The limitation of a graphic EQ is that the user cannot alter the frequencies that they want to manipulate. For instance, if you have a 3-band graphic EQ you will not be able to adjust where the frequencies are centered (i.e. The low-frequency control is preset to center over 60hz, mid-range is centered over 500 hz, and hi-range centered over 8khz, and there is no altering their positions. Graphic EQs can have as little as 2 bands and in some cases hundreds of bands.
When your are given “parameters” that describe what the EQ can do, including where the boost/cut is centered, how much boost/cut you can provide and how selectively it is applied, then we refer to the controls as parametric, hence the name “Parametric EQ.”
Parametric EQs: Parametric equalizers have controls (or parameters) for selecting where a frequency is centered, the level of the frequency, and the quotient of the change or “Q.” The smaller the “Q,” the more subtle the change. It is a much more complex processor than the graphic EQ but it is a valuable tool for manipulating very specific and focused sound waves across the entire frequency spectrum.