One of the confusing things for many people new to DSP equalization (EQ) is the choice between graphic and parametric EQ. In this application note, we explain the difference so you can choose which suits your application best.

Parametric EQ

In a parametric equalizer, each filter cuts or boosts a range of frequencies. Each filter has three controls:

• frequency: the center of the frequency range to be cut or boosted
• gain: the amount of boost or cut
• Q: the "sharpness" of the boost or cut, with higher Q meaning a narrower filter

Parametric EQ thus allows a single filter to be very narrow or quite wide, and it is therefore very useful for correcting frequency response errors in a loudspeaker or reducing peaks caused by room modes. When implemented digitally, parametric filters can also take the shape of a "shelving" filter, which boost or cuts frequencies above or below the filter frequency.

The screenshots below shows four filters in use: a low-shelf filter (boost), a narrow notch, a broad boost, and a high-shelf filter (cut). First the frequency curve as shown in the miniDSP plugin:

Here is the frequency response measured with Room EQ Wizard:

Many of the miniDSP plugins support parametric EQ on each input channel and each output channel, with five or six filters in each. Check the miniDSP plugins page for e.g. "6 band PEQ".

Graphic EQ

A graphic equalizer has a number of filters spread evenly across the audio bandwidth. Each filter is the same shape, and has just one control: the amount of boost or cut. The filters overlap, so the combined response forms the shape given by the positions of the sliders. Most common is the third-octave, or 31-band, graphic EQ, with center frequencies from 20 Hz to 20 kHz.

Graphic EQ can be implemented with analog circuits or with digital signal processing. In the miniDSP lineup, graphic EQ is a 31-band equalizer with up to 12 dB of boost or cut in each band. Here is an example of a miniDSP graphic EQ setting, set to approximately the same response as the PEQ above:

Here is the frequency response measured with Room EQ Wizard:

Check the miniDSP plugins page for "31 band EQ" to determine which plugins have a graphic EQ.

Which to use?

The type of EQ that you use depends on your application. Provided that you have a measurement microphone and software set up, the parametric EQ is ideal for correcting speaker and room response. If you want to be able to adjust the overall response by ear, then the graphic EQ may be better suited to your needs. You can of course try both and pick the one that suits you best.

A point to note is that the parametric EQ gives more flexibility for the same processing power. Each band of a graphic EQ requires the same processing power as a parametric EQ filter. So a 31-band graphic EQ uses the same processing time as 31 parametric EQ filters. That is why the parametric EQ plugins have EQ on every input and output channel, while the graphic EQ plugins have EQ only on the input channels.

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