An EQ Primer

There are a few different types of equalization. This post is an overview of them. In part two of this topic, I will talk about the specific uses for these types in a Live Audio setting.

TYPES OF EQ

  1. Bandpass. This is the method used in crossovers, High-Pass Filters and Lo-Pass Filters. Full bandwidth material enters the circuit, and at a given frequency, the material is processed separately. In a HPF set to 80 Hz, for example, the material below 80 Hz will be lowered dramatically (typical would be 12 dB per octave), while material above 80 Hz will not be processed at all.
  2. Shelving. This is similar to bandpass, but it is variable and can be adjusted for boosting as well as attenuation. The simplest way to think of shelving EQ is your stereo system’s Bass and Treble knobs.
  3. Graphic. A common EQ type which can be found not only in pro audio, but in car audio, home theater, and computer applications. It could be as simple as 3 filters (what an average Joe might call “sliders”), or it could be as complex as 31 filters. Depending on how many filters there are, the manufacturers use different names for them. An EQ with 15 filters (also called 15-band EQ) is often described as a 2/3 Octave. This is because each individual filter affects 2/3 of an octave of frequencies. A 31-band EQ is also known as a 1/3 Octave. If you are a musician, imagine a filter that is centered on middle C. If you decrease that filter, you would not only affect C, but Bb through D as well.
  4. Parametric. A much more precise tool, a parametric EQ allows you to manually select the exact frequency you wish to change, how wide a bandwidth you want to affect, and the level of change you want to apply. With a parametric, you can select a slice of the frequency spectrum as narrow as 1/10 of an Octave or more.
  5. Notch Filters/Feedback Supressors. Sometimes going as narrow as 1/30th of an Octave or more, these very precise tools are used primarily for eliminating feedback from a system while trying not to damage the frequency response of the overall system. In recent years all the manual models have been discontinued and the feature has been included in system processors. But there are also many “automatic” feedback detection devices which claim to be able to set themselves. Your mileage may vary :-)
  6. Software-only Special EQs. In the world of digital audio and mastering, there are a couple of newer types of EQ – specifically the Paragraphic and Dynamic EQ. Neither of these types really exist as a piece of hardware, but they can be useful tools in the digital realm. *CORRECTION: The BBE Sonic Maximizer appears to be a Dynamic EQ, which applies what is essentially a Fletcher-Munson Curve dynamically (meaning the louder the signal, the more EQ is applied. This is how they disguise the hiss from the boosted high frequency EQ. When the level drops – so does the correction.)
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