An Introductory Guide to Subwoofers 
Understanding equalizers and EQ

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NOTE, neither bass traps or equalization alone are a cure for room related issues. When possible, a combination of traps and equalization will provide optional performance. The usual approach is to start with acoustical treatments, then use EQ to fine-tune everything.
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Understanding what an equalizer does

Equalizers take individual slices of the audio spectrum, these slices are created by the use of filters. The equalizer can then apply cut or boost to these filters individually.  

Cut or boost is created by adding or subtracting amplifier power to the specific filter frequencies. Remember that SPL is measured in decibels (dB), and that changes in dB is a logarithmic function. For example: adding  +10dB of boost, means 100 times more amplifier power will be added. So be judicious with your use of boost, or you'll be clipping your amplifier.

The size of the slice (filter) is called the bandwidth. These range in 'size' according to the settings in the equalizer. A narrow  band filter is 1/3rd octave or less. Wide band filter is a full octave or more. 

Below are examples of different bandwidth 'cut' filters.

1/3rd octave

1 octave

2 octaves

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So how do I use this EQ?

Here's an extremely simple example. Let's say you have a 10dB peak at 40Hz (black line). You dial in set a filter with 10dB of 'cut' (turquoise line) and the result is a flatter response (grey line).

Also it's important to understand that depending on the frequency and bandwidth, EQ filters will  interact with each other. Below is an example.  Start with a 1/3rd octave filter at 40Hz with -10dB of cut.

 Now combine that with a 1/3rd octave filter at 75Hz with -10dB of cut. The turquoise plot shows the overall effect of two filters.

It's important to take the interaction of the filters into consideration. Showing these types of interaction is one reason Room EQ Wizard (used to generate the above plots) is such a great tool.

Later in this guide, there's a tutorial covering the use of the Room EQ Wizard software program. Among other things this program allows one to visualize the effect various filter combinations have on the frequency response plot.
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There are 2 basic forms of equalizers, graphic, and parametric.

Analog graphic equalizers are the easiest to use, and are the least useful. This is due to the fact that they have fixed bandwidth filters, that can only be adjusted for boost or cut with a slider.

This is an example of an analog 1/3rd octave graphic stereo equalizer

These are called 'graphic' equalizers because the sliders when in position, provide a graphic representation of the EQ curve.

These have limited utility in today's world because of the fixed nature of the filters. If you need cut or boost between the preset frequencies you're out of luck. For example standard 1/3 octave increments are 20Hz, 25Hz, 31.5Hz, 40Hz, 56Hz, 80Hz etc..... If you have a problem that would be 'fixed' by having a filter at 38Hz that can't be accurately done. Also if you need a filter that's narrower or wider than the standard 1/3 octave spacing that's problematic with this form of EQ.

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Analog parametric equalizers are more difficult to operate than graphic EQ, since they offer complete flexibility with regard to the specific frequency and the width of the filter.

This is an example of an analog parametric stereo equalizer

These offer very good sound quality, but require fancy test gear to accurately set all the little dials. And unless a security cover is placed over the dials, or they're hot glued in place, it's all too easy for the adjustments to be accidentally changed.
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Digital parametric equalizers are less intuitive to use. But the settings are highly accurate since a digital readout is available. And there are low cost extremely effective units readily available. The most popular digital parametric stereo equalizer for home audio use, is of course the Behringer DSP1124P.

This is a very low cost and very powerful equalizer. People run into problems using it because they fail to understand a few simple concepts. We'll cover it's operation next.
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