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Signal Processing Fundamentals: Equalization

There are many different ways of changing the tonal balance of whatever you're controlling...
This article is provided by Rane Corporation.

You may have heard it said that equalizers are nothing more than glorified tone controls. That’s pretty accurate and helps explain their usefulness and importance.

Simply put, equalizers allow you to change the tonal balance of whatever you are controlling. You can increase (boost) or decrease (cut) on a band-by-band basis just the desired frequencies.

Equalizers come in all different sizes and shapes, varying greatly in design and complexity. Select from a simple single-channel unit with 10 controls on 1-octave frequency spacing (a mono 10-band octave equalizer), all the way up to a full-featured, 2-channel box with 31 controls on 1/3-octave frequency spacing (a stereo 1/3-oct equalizer).

There are graphic models with slide controls (sliders) that roughly “graph” the equalizer’s frequency response by the shape they form, and there are parametric models where you choose the frequency, amplitude, and bandwidth desired (the filter parameters – see diagram below) for each band provided.

Far and away, the simplest and most popular are the 1/3- and 2/3-octave graphics. They offer the best combination of control, complexity and cost.

In selecting graphic equalizers, the primary features to consider are the number of input/output channels, the number of boost/cut bands, the center-frequency spacing of each, and the accuracy of the output vs. the front panel settings.

Bandpass filter parameters.

Up until the recent development of true response graphics, the front panel settings only approximated the equalizer’s actual response. Prior to true response graphics, adjacent band interaction caused the actual output response to deviate from the front panel settings.

Described as either constant-Q or variable-Q, the individual filter bandwidth behavior determined the interaction.

Variable-Q graphic.

In the early 1980s, Rane developed the first constant-Q designs to preserve the same shape (bandwidth) over the entire boost/cut range. In contrast, variable-Q designs have varying bandwidths (the shape changes) as a function of boost/cut amount.

Rane’s constant-Q design offered a big improvement in output response vs. front panel settings and became the most popular design until Rane and others developed the first true response graphic equalizers. Now true response graphics offer the best response.

Constant-Q graphic.

Using Equalizers
Equalizers can do wonders for a sound system. Let’s start with loudspeaker performance.

An unfortunate truth regarding budget loudspeakers is they don’t sound very good. Usually this is due to an uneven frequency response, or more correctly a non-flat power response.

An ideal cabinet has a flat power response. This means that if you pick, say, 1 kHz as a reference signal, use it to drive the speaker with exactly one watt, measure the loudness, and sweep the generator over the speaker’s entire frequency range, all frequencies will measure equally loud. Sadly, with all but the most expensive speaker systems, they will not.

Equalizers can help these frequency deficiencies. By adding a little here and taking away a little there, pretty soon you create an acceptable power response—and a whole lot better sounding system. It’s surprising how just a little equalization can change a poor sounding system into something quite decent.

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