2 Signal processing
2.2 EQ - Equalization
This is where an important part of the tone shaping takes place. The term "equalization", "EQ" for short, originates from the PA field where equalizers are used to obtain an even frequency response in a given room. Because every room is shaped differently and thus sounds differently, a device is needed to correct these differences, in other words: to equalize the response. For bass guitar use, the device should really be called "tone controller" instead of equalizer, but that aside. The working principle is simple: certain frequency ranges can be cut (turned down) or boosted (turned up).
Equalizers mainly come in three forms:
- Tone controls: rotary knobs labeled "bass", "mid" and "treble", or similar, control fixed frequency ranges. Other labels may exist and more knobs may be present. Some equalizers have "pre-shape" or "tone matrix" or similar controls. In essence they are equalizer presets. They preset an equalizer setting that has proven to be the preference of many bass guitar players.
- Graphic EQ: sliders labeled with frequencies control cut and boost (aka "gain") of the frequency range with the labeled frequency as a center. Each slider controls a socalled "band". The frequencies are fixed, as is the width of each band ("bandwidth"). The more bands an equalizer has, the narrower they are.
- Parametric EQ: this type lets you control all parameters of each band through separate controls: frequency, bandwidth or "Q", and gain. Some parametric equalizers have additional shelving bands. They control everything below (low shelf) or above (high shelf) their center frequency. When a parametric equalizer does not have a "Q" control it is said to be "semi-parametric".
Boosting low frequencies can have dramatic consequences for the power amplifier. For bass guitar, obviously, power is mostly needed in the bottom end. A jazzy funk sound with an emphasis on the midrange may require 1/10th of the power a dub or reggae bassist needs who's more likely to dial in void deep bass. The usual cut and boost range for equalizer bands is -12 to +12 dB (though -15 to +15 dB is not uncommon).
More general info on equalization and filters can be found here: 
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© Joris van den Heuvel 2001-2009