The equalizer influences the relative amplitudes of various frequencies within the audible spectrum.
Reducing or boosting the amplitudes of different frequencies changes the timbre of an instrument or a mix. The most common frequency ranges that are changed by an equalizer include the:
Equalization may need to be applied to a single recorded channel, to a group of channels, or an entire mix.
There are four primary types of equalizer, including:
The simplest of all equalizers are filters.
Filters usually take the form of a single button or rotary knob. There are three varieties, including:
- The low pass filter cuts the level of those frequencies above a set threshold and rolls the top end away from the signal
- The high pass filter cuts the level of those frequencies below a set threshold and rolls the bottom end away from the signal
- Bandpass filters pass a set range of frequencies between two pre-selected points.
The affected frequency range on filters can not be adjusted.
The shelving equalizer is similar to the high pass and low pass filters in that they work on fixed frequency ranges. They come in two types:
- Low-frequency shelf
- High-frequency shelf.
Unlike the high pass and low pass filters, the shelving equalizer allows for cutting and boosting of the signal using a gain control.
In general, a high-frequency shelf will start rolling the signal off at about 12 kHz, and a low pass shelf at around 150 Hz. The shelving EQ is more commonly found on home HI-FI as the bass and treble controls.
The graphic equalizer affects a range of frequencies evenly spaced through the audio spectrum (20-20,000 Hz).
Frequencies are changed using slide controls which cut or boost the relative signal amplitudes.
A simple graphic equalizer will have 2 or 3 bands (frequency areas) and can be found on guitar amplifiers and small Hi-Fi. More advanced graphic equalizers can provide more than 30 bands.
Using a graphic equalizer for mixing is generally not that effective, as there is no control over the bandwidth of each individual band. More sliders provide more control, but ultimately, for optimum control, a parametric equalizer should be used.
The parametric equalizer is the most flexible form of equalization and usually has three controls.
Parametric controls include:
- Center frequency to work on
- Gain control for cutting or boosting the frequency signal gain
- Bandwidth control (Q) for controlling the bandwidth of frequencies influenced.
- A sharp peak is usually determined from a high Q
- A gentler curve has a low Q
Q is worked out from the equation: Q = center frequency/bandwidth
The Parametric equalizer offers full control over the audio spectrum and is the best type of equalizer used for mixing.