Equalizers

The equalizer influences the amplitudes of frequencies within the audio spectrum.

Graphic equalizer
Fig 1 - Example Hardware Graphic Equalizer

Attenuating (reducing) or boosting the amplitudes of different frequencies changes the timbre of an instrument or a mix.

Equalization may need to be applied to a single recorded channel, to a group of channels, or an entire track.

There are four primary types of equalizer including:

Filters

The simplest of all equalizers are filters.

Filters usually take the form of a single button or rotary knob. There are three varieties: 

  • The low pass filter attenuates the level of those frequencies above a set threshold and rolls the top end away from the signal.
  • The high pass filter attenuates the level of those frequencies below a set threshold and rolls the bottom end away from the signal.
  • Band pass filters pass a set range of frequencies between two pre-selected points.

It should be noted that the affected frequency range on filters can not be adjusted, and that filters can only attenuate a signal.

Shelving

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.

Graphic

The graphic equalizer affects a range of frequencies evenly spaced through the audio spectrum (20-20,000 Hz). The frequencies are changed using slide controls which boost or attenuate the relative signal amplitudes.

graphic-eq.png
Fig 2 - Software example of 15 band graphic equalizer

A simple graphic equalizer will have 2 or 3 bands (frequency areas). These can be found on guitar amplifiers and small Hi Fi. A more advanced graphic equalizer 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.

Parametric

The parametric equalizer is the most flexible form of equalization and usually has three controls:

  1. Center frequency to work on.
  2. Gain control for boosting or attenuating the frequency signal gain.
  3. Bandwidth control (Q) for controlling the bandwidth of frequencies influenced.

The bandwidth control has command over the width of the frequency range and is normally labelled 'Q' (short for 'quality').

  • 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 EQ offers full control over the audio spectrum and is the best type of equalizer used for mixing.

parametric-eq.jpg
Fig 3 - Parametric Equalizer