What is an equaliser and how does it work

What is equalisation, how it works and how to use it.

Dynamic Processors


Purchase to view this tutorial

By purchasing this tutorial, you'll get immediate access - your purchase helps create new and exciting content and this site survive!

£3.00Add to basket


Believe it or not you have been using an equaliser all your life. Every time you turn the volume knob on your stereo system you are in effect using equalisation. A gain knob is sometimes referred to as a single band gain control. BUT, even more common are the equalisers provided with stereo systems that can be used to refine the sound to the listener’s tastes. In the studio we take this many steps further as we need very specific tools for specific tasks and a single all encompassing equaliser might not be enough.

Separating the frequencies of instruments by the use of EQ is a traditional, yet subtle, method of creative and corrective EQ.

In fact, this method is not limited to the mix and production stages but is also prevalent at the tracking stage. There are many instances whereby EQ has to be used to compensate for a poor microphone (or it’s use in a given environment) or a poor room (poor acoustics resulting in boomy, resonant, ringing etc anomalies).

EQ is also used in live situations to compensate for the environment, the singer, the microphone, background noise, colour and so on.

What is an equaliser (EQ)

To define what equalisation is would take quite a while but a condensed definition would be:

A dynamic processor which uses filters to alter the balance of frequencies in a sound.

This is achieved by using a number of filter circuits, which apply positive or negative gain to selected frequency ranges. The positive gain is referred to as ‘boost’ and the negative gain is referred to as ‘cut’.

Corrective or Compensatory eq

Actually the first ever instances of equalisation was in the communications industry. EQ was used to counteract some of the problems in telephone systems. It then transgressed into the broadcasting industry.

Tone controls were created and used to compensate technical inaccuracies in the recording chain, more notably, compensating for microphone colouration and room acoustics. EQ was used as a means of controlling the gain of a range of frequencies.

This form of equalisation is termed as ‘corrective or compensatory’ EQ.

We use the same principles when using equalisation to remove problematic or redundant frequencies.

Creative equalisation

Creative equalisation is used to ‘colour’ a sound or to reshape it’s characteristics to create a new sonic texture (boosting the mid-range frequencies of a kick drum to accent the punch and body is a good example). This is where we get creative and reshape sounds to suit the mix. Sometimes we use minimum phase designed equaliser that impart a ceratin colour on the sound.

Linear phase and Minimum phase

The distinction between Linear Phase and Minimum Phase topologies is that Linear Phase has an equal phase response whether cutting or boosting whereas Minimum Phase doesn’t. This might seem like a simple distinction but it is never that easy when it comes to physics. Certain filter types will exhibit a different phase response even in Linear Phase mode.

Here is a video I created explaining the differences between linear and minimum phase: Linear Phase versus Minimum Phase EQ

Redundant frequencies

Redundant frequencies can be visible when using an equaliser with a graphical display but the important distinction here is that we cannot hear them. Any frequency that is visible but not heard is regarded as redundant – in that we don’t need them.

In the image above you can see the redundant frequencies sitting at around 10 kHz. They are visible but cannot be heard. So, why keep them?

Equaliser features

Cut-off frequency – This is the point (frequency) at which the filter begins to filter (block or cut out). The filter will lower the volume of the frequencies above or below the cut-off frequency depending on the type of filter used.

Attenuation – This ‘lowering of the volume of the frequencies,’ is called Attenuation. In the case of a low-pass filter, the frequencies above the cut-off are attenuated. In the case of a hi-pass filter, the frequencies below the cut-off are attenuated.

Resonance – Boosting the narrow band of frequencies at the cut-off point is called resonance. Also know as Q and bandwidth, in effect, he higher the resonance, the narrower the bandwidth.

Cut – To reduce the level of a signal using a filter. To lower the frequency amplitude of a band.

Q – Also known as ‘width of the filter response’, this is the ‘centre frequency’ of the bandwidth and is measured in Hz. Also know as bandwidth and resonance.

A high Q value denotes a narrow filter width (bandwidth). A low Q value denotes a wide filter width (bandwidth).

This is actually a very important piece of information because with the Q control alone you can make your audio sound high and brittle or warm and musical. This does not mean that you must use low Q values all the time, in the hope of attaining warmth, but you must understand what frequencies need filtering. If your intent is to use EQ as a musical tool, then be aware of what the Q value can do to audio. For creative EQ, this is a weapon often ignored.

Slope – The rate at which a high or low frequency EQ section reduces the level above or below the cut-off frequency is termed as the ‘Slope’ and the shape and parameters are denoted as dB per octave and are usually: 6, 12, 18 or 24dB/octave.Slope also determines the characteristic of the filter and can range from smooth to extreme (gentle to aggressive).

HF – High (Hi) Frequency

LF – Low (Lo) Frequency

Mid – Mid Frequency

Boost – To apply gain or to increase by level. Measured in dB.

Band – A single filter in an equaliser or a range of pre determined frequencies.

Centre Frequency – The frequency at which a peaking filter applies maximum gain.

Filter – A circuit which alters the level of a limited range of frequencies.

Octave – Doubling of a frequency (explained earlier).

Filter Shapes

Bell – An EQ with a peak in its response denoted by it’s shape and quite common nowadays on most virtual EQs. The Bell shape has symmetrical response characteristics. In other words it has the same response whether boosting or cutting (asymmetrical).

Shelf – A high or low frequency EQ where the response extends from the selected frequency to the highest or lowest frequency values in the audio range.

Low-Pass and Hi-Pass – A low-pass shelving filter passes all frequencies below its cut-off frequency, but attenuates all frequencies above its cut-off frequency. Similarly, a high-pass filter passes all frequencies above its cut-off frequency, but affects all frequencies below its cut-off frequency.

In the video I explain everything you will ever need to know about how an equaliser works and how to use it. All of the equaliser’s features are explained and demonstrated. I show you how to read the GUI of an equaliser and how to use the various filter shapes to eq any sound. I walk you through each type of filter including high pass, low pass, band-pass, notch, shelves and so on and using audio examples to demonstrate how each type behaves.

Plugin used in this video:

FabFilter Pro Q2

Topics covered in this video are:

  • What is equalisation and how does it work
  • Band, Q and Gain
  • Fixed versus Peaking
  • Cut versus Boost
  • Phase – Minimum and Linear
  • Slopes and Gradients
  • Octaves and Frequencies
  • Dynamic versus Static/Passive
  • EQ Modes
  • Spectrum Analyser
  • Splitting Bands

Alternatively, grab the book EQ Uncovered which covers all aspects of Equalisation in detail with working examples.