Audio: equalisation (0:03)

Figure: equalisation
Table: equalisation

equalisation plays equalised white noise. Each of the three frequency bands shown in the equalisation table is equalised. Each band is bandpass filtered using a second order Butterworth filter whose centre frequency is fixed at the mid point of the band: 135Hz bass, 2125Hz mid and 12000Hz treble. The bandwidth of each band varies at a different rate: the bass band ranges from 1Hz to 230Hz and back once; the mid band ranges from 1Hz to 3750Hz and back twice; and the treble band ranges from 1Hz to 16000Hz and back three times.

The equalisation figure shows the shape of one of the equalisation presets built into the Audacity music programme. This EQ preset recreates the tinny vintage sound of AM radio.

Equalisation (EQ) increases or reduces the amplitude of bands of frequencies.

There are no new techniques involved in EQ. The same outcome can be achieved by using any or all of the filtering techniques covered in this section and the normalisation and enveloping techniques covered in the dynamics section. EQ provides a convenient opportunity to use all of these techniques at the same time rather than individually.

EQ is simply another word for tone control.

The simplest EQ consists of a single knob that is turned to make the sound more bassy or more trebly. A more sophisticated set up has two bands and two knobs to separately control bass and treble. Three band equalisers have three knobs to separately control bass, mid and treble. A really sophisticated multiband equaliser has 30 or more bands.

Surprisingly, there is no universal definition of the range of frequency bands. The bass, mid, and treble band frequency ranges in the equalisation table are as good an approximation as any.

EQ is commonly used as part of the audio post-production process. This happens after the music has been written and recorded but before a final master version has been prepared. Typically, EQ is applied to a complete piece of music played or sung by multiple instruments and voices. EQ is used to make each instrument or voice more prominent, or less prominent, in the final mix by adjusting the amplitude of its frequencies

EQ is knob twiddling paradise. It is such a common and popular technique that most music programmes have lots of EQ presets to choose from. Many also enable you to create a custom EQ shape of your own choice. Straight lines and curves can be used to create as many shapes as you want for as many bands of frequencies as you need. The amplitude of each individual band can be boosted or cut by any amount. EQ can be applied to a single instrument or to a group of them or to the entire sound. There are so many options to explore that post-production EQ is a little industry of its own.

EQ also provides an opportunity to create sound effects. equalisation is an example of a standalone EQ sound effect that does not have a real world counterpart. It might sound vaguely like two jets taking off, it might not. A large part of the fun in writing sound effects is in trying to describe what they sound like in everyday language.