An equalizer is an audio device with multiple frequency controls for adjusting sound tone quality. There are many reasons for using an equalizer.  This article will look into a few of the common applications for the equalizer.

To correct problems in a recording

An equalizer can be used to compensate for deficiencies in a sound pickup or to reduce extraneous sounds, such as noise.  It is far better to get the recording right first time around, rather than trying to fix it in the mix.  Equalization should not be used as a way out of a bad recording when mixing. 

Some tips to consider during, and after a recording include:-

• Switch high pass filters on when needing to eliminate unwanted low rumbles, for example, from local traffic.
• Switch low pass filters on when needing to eliminate interferences and high frequency noise, such as hiss.
• For bassy instruments, filter off any frequencies above 8 kHz, as they are generally not audible but can induce extra noise.
• Be aware of the proximity effect, especially when miking drum kits.  If added bass energy becomes a problem, roll off some of the low frequencies.

To improve instrument clarity

Equalization can help enhance an instruments sound.  Generally, boosting any sound's harmonics gives the impression of more presence and brightness; whereas decreasing the harmonics gives the impression of a dull, less dazzling sound. Boosting a sound around its first fundamental frequencies will promote warmth and depth; whereas decreasing the fundamental will promote a colder, less powerful sound. 

The main problems in a mix are usually excess muddiness and honk. Before beginning to equalize any instrument, it is important to listen to the sound in order to identify any nuance/unwanted frequencies. 

Muddiness normally comes from bass heavy instruments, such as the kick drum, bass guitar, and the lower end of the piano. The frequencies responsible are usually centred between 100-400Hz.  However, simply cutting these frequencies will make the sound thin as this area does contribute to the body and rhythm of a mix.

The best way to deal with muddiness is to 'scan' the lower frequencies with a high Q setting and a moderate boost level of about 8dB.  Once excess mud is found, cut the signal right down, and then slowly bring up the gain control until there is a good balance between body and muddiness. Exercise caution when cutting frequencies, though, and do not cut the same frequency on all of the sounds. The idea is to fill up the audio spectrum and applying cut to several sounds in the same frequency area will just leave a hole in a mix.  If too much bass is lost from the sound, try adding some moderate boost to the sub-bass region at around 60 Hz to compensate.

The honk area is focused between 500-3000 Hz and determines how ‘honky’ and prominent an instrument is in the mix.  Excess output at this range can sound cheap, boxy and can cause unwanted ear fatigue. If boosting in this area, be very cautious, especially on vocals. Human hearing is extremely sensitive at these frequencies with the slightest boost or cut resulting in a huge change in the sound.

If there are any irritating frequencies, sweep through the sound with a medium curve at about 10dB cut.  Once the frequencies are located adjust the amount of cut as desired.  It may be necessary to compensate for any cutting in this area by applying a small amount of boost around the 5-8 kHz area to liven up anything that may have been severely affected by the cutting. This will help to preserve the overall brightness.

When applying equalization, it is advisable to use large amounts of cut or boost initially. This helps give a better idea of the frequencies being affected. The human ear can quickly become used to an equalized sound, so quick successive bursts on the gain knob until the frequency sounds about right is best.

To create room and balance

The equalizer can be useful for creating space in the mix by balancing frequencies.  If certain instruments occupy a similar frequency band they will end up masking each occur within that particular area of the audio spectrum, often resulting in a muddy sound.

To allow elements to best fit together, there has to be some juggling of frequencies so that each instrument has its own predominant frequency range.  For example, if a kick drum is heavy and powerful in the 80 Hz but getting muddied up by the bassline, attenuating the bassline around this frequency will free up valuable mix room allowing for the kick to shine through.  The result being that the mix will sound more clear and distinct.