Before we get in to detail technical discussions let me update you with the process of learning. I prefer to explain things in the simplest manner and use day-to-day examples, which every individual can relate to. “A good teacher is the one who explain the most complex topic in the most simple way.”
Equalizer is a tool used to manipulate the frequency response of an audio signal. (I assume that you have gone thru the Audio fundamentals tutorial before coming here to understand Frequency.) An equalizer is made of several filters. If we bifurcate the human hearing range in to 4 bands then to control them we need a 4-band eq. we can also have several bands in an equalizer.There are primarily two types of equalizers, Parametric Equalizer and graphic equalizer.
Graphic Equalizer.
As the name suggests it can give you a visual graphical representation. Usually these equalizers are 32 band 1/3rd octave equalizers. (Meaning the frequency bands available are as per 1/3rd octave) In a graphic equalizer you have only one control per frequency, which is the gain .you can either cut or boost the gain of a particular frequency.
It is a dual channel third octave graphic equalizer, occupying 3U of rack space. Oil damped precision slide controls (30mm) are arranged in two identical rows of thirty ISO frequencies between 25hz and 20kHz. Each channel also has +/ 6dB adjustment range selection, 50Hz high pass filter, and overall gain adjustment.These equalizers are generally used in Live Sound reinforcement.
They are very useful to do quick adjustments and are mainly used to tune up the PA to a room and also used for feedback elimination during Monitoring on stage. The quick view of all the frequencies giving a graphical representation of the situation is very helpful for fast action.
Parametric Equalizer.

Parametric Equalizer

A parametric equalizer gives you more controls to shape up the tonality of the audio signal. In a fully parametric EQ, one can change the band’s center frequency (a), gain (b), and bandwidth (c) or Q, depending on the EQ’s design. A parametric EQ is generally used on individual channels or instruments to set the desired ton of the instrument within the Mix.
In an Analogue Parametric EQ you have 5 bands available, which covers the following bands of Frequencies.
1) 15 Hz – 800 Hz, Q of .4 – 4 or shelving, 15 dB boost/cut.
2) 15 Hz – 800 Hz, Q of .4 – 4, 15 dB boost/cut
3) 120 Hz – 8 kHz, Q of .4 – 4, 15 dB boost/cut
4) 400 Hz – 26 kHz, Q of .4 – 4, 15 dB boost/cut
5) 400 Hz – 26 kHz, Q of .4 – 4 or shelving, 15 dB boost/cut

This Is Considered as a High Precision Mastering EQ. The control provided by this EQ to shape the Audio is superb.
In an Audio console each channel is provided with a parametric EQ. These are normally 4 Band. One of the Most Popular is SSL E Series EQ.
1) HF: 1.5kHz – 22kHz, Shelving Type Gain ±20dB
2) HMF: 600Hz – 7kHz, Q (0.7 – 2.5). Gain ±20dB
3) LMF: 200Hz to 2.5kHz, Q (0.7 – 2.5). Gain ±20dB
4) LF: 40Hz – 600Hz, Shelving Type, Gain ±16.5dB

In today’s time when people are so used to Eqing while looking at the EQ curve displayed on the EQ plugin or by using an audio analyzer, the art of Eqing by listening is getting faded. You need a trained ear to Eq by ear. I generally always suggest a method to practice. Before touching any knob on the Eq, try to analyze the target audio just by hearing. When you can identify the problems in the existing tonality of the audio, Note them. For example lets consider a vocal track. What can be the potential issues in a vocal track for which you need an EQ to fix them?

  • The Vocals are too Boomy!! (Meaning the performer was very close to the microphone. due to the proximity effect the recorded audio has a lot of unwanted Low frequency.)

Solution: apply a cut of around 3dB at 100 Hz with a wide Q.
  • The vocals are sounding too boxy!! (The performance might be recorded in a home studio with little or no acoustic treatment, hence a lot of Low Mid reflections are recorded with the Main signal OR the vocalist was having a cold!!)

Solution: (Please keep in mind there is No optimum solution for a badly recorded track. if its recorded in a not good room then the room is heard with the audio signal. there is no tool which can remove the room from a Audio signal without affecting the Main Audio. Hence effectively you will end up having a compromised vocal track. )
Try removing 400-500 Hz from the signal with a wider Q.

  • The vocals are very sharp. (You can hear the minute click sounds, and esses very distinctively.)

Solution: try cutting 6KHZ with a sharp Q. also desser is a perfect tool for this. The above example is a good exercise to follow, write the solution or think of it before you touch the Knobs. Ideally when you know the problem as well as the solution you should be able to fix it with not more than 3 Moves.
Having said that I don’t mean to say that using visual representation is bad, But yes we as engineers need to hear what we do rather than just be Psychologically satisfied by seeing the graph and feeling that Audio has changed. I have been shocked to see some crazy EQ applied on vocals, which clearly shows that the basics were not understood.
I also follow another rule. If you need more than 3 filters and 6dB of Eqing then some thing is wrong at the source. Try and fix that. I strongly want to put a point across .EQ is not a tool which can fix any thing. There is no substitute for a good recording. Some people think that it’s necessary to EQ and compress every track.
I have a simple explanation for them.
Every automobile comes with a Tool Kit. You are explained about how to use them when you buy the vehicle. Now since you know it, do you, in your free time, open up your Engine? Then why do you feel the same with audio?
“Don’t try to fix something Which Is Not Broken “
Pramod Chandorkar

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