Equalization is used for mainly 2 different reasons. It's used for cutting a section of frequencies that you do not want enhanced. The second reason is to boost a section of frequencies that you do want enhanced. An amateur recording engineer will add equalization to boost frequencies, usually in the high and low range. They will not critically listen to the sounds to pinpoint the range ware to cut.
CUT AND BOOST
An EQ with a Boost of 3dB at 1200kHz with a Q setting of 1.0
An EQ with a -3dB cut at 1200kHz and a Q setting of 1.0
BANDWIDTH - The Q
Bandwidth is the width gauged in octaves in the spectrum of frequency's. A human can hear form about 20Hz to around 20kHz and that equals about 10 octaves. When you cut or boost a specific frequency range, you modify more that just the point you cut or boost. The range at ware you cut or boost is just the center. Lets say you boost at 250Hz, that is just the middle band and most Eq's have an adjustable width called a Q setting. This Q setting lets you adjust the width of the curve.
Below, there are 4 different Q settings and each one is labelled by the octaves they cover
There are 4 different Q settings on the EQ above and each curve is labelled by the octaves they cover.
You can see how each curve's bandwidth gets smaller as the Q setting decreases.
The Gain setting changes the amount of how much the filter is cut or boosted and is measured in decibels. 0dB is flat and means that there is no cut or boost for that set point.
Here is an example of a Gain of -2dB at 100Hz and a gain of 2dB at 5kHz
The Slope of the filter determines the accuracy of the cut or boost without interfering with the other frequencies. The slope is essential in telling how fast or slow the EQ will cut or boost a frequency.
You can see how fast and slow the frequencies react to the the different slopes and how they effect the surrounding frequencies from the set point.
The Frequency setting changes the placement of the middle point of the frequency (set point) of the cut or boost.
Just remember that the EQ will cut or boost from both sides of the middle point according to the Q settings and slope setting. The attenuated area is highlighted in red.
DIFFERENT KINDS OF EQ
The different kinds of Equalizers and each one is made for very specific needs and to fix different problems in the audio's frequency range.
1.) Semi Parametric EQ (Sweepable EQ)
These EQ's are commonly found in mixing boards. In this kind of EQ, there are 2 main controls. The cut & boost knob and a frequency selector knob that enables you to sweep and fix problematic frequencies. This EQ can be your best friend when trying to get rid of the mud in the lower end so your bass guitar and kick and live in piece. Most (sweepable EQ) semi para EQ's have separate knobs fr each band for the lows, mids and highs and other times the highs and lows are fixed to a specific frequency and the mids can be swept.
2.) Parametric Parametric EQ
This EQ is just like the semi parametric EQ, but it consist of one other control, the Q setting (bandwidth). This makes the parametric EQ very flexible and the all out favorite among most of the audio engineers.
With the addition of the Q setting, you have the control to to adjust the width of your cuts and boost. This enables you to hone in on a very distinct frequency range and fix any issues. This is great for removing problem frequencies from an instrument as you can adjust the Q to a very narrow setting to only remove a certain frequencies.
3.) Graphic EQ
They got the name because its the most visual of all the other EQ's. Just by looking at it, you can see what has been cut, boosted and by how much.they probably would have put these in the mixing consoles if they were not so big. They take up a lot of space and that space is precious. They usually have at least 10 sliders to adjust each band's the level of boosting or cutting. All these EQ's have a fixed Q setting. A 10 band graphic EQ, the bandwidth is set at 1 octave ( Q of 1.4141). A 31 band graphic EQ's bandwidth is 1/3 of an octave (Q of 4.318)
4.) Linear Phase EQ
Equalizers have filters and these filters are used to change change the frequency of sound. The changing of these frequencies within a defined range causes a very slight delay and this delay can cause phase. A linear phase EQ compensates and fixes these slight delays caused by the frequency changes. This is why this delay is described as the most smooth, silky and translucent of all the EQ's.
An EQ filter is a mechanism that takes out certain frequencies that are defined by the person while passes other frequencies in audio that is in the digital or analog realm. These EQ filters determine what passes through the EQ unharmed and what gets cut or boosted.
There are 6 types of filters that can reside within an EQ
1.) Band Pass - This filter lets the set frequencies that are in the center range go through without being affected by the EQ. It is a combined low and high pass filter. Everything to the left and right will be cut out and everything between the 2 points will be left alone.
2.) High Pass - The high pass filter will let everything pass above the user defined set point. This filter has a slope and the slope is measured as decibels per octave. You will find that most tracks in any mix will have a high pass filter in them. This is to get rid of the unwanted frequencies in the low and even the lower mid end. When used properly, its great for cleaning up the mud in your mix and making it sound clearer and in return it will make it possible to get your tracks louder.
3.) Low Pass - This filter is the direct opposite of the high pass filter. Need I say more? Ok, I will... The low pass filter reduces the frequencies of everything over the set point that the user defines. It attenuates the high frequencies while letting the lower frequencies pass without being affected. Its great for using on instruments with a lot of bass, like the kick drum and floor toms. Its also good to use to tame the extreme sizzle from cymbals, hi hats and other instruments that may be over saturated with upper high frequencies.
4.) Shelving - The shelving filter gets its name for how it looks. Imagine you are diving in the ocean and the ocean floor is flat, then it drops of 25 feet and then it levels off flat again. That is called an ocean shelf and its exactly how this filter looks when its set up. The shelving filter leaves everything flat and then it will raise or lower all the frequencies from the specified set point that the user defines.
6.) Bell Filter - Also called the peaking filter, this filter gets its named because of the bell shape it makes when you cut or boost its center frequency. This filter is used for boosting and for cutting a specified frequency range and is the most popular filter used in an EQ to date.
EQ is a very important part of carving out the sounds you want for your instrument tracks and mixes. It is used for blending your tracks together and constructing a cohesive sound. Most if not all your tracks in a mix will have an EQ in it as it's the most popular audio effect in mixing. Your natural tendencies are to boost, but please keep in mind that is often better to cut then to boost. It will make for a cleaner more crisp mix. One last piece of advice, always try to avoid doing the same EQ edits for every track.