Equalization, better known simply as EQ, is a crucial tool used in mixing and mastering audio. An equalizer is, whether digital or analogue, is designed to shape the tonal qualities of a sound by “cutting” or “boosting” specific frequency ranges. Having an understanding of EQ types and how to use them with various instruments is important if you choose to get the most out of your mix. This blog post explains the different types of EQ and how they are used in a professional setting.
High-Pass and Low-Pass Filters:
There is a range of different equalizers available to the modern producer which can make it difficult when selecting the right one for the job. The first and most simple are the High-Pass (a.k.a Low-Cut) and Low-Pass (a.k.a High-Cut) Filters. A High-Pass filter will completely cut out all frequencies BELOW a specific “cutoff” frequency selected by the user. A Low-Pass filter will do the opposite by cutting out all frequencies ABOVE the cutoff frequency as shown on the graph below.
You may also notice that the cutoff line is not completely vertical but an exponential curve which is determined by a value called the Q factor. A curve might have a Q factor of 24dB/octave which means that every octave the signal is reduced by 24dB. This example will produce a steep slope, which is good for surgically removing the frequencies, however a less steep slope of 12dB/octave or even 6dB/octave will produce a more natural sound. These kinds of filters are used mainly for correctional purposes by removing unwanted low frequency hum or high frequency hiss. It is not uncommon for more mid-range instruments such as guitars, synthesisers, pianos and vocals to have filters applied to them to remove those unnecessary frequencies to keep the mix as clean and open as possible.
Shelving filters are very similar to HPFs and LPFs in the way that they reduce frequencies below or above a cutoff frequency. However, unlike the HPFs and LPFs they do not completely cut out the frequencies. Shelving filters allow the user to cut or boost all frequencies equally above or below the cutoff frequency, meaning that the cutoff line levels out or “shelves” at a certain gain level selected by the user. This is visualized in the graph below.
These types of filters are used when the complete cut of the HPFs and LPFs is not desirable. They can be extremely useful for example if a bass track has a little too much sub bass and the user wants to reduce it slightly. This doesn't completely get rid of the sub, but controls it, making the sound more balanced. Similarly, they can be used to boost those sub bass frequencies if the low end is lacking.
Graphic vs Parametric Equalizers:
Graphic and Parametric equalizers are the most versatile types of EQs as they allow the cut or boost specific frequency ranges or “bands.” A Graphic EQ possesses a series of 10 to 30 evenly spaced individual filters that are cut or boosted by gain sliders. These sliders can serve a visual representation of the EQ curve. Although, it is important to understand that as these sliders interact with one another, they may not be entirely accurate in showing the actual EQ curve if extreme values are used. Because of the versatilely and intuitive nature of Graphic EQs, they are used most commonly in live situations rather than in the studio. The photo below shows a typical Graphic EQ set up alongside the EQ curve.
Parametric equalizers are similar to Graphic ones in the way that they control the gain of specific frequency ranges. However, unlike Graphic EQs where the frequencies are fixed, Parametric EQs allow the user to select at what frequency each filter lies. This additional control makes them one of the most precise and popular types of equalizers used in the studio environment. The user is able to select the centre frequency, bandwidth and gain level of each filter. Multi-band parametric EQs are the most commonly used but can be the most complex due to the amount of control given to the user. However, used correctly, they can help get rid of unwanted frequencies and boost the desirable frequencies in order to get the best out of each instrument whilst allowing them to fit neatly in the mix. There are also Semi-Parametric EQs, these allow all the control of Parametric EQs apart from the bandwidth control. The picture below shows Pro Tools' Multi-band Parametric equalizer:
When using EQ in the studio there are a three golden rules that should be followed to get the best out of your mix. The first and most broken rule amongst bedroom producers is to use EQ sparingly. Ideally, you should aim to avoid using EQ by ensuring the recording is as good as possible in the first place, mic placement is a big factor in achieving this. You shouldn't go into a recording session with the “I'll fix it in the mix” attitude. If you do end up having to use EQ, make sure not to make any extreme boosts or cuts. For example, a boost of 6dB may not seem a lot, but in reality +6dB is double the volume. Boosts like this can quickly contribute to the mix bus peaking.
The second rule to follow is the cut narrow and boost wide rule. When cutting frequencies it is good practice to use a narrow bandwidth. When boosting it is good to use a wider bandwidth. By following this simple rule it will help to keep the EQ more subtle and natural sounding.
The final rule that you should follow is cutting is almost always better than boosting. If there is a guitar that sounds a bit dull, the first thing a beginner would think to do is to boost the high frequencies. Although this would achieve a brighter sound, it would also add more volume to the high end which will in turn mask the other instruments lying in that range. However, by cutting unwanted frequencies from the low-mid range it will help to balance out the overall sound and bring the higher frequencies forward without any unnecessary boosting. This will also help to make room for other instruments that also lie in the low-mid range.
There is no easy way to create great mixes straight away. It requires practice and a meticulous ear for audio. If you are using digital plug ins, do not rely on the visual representation of the EQ curve, it will trick your brain into hearing things that aren't there in reality. By following these simple rules and training your ear, you will be able to make the instruments work well together to produce successfully polished mixes.
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