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How does EQ work?

For audio professionals, equalisation (known in the trade as 'EQ') is a standard part of the daily workflow when recording and mixing sound. However, for beginners and those that are new to the concept of EQ, it can be difficult to know where to start.  

Many people don't realise that we encounter and use basic forms of EQ almost every day. When we change the bass, middle or treble on our car stereo, or select 'bass' or 'vocal booster' on our smartphone's music settings, we're actually using EQ to make our tunes sound as we want them to.

In simple terms, EQ is the process of boosting or attenuating certain frequencies of a given audio signal - but how exactly does it work?

Audio frequencies

Every single sound that we hear is made up of broad range of different sound frequencies, which are measured in hertz (Hz). Humans are capable of hearing frequencies as low as 20​ Hz, right through to the extremely high 20,000​ Hz, which equates to 20 kilohertz (kHz).

A frequency between 20​ Hz and 500​ Hz would, in general terms, be considered as bass. Frequencies falling between 500 Hz and 4​ kHz would be bracketed as 'mid', whereas anything above the latter up to 20​ kHz is known as treble.

Of course, when we record, external factors, such as the microphone that we are using or the room that we are working in, can influence the sound. If we were to listen back to a recording of a voice or guitar and find that the audio contains higher frequencies than we intended, EQ can be used to reduce them, returning audio signal closer to its natural state.

Many people don't realise that we encounter and use basic forms of EQ almost every day.

Let's look at another example. If we record a bass guitar, but discover that the recorded signal does not contain as much low or bass frequency as we'd like, these too can be increased by using EQ.

It's important to note that there are several kinds of equalisation methods available to help you contour your audio signal, so it is imperative that we make sure to use the best type to suit our intentions. Here are a few of them:

Shelf EQ

When using a shelf EQ, all frequencies above or below a certain level are boosted or reduced by the same selected amount. Visually, this creates what look like a shelf on the frequency spectrum,  as we can see here:

PICTURE (shelf diagram)

High and low pass filters

This is fairly similar to shelf EQ, with a few notable differences. To illustrate this, a shelf EQ will only boost or reduce a range of frequencies to a set level, but a high pass filter (HPF) or low pass filter (LPF) will gradually reduce them above or below a certain threshold. When the frequencies reach zero, they'll be eliminated from the audio signal completely.

HPF is used to completely eradicate low bass frequencies. whereas LPF will edge out treble. Take a look at this visual aid - you can see the steep drop-off for all frequencies below the selected threshold.

Controlling the EQ properly can help you achieve a preferred sound.Controlling the EQ properly can help you achieve a preferred sound.

PICTURE: Graph 

This is especially useful when we're looking to completely eliminate any potential problematic frequencies from ruining the recording. Several RØDE microphones, such as the NTG2 and VideoMic Pro, have a HPF option. This eradicates bass frequencies that could be caused by a raft of factors, such as traffic or air conditioning whilst on location. 

Graphic EQ

Using graphic EQ gives us more control over the entire frequency spectrum of our track. It's a very common form of EQ, often used in music playback software. 

PICTURE: Graphic EQ Control Deck

Laid out from from left to right, we can see separate volume faders representing a pre-set range of low to high frequencies in the audio spectrum. The more faders we have, the more specific frequencies that we can adjust, which lends us more control of our EQ.

The faders can be raised or lowered to either boost or reduce the selected frequency. This makes it particularly easy to visualise the changes we are making to the audio track, and also to dial in certain adjustments to target specific instruments or sound sources. 

Parametric EQ

Parametric EQ is perhaps the most complex of all the EQ types, but it gives ultimate control over the frequency responses of our tracks. As with graphic EQ, the parametric variety allows us to make adjustments to multiple frequencies, but with a notable difference.

Using parametric EQ, we can choose the exact frequency being adjusted, as well as how much it is being boosted or reduced by. What's more, we can also alter the bandwidth, boost or cut being made, which can influence neighbouring frequencies. This bandwidth is referred to as the 'Q' setting.

As with many music production tools, EQ can be used creatively or in a correctional sense.

A narrow Q setting means that the selected frequency will be boosted or reduced with minimal impact on neighbo​uring frequencies. This can be really useful for focusing on an exact frequency, and removing it from the mix, or giving it a specific alteration, such as a boost.  

A wide Q setting is similar to the narrow one in that the selected frequency will also be boosted or reduced, but it will gradually bring the nearby frequencies with it. This will result in a smoother-sounding EQ, and the final effect will not be as drastic if we had been using a narrow Q.

What's the frequency?

As with many music production tools, EQ can be used creatively or in a correctional sense. Despite the fact that EQ is a powerful sonic device, it is widely accepted that better results will be achieved if we make more of an effort capturing the best sound at the source, before after-affects are applied. This lends us a better starting point before we've even begun to apply EQ, and will ensure that adjustments are subtle, tasteful and natural.