Measure Once, Cut Twice: EQ Basics

Equalisation, more commonly refereed to as EQ, is the process of balancing frequencies to give a more pleasing overall sound to an individual track, group of tracks or master. It’s a mandatory part of music production used in tracking, sound design and mixing and you’re probably more familiar with it than you realise.

Graphic equalisers were a common feature of old stereos/boom boxes, splitting the frequency range into bands that you could attenuate or amplify. On mixing desks the EQ section is normally 3 or 4 rotary pots that adjust the spectrum more broadly. However, I want to focus on digital EQs, as they’re far more widely available and (arguably) easier to use than their analog forefathers.

EQs are a staple of every commercial DAW. Although there are variations, most adhere to a similar structure; a ‘shelf’ at either end of the spectrum, sometimes with a low/high cut either side of that, and a number of bell-filters in the middle of them. Other types of EQ include graphic EQs (mentioned above), linear-phase EQ, mid-side, parametric, and various filter types like notch and bandpass.

This wont really serve as a guide as to how to EQ. Mixing your music is very specific and whilst you can read some suggested dos and don’ts, you really need to use your ears and A/B your work against other commercially released records within the genre you are working in. With that in mind I’m going to try and briefly sum-up the plug-ins I’ve run into recording and mixing, this is certainly not all of them, but hopefully serves as a useful guide.

Bell Filter

An absolute staple in every DAW and incredibly useful for what’s called corrective EQing. With digital technology we can fine-tooth-comb our mixes with these. The common parameters found on these are the peak or trough defined by the frequency (fairly self explanatory), Q or resonance, which refers to the width and gain, which can either be +/-, and is measured in dB.

These are perhaps the most useful filters for corrective and creative EQing, as you can quickly identify problematic frequencies and attenuate them. There are several ways of doing this if you can instantly identify those areas.

Firstly, you can use the age-old method of sweeping a high-Q resonant peak. If this means nothing to you, then don’t worry. Q is the amplitude applied to the point at-which we are attenuating or amplifying. A wide Q (like the image above) makes broad strokes, a narrower Q makes more surgical incisions.

It’s more common to boost wider Qs but attenuate narrower ones, but this is just a rule of thumb and I am sure someone will find exceptions!

Sweeping an amplified narrow Q band across the spectrum will help you isolate harsh sounds, as when the band rests on one of these, you will hear it resonate or distort. This is a really useful method for helping identify troublesome frequencies in a complex sound or mix.

Another method is using a spectral analyser as many digital EQs have these built into them now (e.g Logic’s EQ, Ableton Live’s EQ Eight, FabFilter’s Pro-Q etc). This can help you visualize peaks particularly in the higher frequencies that may be harder to first hear. Let’s look at an example.

Where to Cut? Think (About It)

Lyn Collins’ 1972 hit Think (About It) is probably one of the most sampled breakbeats of all time. It’s been used on countless record (some of which you can read about here). It’s popularity in the sampling world is largely down to its various drum breaks in the record at 1.21, 1.34, 2.02, 2.14 and 2.21.

The break is characterized by the instantly recognisable vocal shouts and tambourine part. Let’s pickup on the first break at 1.21, here it is without any EQ applied:

N.B These examples aren’t going to sound all that impressive on laptop speakers, so to hear what’s going on properly I would suggest half-decent headphones or studio monitors.

Nothing seems untoward about this at all. However let’s look at at that with a spectral analyser. Normally I’d reach for Voxengo SPAN at this stage (as free and very customizable) but Live’s own EQ Eight has a built-in spectral analyser.

The tambourine has thrown up a couple of sharp peaks at 8.7 kHz and 11.4 kHz. These may not sound that offensive as is, but let’s amplify them to hear what’s really going on there, one by one:

As you can hear there’s quite a squeal there. Let’s drop both frequencies down then compare the original with the EQd versions:

Original:

EQd:

A marked improvement if you ask me, and using the spectral analyser helped us get there in half the time. Of course this should be no replacement for a good musical ear, but sometimes visual tools can help us EQ our material, especially in higher frequencies where it’s much tougher being as accurate as low mids, for example.

There is of course more we could do with this break, but we’ll leave it there for now, what we might do next would largely depend on the style of music, what (if anything) it’s being layered with, how present it is in the mix, what the bass is doing, other percussion, reverbs etc.

Some love has to be shown for FabFilter’s Pro-Q 2 which allows you to isolate frequencies and actually audition them, so you can hear in solo what you are removing, using an extremely tight band-pass filter; a very clever idea!

Low Cut/High Cut

A low or high cut means that nothing* will pass beyond that point. For example, a low-cut EQ at 100 Hz means no sound will pass below this frequency. *Okay, not quite nothing, but it’s attenuated. The strength of this curve is measured in dB/Octave. In practise a 6 dB/Oct will be softer than a 24 dB/Oct.

Low cuts (the same as a high-pass filter) is probably the next most important type of EQ after bell filters. You will read countless tutorials telling you to roll off bottom end, and there’s good reason for this.

Clearing space in your mix for kick drums and basslines is key to getting your track sounding optimum, particularly on larger sound systems. Adding too much bass to your sounds is one of the most repeated amateur mistakes I’ve heard in demos submitted to me or trawling SoundCloud.

Shaving off as much as you can from other instruments (pads, vocals, snares, hats, leads etc) to clear the path for your kick and/or bass line can sometimes seem like a fruitless task, because often those frequencies you’re getting rid of, you can’t hear. So, are you making a difference?

Let’s think of a practical example. Often a low rumble is picked up with vocal recordings. This can be from the mic stand touching the floor and picking up vibrations, heaving breathing, a cheap pop shield, CPU noise or air conditioning units.

A handy tip is to low-pass (yes, low-pass!) your recording at about 80 Hz and turn up your soundcard, carefully!

The lower frequencies will begin to jump out at you at this volume, so it’s clear that they’re there – just harder to hear at unity gain. I use this technique to identify where I should start my high-pass. Once I’ve identified the frequency I’ll turn the soundcard down and disabled the low-pass, adding a high-pass to that point instead of it.

Be careful not to high-pass your tracks too aggressively as they can become unnatural, experiment with different slopes especially. Again, there’s a case to be made for spectral analysis here; always have one placed across your master channel and you can solo individual instruments to see whether or not there’s a need to high-pass filter them.

Low Shelf/High Shelf

These are similar to a low or high cut filter, but they can either amplify or attenuate below/above the given frequency. I tend to use these to make broader strokes, mildly boosting or cutting the high or low frequencies perhaps on a group, return or master channel.

Shelfs are normally included in most digital EQs and can easily be confused with low/high cut, but the main difference is that cuts use dB per octave slopes where are shelfs just increase or decrease at that pointDue to their not-so-subtle nature I use them sparingly, a dB or two here or there. With smaller, less optimal monitoring environments it’s easy to add too much bass or top-end to your mixes.

The reason for this is smaller speakers will struggle replicating the definition and weight in the subs and high end is just too tempting to pile on, especially when starting out with mixes and not feeling they shine enough.

Graphic Equaliser

Normally found on old hi-fis and used a lot in the live music environment to eradicate feedback. In the digital domain I’ve found less use for these, especially as native EQs bundled with DAWs have made such leaps and bounds. These are less flexible than a bell filter as the bands are fixed.

The bands are usually determined by octaves, or divisions of that. Our hearing range spans from 20 Hz to 20 kHz, which is 10 octaves, the reason we often get 10 band, (one slider per octave), 20 band (a slider every 1/2 octave) and 30 band (a slider every 1/3 octave) graphic EQs.

I very rarely use these in a DAW environment, as for corrective EQ purposes an ordinary digital bell filter will get me a lot further. However I wouldn’t rule them out completely as sometimes this less flexible approach can lead to interesting tones especially on distorted guitars, drum busses and even in mastering.

Mid-Side EQ

This allows separate controls for the mono and stereo image, the ‘middle’ is information that is consistent between the left and right speakers, whereas the ‘side’ is anything that is different between the two.

The two main uses I’ve found for mid/side EQing is in mastering and in place of splitting various sounds when layering and mastering. In both examples it can be beneficial to high-pass the bass frequencies (say, below 120 Hz for example) in the side channels, giving more energy to middle (not to be confused with midrange).

Linear Phase EQ

Equalisation introduces a small amount of phasing to our sound, normally not enough to concern us, especially when working with stems. However, when we’re working with groups or a busses the phasing can be far more problematic. Linear phase EQs aim to compensate for this.

This is used a lot in mastering so as to incur the least amount of phase cancelation. On a single track or stem this would be largely unnoticeable, but on a whole track, especially making changes to the sub regions, this is more useful.

A disadvantage of this is that it can be mildly latent, so when exporting or bouncing, ensure there’s a gap of silence before your track starts, or you can get a nasty glitch right at the beginning.

Honestly I don’t use a lot of these in my production, but that’s probably because I don’t deal with a lot of mastering. Some EQs (such as the Pro-Q) allow you to switch the algorithm from most-latent with minimum phase to least latent with most phase.

Parametric EQ

These EQs use pre-defined bands, normally low shelf, one or two mid controls (that have a ‘sweepable’ frequency range) and a high shelf. Often found on mixing desks and older outboard gear.

Vintage mixing desks such as Neves, SSLs and APIs have classic sounding EQs which are much sought after and have many emulations in the plug-in world, which I am sure some purists would turn their nose up at ( ͡° ͜ʖ ͡°).

Sometimes the term is interchangeable with channel strips. The Waves SSL E is a favourite of mine, as I find it easy to use, the bands seems to be places in logical placed, you can overdrive it to pleasing sonic effect and it has a compressor and gate included.

Each pot may be either a bell or shelf (or cut in some cases) but the frequency is defined. It could be that the midrange pot could attenuate/boost at several different points but not between them. These are often selected for their harmonic distortion characteristics when driven, or other sonic properties, rather than their accuracy and fine tuning.

Other EQs that might fall under this umbrella are passive and active but I’m not really in a position to talk in depth about these, I would much rather direct you to this good SoundOnSound Q&A about the topic.

Plug-ins such as Universal Audio’s Manley® Massive Passive, Native Instruments’ Passive EQ and Waves’ RS56 and PuigTecs are examples of passive EQs available in your DAW.

Conclusion

In the digital domain, EQs tend to fall into two categories – those that you might use for surgical-like sculpting of sound and those that emulate classic EQs. This isn’t really the platform to comment on the latter so I’d recommend checking out this YouTube video from Produce Like A Pro (really good channel), which goes into some detail about some hardware EQ.

The important thing to grasp about EQing is that it’s not only about balancing the frequencies of an instrument or track, but balancing all of them together. Frequencies can compound and interact, so whilst removing a band from your kick (for example) may work in solo – how does it sound with your snare and bass?

EQ really is a balancing act (also a dreadful music tech journalism cliché) and it’s as much about listening and stepping back from the mix as it is about zooming right in and analysing things in forensic detail. Hope this has demystified some of the clutter around EQs. Enjoy!