The A-Zed of Audio
Gold is one of the most durable value keepers in the world. Yet long ago, sneaky folks realized that gold could be mixed with less valuable metals to dilute its purity and create something that may have appeared similar, but wasn't the same. In some movie you've probably seen a pirate or some market vendor bite a gold coin to figure out its purity –that's because pure gold is relatively soft, and a bite could [roughly] indicate its purity.
Nowadays we use the carat measurement to determine gold' purity instead of our teeth, and although it might give them a bit less business, our dentists are probably thankful for it. 24 carats is the goldest gold you can get and if there's any other metal present it’s of such a negligible quantity that it’s not even worth mentioning.
In the world of digital audio – specifically when it comes to frequencies and EQing – we have something similar that we call Q. What is Q? The Quality factor. Yet aside from having a technical role in mixing the [very] occasional gold-certified record, what can an EQ possibly have to do with gold?
Recall that an EQ (or equalizer) is a frequency-specific volume knob. This means that we can augment or reduce specific frequencies whereas a normal volume knob augments and reduces every frequency together. Suppose that you're recording a guitar and after listening back you're just going crazy that it sounds like a bit of a clown’s horn. If you've got experience in the ways of the EQ, you might know that these culprit frequencies lie somewhere in the range of 1 to 2 kHz. Knowing this, you can then apply your trusty equalizer, find the frequency that's causing you such grief, and reduce that honkiness until you can once again listen without feeling like you're at the Ringling Brothers circus.
Yet how many other frequencies are going to be affected by this move? What's the collateral damage? This is where Q – the audio carat measurement – comes in. As I've mentioned, carats measure purity, and Q stands for 'quality factor' (it has nothing to do with the Q in 'EQ') – even though the units don't line up, we can pretty reasonably interchange the terms. A low carat value indicates that there is a lot of other stuff beyond the metal we're looking for, and as we'll soon see, a low Q value indicates something similar. Precious metal and treasure chests aside, we can say that Q is the area of action of any band of an EQ. It's one of the 3 main parameters of an EQ – the other two are frequency and gain; any EQ which allows its user to adjust these 3 parameters is a fully parametric EQ.
As you sit in your studio listening to that crazy honking guitar sound, you may discover that it's most concentrated at 1370 Hz. As you begin to lower the gain, you hear that offending sound decrease. Yet you also notice that the recording is losing a lot of definition. In this case the collateral damage is high because you've not only taken out the offending frequency, but a number of the surrounding innocent ones. In other words, there's a lot of dirt; if the frequency we're searching for is gold, what we've just done has a lot of other cheap metals mixed up in it.
So now you adjust the Q. The higher you raise the number, the narrower and more precise the frequency cut becomes until the definition has returned and the honkiness remains in check.
The technical measurement of Q is centre frequency divided by bandwidth between the peak and -3 dB, but in this discussion of Q values it isn't necessary to work with precise calculations – simply remember that the higher a Q value is, the tighter and more precise its effect will be. If you're using a parametric EQ on your computer (as most of us are), then you'll very clearly see the difference as you adjust the Q and the curve changes from a very shallow hill to a very sharp point. As a rough figure, any Q value below 1.41 (and definitely anything that’s less than 1) can be called wide, and any value above 1.41 can be called narrow. The reason for the seemingly arbitrary figure of 1.41 is that this amounts to a bandwidth of one musical octave, and it's always nice to be able to insert any sort of musicality into our work when we can.
But really, there's nothing significant about a Q of 1 octave if it doesn't sound right. The general approach is that you use a narrow Q when you want to do precise, surgical work, and you use a wide Q when you want to create a warmer, more general change. Kind of like how you use pure gold if all you want it for is its value, but you create alloys (with lower carat values) if you want some sort of appliance or tool that can serve a more general purpose.
Two really important suggestions to setting a Q value (and which apply as best practices for pretty much all audio work) are:
- Always work with a reference. Very quickly you can distort your perception of what sounds good – an easy test is to bypass the EQ and listen to the unaffected sound to recall what it sounds like, or better yet, have a comparable recording that you know is good, to which you can compare your work.
- Close your eyes! I can't tell you the number of times I've tricked myself into believing I've done good work just because it somehow looks right. This is one of the biggest dangers with using a 'visual' EQ. In the end, it's your ears that should be doing the work, because it's your ears that'll be enjoying it.
Q is a pretty minute and technical matter, and when it's used well we don't notice it. It's a lot like gold – if we use the appropriate level of purity for the job we're aiming to accomplish, we end up with an incredible result. Yet if we use 24-carat gold to build a hammer just because we like the idea of using pure gold, we might find ourselves with nothing more than a very expensive paperweight because the tool is softer than the nail it's meant to hit. With Q, it's the same story. If you use it appropriately, things can work amazingly. But beware! There's such a huge danger of looking at your computer screen or EQ curve and thinking it looks good, but you need to work blindly in order to do really good work. This (like pretty much every other area of audio) is where your ears will lead you to success. If it sounds right, it is right.
Jordan Mandel is a Digital Media Lab Instructor at the UW Stratford Campus, and writes for this blog regularly. His hobbies include bonsai trees, knife sharpening, and polka dots. More of his work can be found at jordanmandel.com/blog, which is home to the award-winning satire rag, The Outa Times.