Dave Moulton /
09.08.2004
Audio 102 for Video Folks: A Primer on EQ
As I promised in my last column, this article will be devoted to the basics of equalization, or "EQ," for those of us who aren't really all that familiar or comfortable with audio (which is most of us in the biz, I've found). I hope you find it useful, or that you can pass the column on to some of your video colleagues.

SO WHAT IS EQ?

EQ refers to the practice of altering the frequency response, and therefore spectrum, of an audio signal. Why would we want to do this? To make the signal sound better, of course. Frequency response is a prime, if not the only determinant of the subjective quality we call timbre, the distinctive quality of a sound that allows us to identify it among many others.

An equalizer is an audio device that permits us to boost or cut the amplitude (level) of any given part of the spectrum, independent of the rest of the spectrum. It comes in many flavors, most notably graphic and parametric.

What all these have in common is that there is at least one control (usually there are many) that boosts or cuts the level of whatever part of the spectrum is specified.

On a graphic equalizer, there are pre-assigned portions of the spectrum, usually specified by octave parts, as in one-third of an octave, octave-band or two-octave bandwidths. An octave is a generic "doubling of frequency" and is the most basic expression of spectral width that we have.

On a parametric equalizer, you get to choose what part of the spectrum you wish to boost or cut, as well as how wide the part you wish to boost or cut should be. It is more complicated to use than a graphic equalizer, to be sure, but it allows you to do a lot more (or less, when that's what you need).

In any case, we use equalizers for two purposes: first to fix problems in our audio signal by reducing the level of portions of the spectrum that are presenting the problems; second, to enhance the signal by boosting the level of portions of the spectrum that sound particularly nice or are particularly relevant.

WHAT'S HOT, WHAT'S NOT

First we need to consider the spectrum itself. Take a look at Fig. 1.

(click thumbnail)
Fig. 1 The 10 octaves we can hear, with lots of information about each of them, from my book "Total Recording," courtesy KIQ Productions.

Note that we humans hear over a range of 10 octaves (1,000:1). That's huge! Vision, by way of comparison, only covers about one octave (2:1). Each octave has a particular sensory quality to it, and each octave seems "equally wide" to our hearing.

Consider the descriptions of each octave. Note that I refer a lot to "fundamentals" and "harmonic content" of sounds. This leads to the most important single concept in this discussion--all sounds (except for one theoretical test signal, the sine wave) consist of broad bands of frequencies, usually spanning at least five octaves, often several more than that. The lowest frequency in a sound is called its "fundamental." The other frequencies are called "overtones" or harmonics. It is the relative loudness of these various overtones across the spectrum that is a prime determinant of timbre.

When you go to EQ a single sound (a voice track, for instance), you will find that changing the levels of different parts of the spectrum have profoundly different effects on the tonal quality of the voice.

To get really good at the art of equalization, you will need to experiment with these issues and practice equalizing a wide variety of signals. If you are so inclined, I have an audio ear-training course available on CD called "Golden Ears," which will help you do exactly that.

A couple of general comments about the spectrum to consider:

For television work, the bottom and top octaves are pretty much irrelevant because playback systems don't reproduce them reliably, and there is not much audio information in them even if they did.

For the bottom octave, it can be a good practice to simply turn it down.

For the top octave, a slight generic boost is often good, if your signal is not unduly noisy. This may add a slight amount of "sparkle" or "airiness" to the sound, and it will seldom do anything bad.

The two octaves from 300 to 1,200 Hz. will, if boosted, add richness and warmth to a signal, or, if cut, will add transparency and clarity. It's up to you to decide which is more important in each case.

The octave centered around 4 kHz. is the most important octave for human voice, particularly in terms of presence and intelligibility.

BOOST OR CUT

As I noted earlier, we cut to take out problem frequencies, and boost to enhance attractive frequencies. There's more to this, though. When we cut a small portion of the spectrum, the resulting level will usually not change. However, when we boost, often the level will change, so that along with altering the spectrum we're also fiddling with level.

In general, it is desirable to use parametric equalizers for cutting out problem frequencies, and to make the band you are cutting as narrow as is reasonable. Further, cut only as much as you need (something like, say, 4 dB) to get rid of the offending problem.

Meanwhile, when enhancing a sound, gently boost a fairly broad portion of the spectrum (an octave or more) by a few decibels rather than a higher, narrower boost.

EQ FOR FUN AND PROFIT

Here are a few basic tips 'n tricks to make your life easier.

Hum is usually at 60 Hz. and sometimes also at its odd multiples (180, 360, etc.). You can get rid of a lot of it using a very carefully tuned and very narrow EQ cut tuned to those specific frequencies. If you do it well, there will be no audible change to the rest of the signal.

You can get rid of a great deal of low-frequency noise (air conditioners, truck rumbles, other stuff like that) using a "shelf equalizer" (which boosts or cuts everything above or below a given frequency) set to 70 Hz. or so and cut 5 or 6 dB (or more, if it doesn't seem to affect the content of the mix).

You can enrich male voices (particularly) with a little gentle boost in the 250-300 Hz. range.

As mentioned above, you can make a sound lighter and more transparent by cutting gently somewhere in the 300 to 1,200 Hz. range, or you can make it richer and fuller by boosting. Feel free to experiment.

You will make voices more present and intelligible by boosting around 3,500 to 4,000 Hz.

You will add sizzle and brilliance to acoustic guitars and a whole variety of other instruments if you boost them in the 8 kHz. range.

A caution: don't get carried away with EQ-ing the living daylights out of everything. A little EQ can go a long way, and when you first get into it, it's easy to overdo it.

Always check your work a little later to see if you're still the audio genius you thought you were when you did the equalizing. I've found that as I get older, I EQ less and less, but usually to greater effect.

Next month, I'll take a look at the fundamentals of compression. Also, remember that I'm happy to take questions on any of this stuff. Got an audio question? Send me an e-mail.

Thanks for listening.


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