What is "EQ"?

EQ is short for equalization. All sounds are made up of frequencies, and changing those frequencies changes the sound. This is done through EQ. So why would you EQ something? For 3 main reasons: to tame problem frequency/frequencies, to enhance the track, and to enhance other tracks. I’ll go into more detail with audio examples for each.

Taming Problems- Frequency imbalances occur due to either a problem with the instrument, problem with the acoustic environment, miking techniques, or a combination of the three. Fixing these problems can require anything from slight dip, to precision cutting with EQ. For this example, I’ll use a snare drum that elicits a nasty overtone. Normally, putting a piece of tape or something called a “moon gel” can solve this problem at the source. But in the event that neither is available, we can use EQ to find and eliminate the problem frequency.
Snare before:

Listening to the “before” track, you can hear a “whooom” that is quite annoying, and so loud it distracts from the rest of the snare. Trying to get this to work in a mix without first resolving this frequency issue will sound horrible.

Snare After:

Using the Waves Q10 EQ, I can zoom in to the exact offending frequency and eliminate it. In this case, there were two problems- one at 197, and one at 202. Once I cut these frequencies out, the snare sounded listenable again.

Enhancing the Track- Every instrument, including voice, has defining characteristics that are represented in the frequency spectrum. For example, a snare between 200-400 Hz contains the “body”. Between 1-3k contains the “snap”, and 5-8k has the “sizzle”. Cutting these frequencies would destroy the essence of the snare drum. Boosting them will enhance it. To get each instrument to stand out better in a mix, we find their essential frequencies and boost them. For this example, we’ll highlight a drum kit.
Before EQ:


After EQ:


You notice that after EQ'ing the kit, each drum piece sits a little more clearly and can be heard better. The Kick is bigger, the Snare has more body and snap, and the Overheads have a little more high end to open up the sound. This will make each drum stick out better in a rock mix without having to turn the whole track up.

Enhancing Other Tracks- This is a continuation of the principal that every instrument has defining frequencies, but instead of boosting, we cut. For example, I know the speakers in a guitar amp rarely produce any tones higher than 5k. If the guitar was miked, that isn’t to say there won’t be any sound beyond 5k. There just won’t be any sound useful to the guitar. And in fact, all that noise above 5k if left in, will only get in the way of other instruments that do use that space. So we cut it out. But to enhance other tracks, sometimes we have to make them sound worse alone to sound better together. For example, depending on the style, an electric guitar does have some useful information below 200 Hz. However, in a mix setting priority should be given to the Kick drum and Bass guitar for sounds under 200 Hz. Cutting that from the guitar might make it sound strange on its own, but in the mix the Bass and Kick have more room to breath, and we really didn’t take too much away from the guitar.