Most great mixers think in three dimensions.
They think “Tall, Deep and Wide,” which means they make sure that all the frequencies are represented, the mix has depth, and finally has some stereo dimension as well.
The “Tall” dimension (which is called “Frequency Range”) is the result of knowing what sounds correct as a result of having a reference point.
This reference point can come from being an assistant engineer and listening to what other first engineers do, or simply by comparing your mix to some CDs, records or files that you’re very familiar with and consider to be of high fidelity.
Essentially, what you’re trying to accomplish is to make sure that all the frequencies of the mix are properly represented.
Usually that means that all of the sparkly, tinkly highs and fat, powerful lows are there. Sometimes some mids need to be cut or other frequencies need to be added, but regardless of what you add or subtract, clarity of each instrument is what you aim for.
Again, experience with elements that sound good really helps as a reference point.
The Effects or “Deep” dimension is achieved by introducing new ambience elements into the mix. This is usually done with reverbs and delays (and offshoots like flanging and chorusing) but room mics, overheads and even leakage play an equally big part as well.
The panning or “Wide” dimension achieved by placing a sound element in a sound field in such a way as to make a more interesting soundscape, and so that each element is heard more clearly.
Every piece of modern music, meaning Rock, Pop, R and B, Rap, Country, New Age, Swing, Drum and Bass, Trance and every other genre having a strong backbeat, has six main elements to a great mix.
—Balance: The volume level relationship between musical elements
—Frequency Range: Having all frequencies properly represented
—Panorama: Every musical element is well-placed in the soundfield
—Dimension: Added ambience to a musical element
—Dynamics: Controlling the volume envelope of a track or instrument, and
—Interest: Making the mix special
Most neophyte mixers have only four or five of these when doing a mix, but all of these elements MUST be present for a GREAT mix, as they are all equally important.
You can read about these six elements in more detail in The Mixing Engineer’s Handbook.