— The Mix Bus —
The art of audio mixing is steeped in science. For a mix engineer to successfully achieve the goals necessary, he or she must be aware of what these goals are and the principles behind them. Today’s music softwares, with their high track counts, would lead a person to believe that tracks can be mixed together ad infinitum with no problems, and this is not the case!
I’ve often described mixing as an attempt to take a room full of perpetually bouncing tennis balls that are all bouncing at different heights, speeds and angles, and trying to confine them all within a medium sized box on the coffee table. As ridiculous as this sounds, it IS an accurate picture of what needs to be accomplished. From the recording phase we remember having to preoccupy ourselves with microphone positioning and phase alignments, and all of this, so that we could capture the wave movements in the room. Then we compressed some of those sounds and gated some of the unnecessary out of our tracks; this left us with a separate image of each instrument or voice which we now want to superimpose upon one another in a stereo mix track.
The first problem that we face is headroom. For every track that we choose to include in this mix, there is another addition to the overall volume of the track, which means: it doesn’t take long before the mix becomes too loud for the mix bus and starts to clip. The more we add, the greater the problem becomes and the more each of the tracks needs to be turned down at their faders. This, then, presents the problem of each individual instrument or voice being lost in the mix and not being heard very well. Keeping track counts lower can help but it also becomes necessary to use a bus compressor to create more room. Today’s software also works in a greater bit depth which allows a better resolution for mixing, in essence, creating more increments of volume to mix to.
Secondly, each track is its own sound with its own wave energy being pushed by the gain stage. What happens when these get laid on top of one another? Having discussed phase differencing before, we can already anticipate that some frequencies will conflict with the frequencies of other tracks. A phenomenon known as frequency masking and one seen mainly where two tracks share the same part of the spectrum. This can be measured with a spectral analysis tool like Voxengo’s SPAN. By using equalization, filters and stereo separation, this effect can be reduced. Think of it like an artist who wants to lay two images together; they wouldn’t just plop them on top of each other and be done with it. The aesthetic eye would try to make them sit well with one another so they complement each other.
Finally, the act of summing raises the noise floor. Additional power is being added to the mix to make it louder and the stuff you don’t normally hear, like microphone hiss and gain stage hum, begins to get louder. An effect that’s cumulative. Meaning, the more tracks that get bounced, (mixed down), the higher these sounds become. Working with high end microphones, controlling room noise, and the use of quality preamps with lots of gain help to ensure a loud and clean signal to begin with; so the track count can get a little more flexible before the noise floor becomes intrusive to the mix. Nevertheless, Bouncing your tracks and mixing some more, only to bounce again, will inevitably lead to it becoming a problem. You should be aware that less is more in the audio world. Getting it right from the beginning means doing less harm later. Afterall, every bit of processing you do is destructive in its own way and it’s only a matter of time before things get over processed; a sound that just doesn’t sound right to the listener, (and is also easily recognized by an audio engineer.) Examples are: things being over compressed until no dynamic range is left, too much frequency boosting which leaves unwanted resonances within the Q, or too much echo from delays or reverb taking away the clarity of the tracks in the mix. All of these factors can make mixing seem a daunting task but the general rule of keeping things minimal can go a long way toward making a better mix. Don’t just be cautious; be deliberate! If someone were looking over your shoulder could you explain why you made that last move?