As live mix engineers, the audio reinforcement systems we operate typically fall into two categories: mono or stereo.
Yes, there are the occasional opportunities to mix surround sound, and for many events, delay clusters or various fill loudspeakers are common, but for the most part it’s all about some version of mono or stereo.
While on the surface it may seem that stereo offers just a version of dual mono, there is a lot more to stereo than just two simple channels.
Stereo offers the significant advantage of allowing a perceptual horizontal source placement of the various signals. The primary challenge with mixing in stereo is avoiding situations where people on one side of the venue do not hear instruments that are panned to the other side.
To fully realize the benefits of stereo systems we need to look a bit deeper into some of the less obvious aspects of sound and human perception. Our ears not only recognize the volume and tonality of the energy that enters our ears, we also clearly perceive the direction from which the audio is radiating.
I like to think of a mix in multiple dimensions. Left to right is the X axis, near to far is the Z axis, up to down is the Y axis, plus bright to dull and loud to soft, and also, there are timing factors that can be added to further modify or perception of what is being reproduced. All of these aspects give us a diverse palette from which to add more character and clarity to the sonic experience we offer.
One of the primary challenges faced when mixing together a multitude of sources is presenting the audio such that the listener can discern each individual source, yet the sources all blend together in a cohesive and complementary manner.
Purely turning up an input signal increases audibility while simultaneously masking the audibility of other signals. It becomes quite tempting and common to constantly cycle through turning things up until the system limitations are reached. But by using a careful and a well thought-out strategy, significant improvements in clarity can be realized.
Everything Piles Up
So let’s start with what not to do. If all instruments and vocals are sent at equal volumes to both the left and right loudspeakers, everything piles up in the perceptual center.
Additionally, if both left and right loudspeakers are reproducing identical signals, there is a maximum amount of comb-filtering issues occurring everywhere except dead center. Comb-filtering is frequency dependant cancellations and summations that are most pronounced when two identical signals combine with a relative time offset between them.
To reduce audible comb-filtering, we either need to run a single mono cluster, have everyone in the room stand exactly equidistant between the left and right loudspeakers, or minimize the tonal and timing similarities between the signals reproduced by the both the left and right side of the system.