Out-of-phase sources
A stereo audio signal with identical frequency and volume information in the left and right channels, while still technically stereo, will be perceived as mono. Take the left channel, alter its time in relation to the right, and the signal will become out of phase, and therefore stereo – you’ve created a phase difference between L and R. A common way to do this is using the Haas effect: delay the left channel later in time by a few milliseconds, and a powerful widening effect is created. This is a popular trick for ‘stereoisation’, but often comes at a price: sum the resulting signal to mono, and the mid signal will feature obvious phase smearing and frequency cancellation. Mix this delay effect in parallel and perform a mono check to balance mono compatibility and width.
We often work with multisampled sound libraries (pianos, strings, etc) and ready-rolled softsynth presets. For obvious reasons, these are designed to impress you straight out of the box – and one guaranteed way sound designers grab the listener’s attention is by giving these sounds super-wide stereo information. This seductive width can become addictive when browsing through patches, but will harm your mix in the long run – one surefire way to expose a rookie production is to sum the master bus to mono, only to hear these amazing sounds collapse into a blurry mess!
Tread especially carefully when using these sounds: watch out for out-of-phase synth patches with loads of spread-out unison voices, chorus or other widening effects; or mega-phat piano patches with Haas delay applied. Overtly stereo signals will phase-cancel when folded to mono, weakening your mix.
If you want to sum the sound down and use it as a mono signal, try soloing the left or right channel, rather than simply mixing down to 0% mono. One side (either L or R) will almost always sound more solid and consistent, compared to the blurred, out-of-phase mono sum of L and R.