What is stereo reverb
"Stereo reverb" may mean different things:
- mono in, stereo out
- stereo in, stereo out
The latter category is further split between "true stereo" and "dual mono" categories, and maybe something in between. In dual mono the left and right inputs have completely independent and separate processing, and sound doesn't "leak" between the extremes, which is a very unnatural situation. In a real room you can't prevent sound from echoing from wall to wall, even if you place the sound source to the furthest location on either side. In "true stereo", a stereo signal is kept as an actual pair and placed in the device's internal stereo field or virtual room or whatever, as a pair. Not left and right separately to their own independent mono reverberators. How "true" it really is, is somewhat debatable, because it's an artificial effect anyway.
The send effect buses in some DAW applications and mixing boards are mono, so if you use such a send for reverb, your signal is first summed to mono before being sent to the reverb. Or the reverb effect itself may sum its two inputs to mono before processing. That means that even if there was actual stereophonic information in the source signal, for example if it was recorded with a stereo mic, it is narrowed down to mono before the reverb gets it. This may or may not be OK, depending on if you like the sound and if the signal is "mono compatible", i.e. can be summed to mono without causing too much phase canceling. You should have your signals reasonably mono compatible anyway. What is too much, is up to your discretion of course. If you want to have a "true stereo" reverb, use it as an insert effect in a stereo track, if you can't set up a stereo bus.
Even if an effect is stereo in, stereo out, it may not be "true stereo" in everyone's opinion, if it's not properly placing the left and right inputs separately in its internal stereo field, keeping the input's own stereo image intact. The reverb may even do an internal mono sum to its inputs, or a phase swap for one channel, or anything. It may or may not be suitable for the type of sound you want. Search for "true stereo" if you're interested.
When to use stereo reverb
Use stereo reverb whenever you think it's good for the genre and the sound. Or simply if you like the way it sounds. A really wide stereo field sounds nice to me, but YMMV.
It's worth noting that (mono-in stereo-out) stereo reverb is one way to artificially create a stereo field from a mono input. Other stereoization methods include comb filtering with a delay on one of the stereo channels, complementary comb-shaped EQ curves for left/right, and special stereo chorus effects.
When to use mono reverb
Use mono, as in "mono out", if you like its sound for the genre. For example if you're after a specific retro/lo-fi sound like a reverb through a guitar amp or lo-fi reggae sort of sound. The method of operation of the reverb is also important, and might be more important than if it's mono or stereo. For example, spring or plate reverbs have their own sound. And a spring reverb cannot really even be "stereo" because of how it's constructed, maybe dual mono, but not real stereo.
I think you'll find out by listening if you like your reverb in stereo or mono. Either way it's your own sound you're creating.
Things to watch out for when using reverbs
You can spoil your mix and make it muddy and unclear with too much reverb, and it doesn't matter if it's a mono or stereo reverb. Both kinds of reverb can be used wrong.
- How to detect: Use your ears. Do you really need that much reverb? Listen to the mix with different loudspeakers etc.
- Hot to fix: (1) Just send less to the effect. (2) EQ out the lows from your send reverb effect, keeping low bass only in the dry signal. (3) Side-chain compress/duck the reverb with e.g. the kick, snare or guitar, so there's lots of reverb only at the quiet spots, and you can have both punchy drums and/or guitars and a very thick reverb effect without the mud.
Things to watch out for with all stereo things
You can spoil your mix with bad stereoization by making it mono-incompatible, or just smearing the wrong frequencies in a bad way. It doesn't matter if the bad stereo field comes from stereo reverb or other methods.
- How to detect: (1) Use your ears, (2) check mono compatibility by summing to mono and listening in mono, (3) check mono compatibility with a stereo correlation meter (if all else fails and you have to mix with your ears closed or something).
- How to fix: (1) Use less of the stereo effect, (2) use a better pseudo-stereoization method that's actually mono compatible and adjustable, (3) split the stereo effect to dry and wet, and EQ and process the wet signal, (4) use M/S i.e. mid/side processing, and cut down the Side signal from the frequencies it's hurting. For example, the bass.