Reverb is the sound that reaches our ears after it has bounced off some other surface first. The characteristics of the reverb will vary based on the frequency range of the source, the position of the source, and the position of the listener.
Reverb amount and “quality” varies based on the frequencies in the source and how far our ears are from the source and surfaces sound is reflecting off.
Reverb is why we sing better in the shower. All those reflections hit our ears at different times, making our voices bigger and more luxurious than they probably are. It also helps us with pitch. Those reflections alert us to being sharp or flat and help us course correct.
In live sound, reverb can also be the enemy. It causes phase issues, unwanted echos and feedback. Because of this we try to minimize the amount of reverb that goes into our mixes so that we can control what the main audience hears.
When we used to monitor (in the old days, like, last year) with floor monitors, we got those reflections from walls, floors, ceilings, and the instruments themselves. Personal monitoring with in-ear monitors changes the equation. Now we get mixes delivered straight to our ear drums with no environmental reverb at all.
If your personal monitor system provides reverb, we recommend placing some on the master mix, just enough to give you that natural feeling but not enough to muddy up the mix we just created. Reverb is another one of those tools that we want to overuse. Our tendency is to drench everything in luscious reflections, but if not done very judiciously you can lose all that separation you worked so hard to create.
If your personal monitor system doesn’t have reverb, I recommend placing a couple mics on stage and adding ambient sound to your personal mix. Ambient sound and reverb will compete with each other when used together, so add them to your mix slowly and stop when you begin to hear them.