Some Dos & Don’ts
Reverb for Drums & Percussion
Close-miked drums have very little room ambience, unless you're working in a nice live room and are adding ambience mics into the mix. The same is true of drum machines where the samples are unprocessed. Kick drums can be problematic if you add any obvious reverb, as the resulting wash of low-frequency reverb can really muddy up the low end of a mix. Using a short ambient setting is generally better and can help make the sound more snappy and punchy. One of the reasons an ambient setting adds punch is because the extra reflections generated by the reverb process add to the dry sound, stretching it out over a longer period. Unlike conventional reverb, however, the ambient reflections die out fairly quickly, before they have a chance to confuse the sound. Gated reverb also works well on kick drums for the same reason, though you should use this effect sparingly unless you want your drum part to have that “sound”.
The snare drums are usually what define the “sound” of the overall drums, except in dance music where the PHAT Kick drives the track. You can usually get away with adding more reverb to snare drums, as they don't generate much in the way of very low-frequency components. Plate settings are commonly used, because they're flatteringly bright and have a fast attack, although you should avoid using an excessively long decay time (over 2 secs) unless the music is slow and leaves lots of space for the reverb. In fact, a good rule of thumb is that the faster and denser the track, the shorter the reverb decay time needs to be.
A typical snare reverb setting can be anything from subtle one-second decay, or even less, up to an obvious three seconds or more, but the perceived effect also depends a lot on the wet/dry balance of the sound. With very short reverb settings you can often mix in more reverb, to create a very live sound, but without allowing the mix to get messy. A common trick is to use a heavy dose of short, bright plate or tiled-room ambience to add life to a dull snare sound. For a bigger sound, pick a short hall setting or longer plate and experiment by adding a little pre-delay to give the drum hit something of a slap-back feel.
Toms tend not to need a lot of added reverb (unless you're after that overstated '80s rock sound, of course), as they have a longer natural sustain than the other drums in the kit. However, short settings can make them sound more powerful and give them a sense of place, so don't leave them completely dry.
If you're miking your hi-hats separately, again try a short ambience or plate setting to make the sound more three-dimensional. What you're after is a sense of space and high-frequency detail, without a long reverb tail.
Percussion sounds also benefit from added ambience, but long reverb on percussion tends to be used only in film/TV music or slow atmospheric music (i.e. New Age), and even then only where there's a lot of space in the composition, so that the reverb can breathe. The musical arrangement and the choice of sounds dictates the amount of space you have to play with, but in TV and film music it's not unusual to hear very spacious passages, where triangles or odd percussion instruments are treated with very long, bright reverb settings.
Traditionally, the kick drum is treated with the smallest amount of additional reverb and the snare with the largest; toms come somewhere in between. However, these days there are more opportunities to break musical rules, so let your ears decide what works. You'll need to experiment with the reverb level added to any overhead or ambience mics, as so much depends on the natural characteristics of the room.
Finally, be aware that the quality of ambience and other short reverb settings varies enormously from one reverb processor to another. So when mixing acoustic material which requires very realistic spaces, you will tend to use your best quality reverbs, however there are times when an obviously trashy reverb sound can work really well in the mix, so don't throw out all your cheap boxes.
Be aware of Reverb Modulation
If you have a reverb unit or plug-in that offers internal pitch modulation, or you want to experiment by putting chorus before a reverb, make sure the treatment is appropriate for the instrument you're working with. For example, a heavily modulated reverb can be quite flattering on strings, but will sound totally wrong on acoustic piano, where even the slightest hint of pitch modulation sounds unnatural. The same applies to solo harp, classical guitar, and so on, whereas brass and other ensemble sounds can sometimes be made to sound a little richer by the judicious application of modulation. For electric guitar (and some synthesizer sounds), heavily modulated reverbs can work nicely, because a natural sound is not a prerequisite for an electric guitar! As a rule, the more 'classical' the instrument or the production, the more you should steer away from modulation and aim instead to create a natural-sounding room ambience.
Note about Guitars
The electric guitar is often associated with a spring reverb sound, not necessarily because springs sound great, but because that's the sound we've grown up with. Many ‘Classic’ Guitar amp had a built-in spring reverb unit, which in some cases really contributed to the status of fame some have achieved (i.e. Fender Twin reverb).
Many reverb units include spring emulation, and some models do a better job at simulating the good or bad aspects of spring!
For certain styles, it's worth experimenting with different reverb types, with and without pre-delay, to see what variations you can get. For example, if you want that big, 'Floyd-y' sound, compress the guitar and then treat it with a large, bright hall reverb setting. This can be very effective in the right context, but the usual rule applies: only use this treatment where there's space for the effect to breathe.
It's also well worth trying gated and reversed treatments. Gated patches, in particular, can be useful for creating spaciousness and impact without filling in the spaces between notes, and they can provide a useful alternative to ambience settings.
Bright plate reverb patches work well with acoustic guitar if you're after that American West Coast sound, but keep the reverb time reasonably short or you could drown the sound.
Ambience
Today's music tends not to use as much “very obvious reverb” as before, but instead relies a lot on so-called ambience algorithms. These recreate the early reflections of a natural room but add little or no reverb tail, thus recreating the effect of a live room. Ambience can be used to make close-miked or synthesized sounds sit more convincingly in a mix. Because the extra reflections add to the original sounds, percussive parts treated with ambience often sound more powerful. Convolution reverbs are ideally suited for that job.
A sense of space
So far I've been talking about creating a natural sound, but in pop music natural isn't always best. For example, you can pan the dry sound hard to one side and all the reverb to the other to provide an enhanced (albeit unreal) sense of space. This works particularly well if you use a generous amount of pre-delay. Furthermore, some producers will deliberately use a mono reverb (panned to the same place in the mix as the dry sound) to provide a more focused position for the sound.
While widely panned reverb increases the general sense of spaciousness, it actually decorrelates directional information to such an extent that it's often difficult to be precise about where the sound is supposed to originate from.
For more control over your reverb sound, patch a delay line between the effects send and the reverb input. This provides more control than the pre-delay found in most reverbs; for example, adding a bit of feedback can create a more complex reverb effect.
The Bottom line
When setting up a mix, try to get the mix sounding close to right before you add any effects or signal processing. Once you've got this right, add further vocal compression if needed and also apply just enough reverb to make the vocals sit comfortably with the backing track. When you're happy with the overall timbre and balance, adding effects for 'effect' should be easier. Remember that, in most cases, effects are there just to add the final gloss, they won't compensate for a poor balance or bad basic sounds. Home demos often use far too much reverb, and the result is a cluttered mix with no space.
For all the technology we have to play with, our most valuable tools are still our ears, so I'll end as I started, by suggesting that you spend some time listening to how different styles of music have been mixed and how the most successful reverb treatments have been chosen to enhance the arrangement.