An Absorbing Occupation
After the basic algorithm, the most important factor affecting the tone of a reverb is absorption. Again, this simulates a property of real acoustic spaces. When sound hits something solid, only some of its energy is reflected. The rest is absorbed by whatever it's hit. Some materials absorb sound much more effectively than others, and this is frequency-dependent: a velvet curtain will absorb high frequencies more effectively than a thin pane of glass, but neither will soak up much low bass. In the real world, absorption is also dependent on the texture of the surface and the angle at which sound strikes it.
For a small space to have a two-second decay time, individual reflections would have to bounce around inside it many times without their energy being lost. This is only possible if the entire room is covered in hard, reflective material. In a large hall, by contrast, each path from wall to wall is much longer, so if the reflections were to bounce many times, the overall reverb would be much longer. The decay time can only be as low as two seconds if a significant amount of energy is lost each time sound hits a surface.
It would take a supercomputer to perfectly simulate the way in which sound is absorbed at each point in a furnished room, so reverb plug-ins typically use a simpler approximation. This works on the fairly universal principle that high frequencies are always absorbed more easily than low frequencies. The key question is, by how much? In a theatre filled with heavy drapes and soft seating, sound will lose a lot of its high-frequency content the first time it's reflected. In a tiled bathroom, it can bounce around many times before the same amount of top end is lost. At its simplest, a reverb plug-in can mimic this variation by placing an adjustable low-pass filter in an internal feedback path so that each time the reflections are recirculated, they get progressively darker.
Valhalla's VintageVerb allows high and low-frequency damping to be adjusted using a simple EQ-like interface.
This is an area where both the implementation and the controls on offer can be quite variable. Sometimes absorption is adjusted using an EQ-like interface, and indeed, EQ can be an equally effective way of shaping the character of a reverb (see box). In other plug-ins, you may see dials for adjusting the amount of absorption (or damping as it's often known) and the frequency above which this should take place.
Super-dark and sizzling bright reverbs, and everything in between, all have their place in mixing. Setting the controls for the least possible high-frequency absorption can seem exciting, in the same way as exaggerated high-frequency EQ boosts do; but things like vocal sibilance, hi-hat spill and finger squeaks on acoustic guitar quickly become intolerable if you're not careful. The key is to set things up so as retain a sense of liveliness without drawing undue attention to these transient nasties. For example, sibilant energy typically lives at 5kHz or above for male vocalists, and perhaps 7kHz or above for female singers; there's still plenty of brightness to be had in the 4kHz region if you want it. Unnaturally bright reverbs are nevertheless widely used on pop vocals, but you'll need to do a lot of work with de-essing or automation to make them sound as good as the pros do.
The more top end you roll off a reverb, the more 'vintage' it sounds, and the easier it is to fit it into the mix at a prominent level. Take care, though, to ensure that you're not just reducing it to ill-defined mud, and don't be afraid to cut the lows as well as the highs.
Whether or not our chosen settings are lifelike is neither here nor there. What matters is whether they're effective.
Modulation
Modulation can be used with surprising abandon on many sources. Lexicon's means of applying it are via the Spin and Wander parameters.Algorithmic reverb plug-ins are only approximations to the real thing, and even those that model the early reflections accurately take shortcuts with the tail. These compromises can lead to tails that sound static and lifeless, or which have an audible repetitive element. To eliminate or disguise this, many reverb plug-ins use modulation to reintroduce a sense of motion and complexity. This is usually, at root, a kind of chorus or ensemble effect, but again, the nomenclature varies: the key modulation parameters in the classic Lexicon reverbs are named Spin and Wander.
Used tastefully, these effects can indeed make an otherwise plain reverb seem richer and more engaging; but the real beauty of them is that they don't have to be used tastefully. As with EQ and other effects, it's amazing how much chorus you can add to reverb before things start to sound seasick! Just be careful on instruments such as piano, which have no natural pitch modulation of their own. If you want to get really experimental, try adding other modulation plug-ins such as flangers, phasers and pitch-shifters, either before or after the reverb.
Wide Or Narrow?
Depending on your reverb plug-in of choice, you may be faced with many other secondary parameters that I've not had space to cover in this article. These are usually used to fine-tune the settings we've already discussed: for example, your room-size parameter may be augmented by control over the shape of the virtual space, or you might get a choice of virtual wall materials with different absorption characteristics. Understand the basics, and it should be apparent how these fit in. I'm going to conclude by looking at one more global setting that might be less obvious than you think.
If we ignore the world of surround sound, plug-in reverbs can accept either a mono or a stereo input. The vast majority nowadays are 'true stereo', meaning they take a stereo input and spit out stereo reverb that reflects the position of any dry sources within the stereo field. If you pan a vocal hard left and route it into a true stereo reverb, the reverb will appear to emanate from the left and spread out into the virtual space. This contrasts with a mono-to-stereo plug-in, where the reverb from every source originates in the centre.
Reverbs often allow you to adjust stereo width, as here on FabFilter's Pro‑R. Don't be tempted to always turn it up, however: narrow or even mono reverbs can sometimes be more effective than wide ones.
Stereo reverbs usually have a control to adjust the apparent width of the output. It's tempting to crank this all the way to the top and enjoy the expansive, enveloping soundfield that results, but there are times when this might not be the best option. For one thing, the range of this control can be quite variable between different plug-ins. Some allow you to push it well into 'outside the speakers' territory, which can bring disappointment when your mix is heard in mono. At the risk of stating the obvious, a wide stereo reverb also takes up lots of space in the mix, and can start to feel detached from the source, or even to overshadow it. Sometimes, turning down the width can make everything gel a bit better. Finally, don't neglect the possibilities offered by mono reverbs. Panned with the source, a mono reverb seems to become part of that source; panned the other side, it can create an interesting stereo panorama out of recordings that are intrinsically mono.
Better Than The Real Thing
If you're not confident with setting up a complex reverb plug-in, an obvious place to start is with the presets. Any reverb worth its salt will ship with a wide selection, and these are sometimes categorised by application, so you have an idea which ones might work for drums, guitars, vocals and so on. More often, though, they'll be named after the type of space they emulate, and you'll find lots of variations on 'drum room', 'studio', 'corridor', 'nightclub', 'church' and so on. I'll end this article by urging you not to take too much notice of these names.
In rock and pop mixing, we're not trying to convince the listener that what they hear was recorded in the Albert Hall or on the top deck of a bus. Rather, we're using artificial reverb almost like an abstract effect. Whether or not our chosen settings are lifelike is neither here nor there. What matters is whether they're effective.
Artificial reverb can enhance a sound, making things come across as bigger, wider, livelier, weightier, deeper, richer, brighter or darker. It can help things stand out from the mix, or bed them into the mix. It can fundamentally change them, or it can bring out the essential qualities that were already there. The potential is limitless, so don't limit yourself by thinking your reverbs have to sound like real spaces.
The Sends Of Time
Whether you're using hardware processors or software plug-ins, there are two ways of plumbing them into your mix. One is to use plug-ins as inserts on the source channel. In this approach, there's no splitting or recombining of signals: you're simply using a plug-in to modify a single recorded track. This is the normal way of applying EQ, compression and any other process that's designed to alter an input signal rather than add something to it.
Reverbs are usually used as 'send effects', rather than as simple channel inserts on individual sources. The latter approach has its place, but you'll need a reverb with a wet/dry control like SoundToys' Little Plate.
The other approach is to place a plug-in on a separate mixer channel, variously known as an 'auxiliary input', 'FX track' or similar. You then add an auxiliary send on a source track's mixer channel, so that source is feeding the reverb auxiliary as well as its original destination. Whereas insert processing is transformative, auxiliary effects are additive: the source track is unchanged, and the effect is generated alongside it.
There's nothing to stop you simply inserting a reverb across your vocal or snare track, as long as the plug-in has a wet/dry mix control. In general, though, it's good practice to use reverbs as auxiliary effects. There are several reasons for this, the most important being that you can send multiple sources to the same reverb, but in different amounts, by varying the level of the auxiliary sends on the source tracks. In an aux send configuration, your reverb should be set to 100-percent wet.
Note that if you have both the aux send and the fader on the auxiliary channel close to zero (unity), you'll nearly always hear far more reverb than you want. I prefer to deal with this by pulling down the fader on the aux channel, rather than by pulling all the sends down to -30 where their resolution tends to be rather coarse. Sending at full level to the aux channel is also important if you're using any non-linear processes such as dynamics in front of the reverb.
Usually, the output of the auxiliary channel would be feeding the main mix bus, but there are times when you might want to route it somewhere else. For instance, if you like to apply compression to your drum subgroup, you might want to route the snare and tom reverb to that same subgroup so that they get compressed along with the dry snare and toms.
EQ On The Way In
A very powerful tool in our mixing toolbox is the ability to condition the signal before it hits the reverb algorithm. The importance of EQ in setting up a reverb can't be underestimated, and I think it's fair to say that this is an absolutely routine practice for most serious mix engineers. It's also fair to say that you can and often should use much more dramatic EQ settings in the reverb path than you ever would on the dry source! The tonality of a reverb is never 'flat' in any case, so don't be scared to sculpt it to your tastes.
Applying EQ before and after a reverb processor can be extremely useful in shaping its sound. Some reverb plug-ins, such as this one from Eventide, even have a comprehensive EQ section built in for the purpose.
Many plug-in reverbs have EQ built in, and the more sophisticated offer two EQ stages: one applying to the dry signal before it hits the reverb, and one to the output from the reverb. It's worth playing around to get a feel for how the same settings affect the sound differently in each case. If your reverb doesn't have EQ, or if you need more control than is available there, just insert your favourite EQ plug-in before or after it in the chain.
One obvious reason to EQ reverbs, especially on the way in, is to remove unwanted low and low-mid content. In any reverb that's vaguely lifelike, low frequencies have the longest decay time, so it doesn't take much 100Hz in a vocal or acoustic guitar to introduce a cloud of woofy bottom end. This usually contributes nothing useful, and makes the whole mix sound muddy. If it works, don't be scared to roll off everything below 200Hz or even higher in a pop or rock mix.
Another application for EQ is to deal with problem resonances. Snare drums, for example, often 'ring' at one or more frequencies in the mid-range. This might not be a bad thing in the dry sound, but can become exaggerated when reverb is applied, so it might be desirable to notch these frequencies out in the reverb path.
EQ can provide a quick and effective alternative to other reverb parameters that affect the tonality. Rather than juggle several different controls relating to high-frequency absorption in order to create a dark reverb, you can simply roll off all the top end on the input. But EQ can also be used to make tonal changes that aren't easily achieved using core reverb parameters. For instance, powerful female vocalists often generate a lot of energy in the 2kHz region. If that frequency balance is reflected in the vocal reverb, it will quickly become harsh and overpowering. You might find that scooping some mid-range out of the reverb path will create a more pleasing reverb that sits more effectively behind the dry vocal.
Quite often, these sorts of problems call for something more sophisticated than a simple EQ. A bright reverb on a pop vocal can add real excitement to the track, but tends to exaggerate sibilants and fricatives to an unpleasant and distracting degree. If it does, try aggressively de‑essing the input to the reverb, or using dynamic EQ to tame any frequencies that are sticking out. Once again, you can go in much harder with the processing here than you can on the dry track!