Reverb plug-ins can present a bewildering variety of parameters. We explain what they're there for, and how to put them to good use!
When we sing or play an instrument, the sound spreads out in all directions. Some of it travels directly to the listener or microphone. The rest arrives there in roundabout ways, after bouncing off one or more surfaces.
Because there are lots of possible paths from the source to the listener, we don't hear this reflected sound as a single echo. Instead, thousands of smaller echoes arrive in a short space of time. These merge to form a wash of sound which we call reverberation. Different spaces reverberate in different ways, and the human ear is very good at picking up on these differences. In recording, the quality and character of the reverberation are vital.
Especially if we're recording in a domestic house, however, natural reverberation won't sound pleasing on everything. We deal with this by placing the microphone close to the source, to capture mostly direct sound, and then adding reverberation artificially.
Most of us now do this in software, and even the most basic DAW program will come with several plug-in reverbs. But, unlike a real room, a typical reverb plug-in offers endless ways to modify the sound. So what do all those controls actually do, and how do you tailor a reverb to get what you want from it?
A Room Of One's Own
Reverb plug-ins usually don't just have one reverb algorithm. You'll often have the option to choose from a list that might include one or more rooms, halls, plates, ambiences and so on. As the names suggest, these are optimised for recreating particular types of reverb. Choosing a new algorithm is often the most important single thing you can do to vary the sound of the plug-in. Algorithms such as room, hall and church are fairly self-explanatory, but a few other common types are worth explaining:
- Plate recreates the sound of electro-mechanical reverbs such as the classic EMT 140. These behave rather differently from real spaces, and have a characteristic dense, 'sizzly' sound.
- Ambience is a term usually reserved for very short reverbs. In a mix, these are often audible less for their spaciousness and more for their effect on the tone of the source. A bright ambience can add the sort of sparkle you might otherwise get using treble boost or an aural enhancer, while a darker one can thicken up drums and other sources.
- Non-linear reverb algorithms are used mainly on drums, and are designed to be inherently unnatural. They reverse the usual dynamic envelope, yielding a reverb that builds up slowly and then suddenly cuts off.
The same controls are usually available in all the algorithms, but they may have different ranges depending on which algorithm you've chosen. For example, the range of decay times available for a hall or church setting might run from 2 to 10 seconds, while an ambience algorithm might run from 100ms to 1 second.
What About Convolution?
Plug-in reverbs use one of two technologies. In a convolution reverb, a gunshot or other impulse is fired into a real space and the results are recorded as an 'impulse response', which can then be imposed onto any source. In an algorithmic reverb, the way in which individual reflections bounce around and recombine is simulated in real time. Convolution reverbs tend to be quite simple to use, as the main thing you can do to change the sound is load in a different impulse response. In this article, therefore, I'm going to focus on algorithmic reverbs.
Pre Possessing
Before the reverb algorithm gets to work, though, we encounter the all-important pre-delay control. This introduces a gap to separate the dry sound from the reverb that follows it. With no pre-delay, source and reverb merge together. With a long pre-delay, the source sound retains its dry character and sits in front of the reverb.
For obvious reasons, long pre-delays work better with fairly expansive reverbs than with tight ambiences. A short or non-existent pre-delay is often perfect for rock drums, where the decay of a snare hit blends with the added reverb to create something bigger and meatier. On other sources, it can help to bed things into the mix, so can be ideal for background guitar or keyboard parts. Long pre-delays, by contrast, are particularly useful on vocals and lead lines, where you want to add lushness and thickness without detracting from the immediacy of the part. Long pre-delays also help to preserve the intelligibility of vocals and intricate solo parts. Don't be afraid to dial in 100ms or more if it works, and don't overlook the option to make the pre-delay time a musically significant interval such as an eighth or 16th note.
Nearly all reverb plug-ins offer pre-delay as a parameter, but there are times where you might want to set it up another way. Back in the day, engineers would implement pre-delay by feeding the signal through a tape machine in record mode, varying the tape speed to change the delay time; this is easily recreated using a tape echo plug-in, for a more retro sound. You might also want different sources to have different pre-delays, even if they're feeding the same reverb. When you record an orchestra, the first violins will typically be nearer the 'listener' than, say, the timpani at the back of the stage. To mimic this positioning with dry sampled instruments, you might want to use a long pre-delay for the strings, a shorter one for the woodwinds and so on. This is best done by setting up several different busses with short delays on them, and routing all of them to the same reverb bus.
Imagine standing in the middle of a hall. A long pre-delay makes it feel as though the sound source is next to you in the middle of the hall. With no pre-delay, by contrast, it feels as though the source is far away on the edge of the hall.
Early Doors
Perfectly simulating the behaviour of sound in real spaces would require immense processing power, so algorithmic reverbs don't model the behaviour of every single reflection. Typically, they use individual delays to simulate the most prominent ones, which reach the listener first. The later, secondary reflections that form the reverb 'tail' are then approximated, for example by feeding the initial reflections into further recirculating delays. Consequently, many plug-in reverbs offer separate controls for the early reflections and the reverb tail.
In some plug-ins, the only thing you can do to the early reflections is adjust their level. Even so, this opens up plenty of possibilities. If your reverb allows it, try feeding it an acoustic guitar track and muting the reverb tail altogether, to leave only the early reflections. What you'll hear might sound more like EQ or treble enhancement than reverberation as such: heard on their own, the early reflections arrive so quickly that the ear can't separate them from the source. It's a virtual counterpart of the old recording engineer's trick of placing a sheet of hard material on the floor in front of the guitarist to add liveliness and sparkle. A short, sharp dose of early reflections can be equally effective on lifeless drum overheads, boring digital pianos, dull-sounding strings and so on.
More advanced plug-ins might give you control over the spacing and tone of the early reflections. In the real world, these are important psychoacoustic cues that help to tell us about the shape and size of the room we're in. For instance, in a tiled bathroom, all the early reflections will reach us at nearly the same time, and they'll all be relatively bright. In a larger space, the early reflections will be more spread out, and may be darker, owing to the high-frequency absorption of the air through which they've travelled. An empty square room will produce a handful of very strong early reflections, while a cluttered, oddly shaped space will generate many more, weaker ones.
In rock and pop mixing, realism is less important than whether your choices work for the track. Dense, bright early reflections can be great for snare drums or overheads, but on a vocal, they might exaggerate consonants and sibilants. Having lots of early reflections spaced quite evenly makes for a smooth, rich effect that benefits many sources, but the opposite can work well on instruments such as electric guitars. If you want a reverb to work 'behind' the source, or if you're trying to imitate a spring or plate sound, you could even try muting the early reflections altogether.
Tone & Time
When it comes to the main part of the reverb, there is endless variation in the controls offered by different plug-ins. This means not everything I'm about to say will be universally applicable, but hopefully it should be possible to translate it to your reverb of choice! Broadly speaking, we can divide the controls into two categories: those that determine the timbre or character of the reverb, and time-related controls that adjust the duration and 'shape' of the reverb decay. These often interact and, as I've already mentioned, will be operating within limits that are set by the chosen algorithm.
Reverberation consists of thousands of reflections, all arriving within a short space of time so that they end up superimposed on one another. We don't hear these as individual echoes, but we can certainly detect variations in their overall pattern. As we've seen, some reverbs allow the spacing of the early reflections to be varied, and nearly all of them provide a similar sort of control over the reflections that make up the main body of the sound. The key parameter here is diffusion, sometimes known as density.
In an empty room, individual reflections from each wall remain relatively coherent as they bounce around the room, so what we hear is more like a series of distinct echoes. In a cluttered space, by contrast, reflections are quickly broken up as they encounter other objects and surfaces, and the pattern becomes much more chaotic. Diffusion simulates this behaviour. At its lowest settings, the reverb tail will sound grainy, thin and perhaps even metallic; you may even be able to detect 'flutter' on transients. At maximum diffusion, by contrast, the tail will be dense, smooth and lush.
You might think that one of those extremes sounds a lot more appealing than the other, and in a sense you'd be right. However, context is everything, and not every source benefits from the thickest possible reverb. By all means crank up the diffusion on a snare drum and enjoy the beefiness and substance that it adds, but don't neglect the possibilities that lurk at the other end of the dial. A thinner, less diffuse reverb takes up less space in the mix, and often has a more individual character; electric guitars in particular often seem to benefit from reverbs that might sound grainy and coarse on other sources.
Size Isn't Everything
Turning next to the fourth dimension, let's consider some of the most important controls that affect the length of the reverb, and how you can use them to your advantage. If you're familiar with basic synth programming, you'll have come across the concept of an envelope. In its most basic form, the envelope of a reverb has two sections: an attack phase during which it builds up to maximum amplitude, and a decay phase during which it drops away to silence. The attack phase is usually short or even instantaneous, while the decay time can be as little as half a second in a small, dead space, or more than 10 seconds in a huge cathedral.
Once again, the most important factor governing the envelope of the reverb is the choice of algorithm. In a non-linear algorithm, for example, the usual envelope is reversed to give a long attack and short decay. Within each algorithm, though, you'll usually have the option to specify both decay time and room size. These work together to determine how long the decay phase of the reverberation lasts, again within limits set by the algorithm choice.
There might be many different combinations of decay time and room size that yield a two-second reverb, but the character and 'shape' of that reverb will be very different. A smaller room brings the listener closer to the action and makes things feel lively and energetic, which can be ideal if rock & roll excitement is what you're looking for. A larger space has the potential to sound rich and expansive. Setting up a large hall for a two-second decay will give you something that sinks into a mix and might help to bed multiple sources together; extending a bathroom reverb to two seconds will produce something that draws attention to itself, and which might be more useful as a special effect on one source only. Some reverbs give you control over the shape of the virtual space as well as its size, and there's a considerable difference between the sound of a cubic room and a corridor of equivalent volume!
So how long should your reverb be? Almost as long as the proverbial piece of string! There are no absolutes, but in general, faster or busier tracks and parts leave less space for long reverbs than down-tempo, sparse pieces. If you're applying more than a smidgeon of reverb to anything rhythmic or repetitive, you'd usually want to keep it short enough that each hit's reverb dies away before the next one comes around. In a punk rock track, you might stick to a tight drum room or ambience on the snare and toms, and a slapback delay on the vocals; in a lush ballad, you might have several different three- or four-second reverbs layered on the vocal alone.
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.
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
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.
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.
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.
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!