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All About Reverb


Despite the introduction of complex multi-effects processors, digital reverberation is still the most important effect in modern music. Paul White explains why.

Ever since early man moved from the open plains to the safety of the cave, he has been aware of reverberation as part of his natural environment. Indeed, reverberant or reflected sound is a vital part of the human survival mechanism, in that it allows us to make certain deductions about our surroundings based on sound alone. Just as our visual world is inhabited by light and shade, colour and shadow, our aural world consists of both direct and reflected sound, and though the sound patterns undoubtedly changed as we moved from caves to something more comfortable with central heating and a satellite dish, the importance of reflected sound has, if anything, increased. Unlike the cave dweller, we encounter numerous acoustic environments which are radically different, from the tiled washroom at the local pub to the concert hall or cathedral.

Sound without reverberation is rather like a picture with no colour and no shadow — stark and unnatural. For psychological reasons which are only partly understood, the appreciation of music is greatly enhanced by the addition of suitable reverberation — one theory is that it causes the notes to sustain, which makes it easier for the brain to compare new notes with their immediate predecessors. In some ways, it's like primitive music which uses a continuous drone to act as an anchor to the more mobile parts of the music. Because natural reverberation is so much a part of our everyday lives, it stands to reason that any artificial substitute must be quite sophisticated if it is to be accepted as natural. In practice this tends to be so, though our perception of reverberation in music is already showing signs of being influenced by the electronic simulations that have been available over the past decade. In other words, when we evaluate a reverb processor, we don't so much compare it with the real world as with the simulations that have gone before.

STATE OF THE ART



Digital reverberation systems have now been developed to such a degree that it can be difficult to differentiate between the electronic imitation and the real thing. If all the parameters available on a modern digital unit are to be used creatively, it helps to have some understanding of the basic mechanism of natural reverb.

In mathematical terms, reverberation is no trivial phenomena — the reflection patterns rapidly increase in complexity as they build up — but if we examine what happens following a single sound-impulse in a reflective space (such as a room or hall), the general process is straightforward enough to follow. Many researchers use a simple bursting balloon to provide a short, high-intensity source of sound. If heard in an anechoic chamber (a specially-constructed, acoustically 'dead' room with little or no natural reverberation), this would be perceived as a single, short crack, but the same sound in a concert hall would radiate outwards in all directions until it encountered a solid surface, at which time some of the energy would be absorbed and some reflected back into the room.

In practice, there would be many such reflections generated as different parts of the sound wave encountered the different parts of the walls, floor and ceiling of the room, as well as any hard objects within it. It is the sound from these multiple reflections that create the effect we know as reverberation.

Such reflections reach a listener in the same room as a series of very closely spaced reflections, though their spectral content will be different to that of the original sound because different materials absorb different parts of the audio spectrum more efficiently than others. Soft furnishings absorb high frequencies very efficiently while panelled walls may reflect the high end but absorb at the bass end of the spectrum. It is also important to note that these 'early reflections', as they are called, are not clean repeats of the original sound but are 'smeared' by diffraction due to irregular surface features. In budget reverb units, the early reflections often sound unduly sharp because this smearing effect is not properly simulated.

Very soon the early reflections encounter the room boundaries and other obstacles, once again resulting in further reflections. Every time a sound is re-reflected in this way, its spectral content is further modified and its energy diminished. As energy is absorbed on every reflection, the total amount of sound energy decreases, which means that the level of reverberation dies away, and as it does so, the complexity of the reflection pattern builds up rapidly. Very soon, a point is reached where no individual reflections or echoes can be picked out from the overall wash of reverberation.

STEREO REVERB



Humans and most animals are equipped with two ears in order to provide stereophonic hearing. Again, this is part of nature's survival mechanism, as it enables us to tell from which direction the predators are coming. Reverberant reflection patterns coming from two different directions can be assimilated by our stereo hearing mechanism, enabling us to perceive spaciousness. Electronic reverberators generate different reflection patterns for the left and right audio channels in order to synthesise the spatial quality of a real, three-dimensional environment, most working on a mono-in, stereo-out principle. If mono in, stereo out seems to be a compromise, it isn't really. Most real life sounds can be considered as mono point sources, and it's only the reflected sound that gives them a true stereo identity.

DIGITAL REVERBERATION



A digital reverberator works by using computer technology to generate a simplified model of the reflective properties of a real room using a network of short delays and filters. The first important parameter is a short delay to simulate the time delay between the original sound source and its first reflection returning to the listener. Because the speed of sound is constant, longer delays tend to be used to simulate larger spaces. This so-called 'pre-delay' is built into most digital reverberation systems, and in a programmable unit may be adjusted by the user.

Early reflections are most often simulated by a multi-tapped delay line (also known as a Finite Impulse Response or FIR filter) which has random tap spacings. Different patterns of early reflections are used to imitate different types and sizes of rooms; the longer the spacing between the individual reflections, the greater the impression of space. In a simple reverberator, these early reflections are taken directly from the FIR taps and so exhibit none of the diffraction smearing found in a real room.

There is a potential problem in trying too hard to simulate early reflections. In a real room, the early reflection pattern created by each instrument in a band or orchestra will be quite different because they are all located in slightly different positions relative to the walls of the room. If a digital reverberator is used to process, say, an orchestral recording and a high level of early reflections is used, the final illusion will be of all the performers occupying the same point in space. In a real-life situation, the individual early-reflection patterns for all the various instruments tend to merge so as to render the individual reflections less obvious. For this reason, when attempting to create a natural room or hall sound, it may be better to use a low level of early reflections.



"If all the parameters available on a modern digital unit are to be used creatively, it helps to have some understanding of the basic mechanism of natural reverb."


In order to simulate the further build-up in complexity of the reverberation pattern, most digital reverberators use a combination of comb and all-pass filters to create multiple feedback paths which serve to regenerate the early reflections into something much more dense. Filtering is also used to simulate the way in which real reverberation becomes less bright as it decays. In electronic systems, this parameter is given the name 'high frequency damping' and the user may vary this to create different environments.

Of obvious significance is reverb decay time which, once again, tends to be longer in larger spaces. In real life, this parameter is governed both by room size and by the absorbent characteristics of the room boundaries and contents. The rate at which the reverberant field builds up and the shape of the decay curve vary from one type of room or space to another. In a real room or hall, especially one designed for music, all the frequencies die away fairly evenly with none hanging on for significantly longer than the others. There may be a difference between the high frequency decay rate and the low frequency decay rate, but the transition should be smooth. Unfortunately, a smooth decay is difficult to simulate, and less well-designed reverberators tend to exhibit a metallic, ringing tone, as some frequencies hang on after the rest have decayed. This is less of a problem now than it was on earlier units, but it is still something that has to be kept in mind when comparing reverb units from different design stables.

The only reverb settings that should have an audible ring are plate and very small room reverb simulations. Because of the mechanical nature of the early plate reverbs (which relied, literally, on a suspended plate of metal), a degree of coloration was part of their sound and, though technically imperfect, it became popular, especially for use with drums and vocals. This bears out what I said earlier about the perception of reverb in music being affected by earlier simulations — every digital reverb unit provides a plate simulation!

KEY REVERB PARAMETERS



From the previous analyses, it would seem that the main parameters used in simulating natural reverb are:

Pre-delay
Early-reflections
Overall decay time
High frequency damping

By adjusting these parameters, and by varying the level of the early reflections as well as the balance of direct and reverberant sound, we can simulate not only various room types but also, to some extent, the subjective position of the listener within that room. Figure 1 shows how the reflections from a single impulse build up to form reverberation in a typical hall. Figure 2 shows a block diagram of a typical digital reverberation unit — this is not based on any model in particular but is shown to illustrate basic principles.

Figure 1: How reflections from a single impulse build up to form reverberation in a typical hall.


Other parameters which might be presented on a more sophisticated unit include the density of the reverberation and the rate at which the density builds up. Some Lexicon reverbs also include Spin and Wander parameters which introduce random time delays into the system by continually varying the tap spacing. This helps to keep the reverb decay smooth and even.

UNNATURAL DECAYS



Virtually all modern digital reverbs, whether programmable or preset, will offer a selection of room types and sizes as well as plate simulations. In addition, most will offer so-called gated and reverse settings. These have no natural counterparts; they simply emulate artificial effects that were originally created by other means. Gated reverb mimics the effect of feeding the output of a reverberator or live room into a noise gate which is set to truncate the natural decay curve of the reverberation. This was originally achieved by using either natural room ambience or the output from a plate reverb, often heavily compressed, which was then fed through a noise gate set with half a second or so of hold time and a very fast decay. The outcome is a burst of reverb of almost constant level which stops abruptly when the gate closes. A derivation of this effect is reverse reverb; a burst of reverb is created with a slow-attack/fast-decay envelope imposed on it. This is reminiscent of a sound being played in reverse, and though the effect is pure illusion, it can be very effective.

Figure 2: Block diagram of a typical digital reverb unit.


PERSPECTIVE



Artificial reverberation is an essential part of making pop music, as many of the sound sources we use these days are either synthesised or recorded in a relatively dead acoustic environment. It is common practice to use several different reverb settings within one mix with, say, a short plate setting on the drums and a longer, smoother setting on the vocals.

The skill comes in choosing the right reverb treatments for the individual elements within a mix and then applying these without the effect swamping the original sounds. Music is about contrast, and there's a fine line between adding enough reverb to make something attractive and filling up all the vital spaces with reverb. One general rule is to avoid adding reverb to bass instruments or kick drums; if you feel that some is necessary, try a short setting.

In real life, the most reverberant sounds tend to be those heard at a distance, with closer sounds being more direct. You can use this knowledge in your mix to create a degree of front-to-back perspective by using longer reverb times on the sounds that are supposed to be in the background. Adding too much reverb to vocals tends to push them back into the mix, though increasing the pre-delay time can help to maintain an up-front sound by separating the reverb from the original sound. Most well mixed records make use of less reverb than you might think, and where it is used in a more obvious way, the arrangement usually leaves room for it. Indeed, it is worth spending some time listening to your record collection specifically to hear how reverb is being used. Try to hear what lengths of reverb are being used on the different instruments and whether they are bright-sounding or warm. Listen specifically to the vocal and drum reverbs, and if the music feels particularly spacious, see if the way in which the reverb is being used contributes to this feel. As you get more experienced, you may even be able to pick out the type of reverb unit that's being used; some models have a very specific character, such as the Lexicon units used in the majority of top studios.

WHY DO SOME DIGITAL REVERBS SOUND METALLIC?

In real life, reverb decay lime is governed both by room size and by the absorbent characteristics of the room boundaries and contents. The rate at which the reverberant field builds up and the shape of the decay curve vary from one type of room or space to another. In a real room or hall, especially one designed for music, all the frequencies die away fairly evenly with none hanging on for significantly longer than the others. There may be a difference between the high frequency decay rate and the low frequency decay rate, but the transition should be smooth. Unfortunately, a smooth decay is difficult to simulate, and less well-designed reverberators tend to exhibit a metallic, ringing tone, as some frequencies hang on after the rest have decayed. This is less of a problem now than it was on earlier units, but it is still something that has to be kept in mind when comparing reverb units from different design stables.


REVERB TIPS

When trying to simulate a concert hall setting for use on several instruments at the same time, take care not to over-emphasise the early reflections. In a real concert hall, the early reflections are randomised due to the different physical locations of the performers.

Avoid putting too much reverb on bass sounds such as bass guitar, bass synth or kick drum. If the mix demands it, try a short setting with the bass rolled off.

Bear in mind that a high level of reverb produces the illusion of distance. Adding too much reverb to the lead vocal can push it back in the mix when you want it to project. Adding between 20 and 80ms of pre-delay can help space the reverb away from the vocal.

Soft sounds such as keyboard pads, flutes and strings can benefit from quite coarse reverb patches with very pronounced early reflections. These help create an ensemble effect giving a bigger, richer sound.

Don't feel that gated and reverse reverb settings are only for use on drums, they can work well on almost anything as long as the context is right.

To create a 'reverse' guitar solo, try using a longish reverse reverb setting but use only the reverb output with none of the dry sound mixed in.

You can sometimes get interesting snare drum sounds by deliberately overdriving the reverb unit's input to create distortion. This works better on some units than others, so experiment.


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Professional Monitor Company LB1

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Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Sep 1993

Feature by Paul White

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