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Reverb by Numbers

How digital reverberation devices work and how to use them.

Digital reverberation is probably the most important effect in the recording studio — yet it's often the least understood. Paul White unravels the ins and outs.

Reverberation is inextricably linked with the way in which we perceive and interpret sounds — it is an integral part of our everyday lives and our brains make subconscious use of it to analyse our surroundings. Sound that is totally devoid of reverberation is rather like a picture with no colour and no shadow — stark and unreal. Listening tests in an anechoic chamber (a specially-treated, acoustically-dead room) serve only to emphasise that a large part of what we hear is actually reverberant or reflected sound. Interestingly, reaction to an anechoic environment is apparently different for people who spend most of their lives in open spaces and who are not accustomed to hearing reflected sound. After all, if you spend your life as a nomad living in a tent in the middle of a desert, reverberant sound is going to play a far less important part in your life.

For psychological reasons which are not yet fully understood, the appreciation of music is greatly enhanced by the addition of suitable reverberation; one theory is that it, in effect, causes the notes to sustain, which makes it easier for the brain to compare new notes with their immediate predecessors. Whatever the explanation, because natural reverberation is so much a part of our everyday lives, any artificial substitute has to be quite sophisticated to convince our brains that what we are hearing is real.

Over the past decade or so, digital reverberation systems have been developed to such a degree that only a person with a highly-trained ear can tell the difference between an electronic imitation and the real thing. However, the studio engineer needs to have an understanding of the basic mechanism of natural reverb if all the parameters available on a modern digital unit are to be used creatively.

Reverberation is a far more complex phenomena than it might first appear, but if we examine what happens following a single sound impulse in a reflective space (such as a room or hall), the process becomes much more obvious. A simple bursting balloon provides a short, high-intensity sound which, if heard in an anechoic chamber, would be perceived as a single, short crack. The same sound in a hall would propagate outwards in all directions at around 300 metres per second until it encountered an obstacle, at which time some of the energy would be absorbed and some reflected back into the room. In fact there would be several such reflections, as different parts of the sound wave encountered the different parts of the walls, floor and ceiling of the room, as well as any objects placed within it.

The Real World

These reflections reach a listener in the same room as a series of very rapid, but nevertheless discrete, reflections, though their spectral content will be different to that of the original sound, due to the fact that 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. Very soon these early reflections encounter the room boundaries and other obstacles, resulting in further reflections — and every time a sound is reflected in this way, its spectral content is modified and its energy diminished. As the reverb decays, the complexity of the reflection pattern builds up rapidly, to the point where all the reflections blend together to form a homogeneous, reverberant field.

Because the human being has two ears, the reflection patterns coming from two different directions can be assimilated at once, and this stereo hearing mechanism enables us to perceive spaciousness and direction. 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.

Figure 1. Reverb build-up from an Impulse.

The Digital World

A digital reverberator works by using computer technology to generate a simplified model of the reflective properties of a real room; this model is then excited by the sound being processed. The first important parameter is a short delay, which is intended to simulate the time delay between the original sound source and its first reflection reaching 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 reverb unit, may be adjusted by the user.

Early reflections are usually simulated by a multi-tapped delay line (also known as a Finite Impulse Response or FIR filter) which has random tap spacings. The outputs from these taps tend to decay in level after the first few reflections, again in an attempt to emulate what happens in nature. Different patterns of early reflections are used to imitate different types and sizes of rooms, and the longer the spacing between the 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.

At this point, it is worth pointing out a potential problem in simulating 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 boundaries 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.

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 multiply the early reflections into something much more dense. Digital filtering is also used to simulate the way in which high frequencies are absorbed more readily than low ones, which results in the reverberation becoming less bright as it decays in level. As a rule, the harder the surface of a room, the brighter the reverberant reflections, and in the vast majority of spaces, the high-frequency energy is absorbed much more rapidly than the low-frequency energy. In electronic systems, this effect is called 'high-frequency damping' and it may be varied to create different environments.

Also of great significance is the 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. Larger rooms also permit the formation of more reverberant modes than a small room, which is why small rooms can sound coloured or resonant.

"A digital reverberator works by using computer technology to generate a simplified model of the reflective properties of a real room"

Comb And All-Pass Filters
A comb filter is constructed from a short delay line, where a proportion of the output is fed back to the input. This causes spectral filtering of frequencies that are time-related to the delay time of the filter; if a spectral analysis is made, the filter response can be seen to comprise a series of peaks and troughs resembling the teeth of a comb — hence the name.

An all-pass filter is also a form of recirculating filter, which has a flat frequency response over its working range but a complex phase response. Both of these filters regenerate the input signal through the feedback mechanism and are useful for increasing the density of reverberation.

Other than the parameters mentioned above, 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, and better reverberation units provide control over these parameters also. 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, this is difficult to achieve in an artificial reverberator and less well-designed models tend to exhibit a metallic tone, sometimes called ringing, as odd frequencies do 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 evaluating electronic reverberators from different design stables.

In practice, the only reverb settings that should have an audible ring are plate and very small room reverb simulations. Reverb plates were used extensively before digital reverberation was introduced and are simple mechanical devices which work by inducing vibrations (via a transducer similar to a loudspeaker) into a suspended metal plate; these vibrations are then picked up by surface-mounted contact mics. Because of their mechanical nature, a degree of colouration was part of their sound. Even though technically imperfect, their sound became popular for use with drums, and most digital systems include a selection of plate programs.

Figure 2. Schematic diagram of a Digital Reverberator.

So far, we have identified pre-delay, early reflections, overall decay time and high-frequency damping as the main parameters in simulating natural reverberation. 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 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 which aims to fulfill the above requirements. This is not representative of any model in particular but is presented to illustrate the basic principles.

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 reverberators 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

The majority of digital reverberation units, whether programmable or preset, will offer a selection of room types and sizes. In addition, most will offer so-called gated and reverse settings. These have no natural counterparts at all, but simulate artificial effects that were originally created by other means. Gated reverb mimics the effect of feeding the output of a reverberator into a noise gate which is used 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, and 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 discrete burst of reverb of almost constant level which stops abruptly. Though used mainly to process drums and percussion, this effect has a number of other creative uses, including treating vocals and electric guitars.

A derivation of this effect is reverse reverb, a burst of reverb with a slow-attack/fast-decay envelope imposed on it. This is reminiscent of a taped sound being played in reverse, and though the effect is pure illusion, it can be very effective.

On Reflection

Artificial reverberation is an essential part of making pop music, as many of the sound sources 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. This would never occur in nature, but then pop music is about creativity, not about copying what already is!

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 sound. One general rule is to avoid adding reverb to bass instruments or kick drums, and 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. Listen carefully to your record collection and try to identify the different types of reverb used — you'll be surprised at what you discover. Most records make use of less reverb than you think, and where it is used in a more obvious way, the arrangement usually leaves room for it.

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JBL 4208 Near Field Monitors

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Modern Art

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


Recording Musician - Jul 1992

Feature by Paul White

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