At one time or another, you’ve probably walked down a large hallway with a polished marble floor and stone walls. Each step you take is followed by an echo that seems to hang in the air for awhile. The echo is not a clear repeat of each footfall, but a smear of sound that dies away slowly.
That lingering sound is called reverberation, or reverb, and it is a vital part of virtually every sound you hear. Your brain uses it to determine the size, shape, and other characteristics of the space in which the sound was produced. It occurs naturally almost everywhere, and it is artificially simulated in the recording studio. In fact, reverb is probably the most common signal-processing effect of them all.
Almost all enclosed and semi-enclosed spaces exhibit some reverberation. The process starts with a sound wave that emanates in a more or less spherical pattern from a sound source and expands toward any listeners in the room, as well as nearby surfaces: walls, ceilings, windows, furniture, etc. (Speakers don’t necessarily radiate sound in a spherical pattern; the radiation pattern depends on the design of the speaker.)
Once the sound wave reaches a surface, it is reflected back into the room, where it’s reflected again and again by various surfaces. These multiple reflections also reach the listener. However, the initial sound waves almost always reach the listener first because the path between the source sound and the listener is shorter than the path taken by any of the sound’s reflections.
Each reflection is generally lower in amplitude than the preceding one, because the sound wave loses some energy each time it is reflected. Most materials absorb some of the sound wave’s energy and reflect the rest. As a result, the reverberation dies away over time.
Normally, you can’t hear these reflections individually because they happen in such quick succession. Most rooms are no more than a few dozen feet long in any direction, and sound travels at about 1,130 feet per second in air. Therefore, the sound waves are reflected many times per second in every direction (up/down, right/left, front/back). Our brain tends to smear all these rapid reflections into a continuous sound, which we call reverberation. This sound has a haunting, ringing quality that lingers for some period of time after the original sound stops.
Several factors contribute to the specific reverberation of a particular space. For example, the larger the space, the longer it takes sound waves to reach the walls and reflect back to your ears. Heavy drapes and thick-pile carpeting absorb much more sound than marble walls and hardwood floors. In addition, people tend to absorb a fair amount of sound energy (unless they’re wearing suits of armor), so an empty room has different reverb characteristics than the same space when it’s crowded with people.
The phenomenon of reverb can be distilled into several distinct parts. The most obvious component is the time it takes the reverberation to become inaudible. This decay time depends primarily on the reflective properties of the room–as determined by the texture of the walls, ceiling, floor, and furnishings–and the amplitude of the original sound. Also, high frequencies tend to fade away more quickly than low frequencies. In very large spaces, such as enclosed stadiums, there could be a perceptible delay between the original sound and the onset of reverberation.
One problem with acoustic reverberation is that you can’t easily control it. The physical size of the space limits what you can do, so studio recordings normally rely on artificial reverb. For maximum flexibility and creativity in signal processing, acoustic musicians are often recorded in a relatively “dead” (that is, non reverberant) environment, and their signal is passed through a reverb unit that digitally simulates various acoustic environments. Electronic sound sources also benefit from passing through a reverb unit.
Best reverb units are actually multi-effects processors that perform reverb, modulation effects, and several other signal processing chores, which I’ll discuss later in this part of the book. Many synthesizers also include onboard multi-effects processors that apply reverb in addition to other effects.
To program a reverb unit effectively, you need to know the basic parameters you will encounter. The most fundamental parameter is reverb type. Most units offer several types of reverb, generally based on different acoustic environments. As you might expect, a room reverb simulates a small to medium-size room, while a hall reverb simulates a large room or auditorium. Some units offer a stadium or cathedral reverb, as well. Select the reverb type that best suits the acoustic space you wish to simulate.
Many reverb units also include several digital simulations of early types of reverb processing. In the early days of recording, engineers sent a recorded signal to a speaker in a small, tile-lined chamber. A microphone placed in the chamber picked up the sound from the speaker along with the subsequent reflections inside the chamber. Reverb chambers are prized for their lush, blossoming quality and were a major component of the classic Phil Spector sound in the early 1960s.
Spring reverbs employ a physical spring with a microphone transducer attached to one end and a speaker transducer attached to the other end. The signal passes through the spring to create a reverb effect. Spring reverb stend to produce a boingy, chattery timbre with accentuated high end. Plate reverbs use a metal plate instead of a spring, with pickups placed at various locations on the plate. Plate reverbs typically produce a sharp timbre that greatly enhances the punch of drum and percussion tracks.
Many units also offer several reverb types that have little to do with acoustic reverb. These types are generally known as nonlinear reverb. For example, a reverse reverb starts from silence and grows to maximum level, after which it cuts off abruptly, which is opposite from the way natural reverb works. A gated reverb starts normally but suddenly is cut off after a user-definable gate time. Gated reverb is often applied to the snare drum in pop and rock music.
Once you have selected the basic reverb type, there are several parameters you can play with. The first and most obvious parameter is reverb or decay time. This parameter determines how long the reverb effect remains audible, and it lets you control the apparent size and reflectivity of the simulated acoustical space. For example, a room reverb might have a decay time of one or two seconds, whereas a hall reverb might last three to five seconds. A cathedral reverb might last as long as eight to ten seconds. If the environment is highly reflective, the reverb time is longer.
Sometimes, you can actually discern the first few reflections separately before the reverberation starts to smear together, particularly in large acoustical spaces. These early reflections have a strong influence on how you perceive reverb in a large space. In a digital reverb unit, the time between the original sound and the very first reflection is controlled by the predelay parameter. As the predelay time gets longer, the apparent size of the space increases and your apparent location within the space gets closer to the “center” of the room. Predelay is generally no more than 500 milliseconds and typically in the 10 to 50 ms range.
Another parameter that relates to early reflections is called, reasonably enough, early reflections (in some units, it’s called density). This parameter controls the time between the first few reflections, which is generally just a few milliseconds. This might seem insignificant, but it determines the “opacity,” or density, of the reverb sound during the first few moments. Like reverb time, density helps simulate different room sizes; higher density values simulate larger spaces, because it takes longer for the early reflections to reach your ears in a large space.
The diffusion parameter controls the separation of the reflections within the main body of the reverb sound, which determines its “thickness.” Reducing the diffusion value produces a thinner sound because the reflections are more widely spaced in time. This parameter lets you determine the complexity of the simulated acoustical space. Keep in mind that many reflective surfaces in a room result in a thicker, denser reverb.
One of the characteristics of acoustic reverberation is that high frequencies typically die away faster than low frequencies. As a result, many reverb units include a high-frequency damping parameter, which lets you control the decay rate of high frequencies separately from the main reverb decay. Some reverb units even include a low-frequency damping parameter. Both of these parameters provide additional control over the apparent size and reflective characteristics of the simulated acoustical space. For example, softer surfaces cause high frequencies to decay more rapidly, while smaller rooms cause low frequencies to decay more rapidly.
As mentioned earlier, one of the primary applications of artificial reverb is to Simulate an acoustical space within which your recorded ensemble “performs.” To accomplish this, send the entire mix through the same reverb unit programmed to re-create the type of space you wish to simulate.
You can also apply different reverbs to individual instruments for special effects. In many pop drum mixes, for example, the snare is heavily processed (often with a gated and/or reverse reverb), while the kick drum is relatively dry. This approach lends an air of drama to the snare backbeat without washing the kick drum in confusing reflections that would diminish its cohesiveness with the bass. Many guitar players like to apply liberal amounts ofreverb to enhance their solo sounds.
Another interesting application is playing in a highly reverberant environment a la Paul Horn’s Inside the Taj Mahal. Use a hall or cathedral reverb type with a long decay time and high aux-send and aux-return levels.
In many synthesizers with onboard multi-effects units, the effects are an integral part of each patch. Unfortunately, most of these kinds of synths can produce only one combination of effects at a time. If the synthesizer is multitimbral, all parts are passed through the same effects. If you’re not careful, this overall effect will be the one assigned to the last patch you called up, which might or might not serve the other parts well. Instead, you should set the synth’s effect mode to “master,” which lets you select the effects you want for all the parts from that synth. (Fortunately, some synths now include several separate effects processors; some effects are global and others are applied as “insert” effects to individual patches.)
Reverb is inescapable. You hear it everywhere, and rightly so. Almost all music sounds better with reverb, which is why church choirs generally sound better than one would expect from the singing skills of their members. Your music will sound better with reverb, too. All you need to do is experiment a bit with a digital reverb unit to discover just how much better.