Music Theory – What is Reverb?

Understanding reverb

When we hear sounds in the “real world”, they are in an acoustic space. For example, suppose you are playing an acoustic guitar in your living room. Not only do you hear the sound of the guitar, but because the guitar generates sound waves, they bounce off the walls, ceiling, and floor. Some of these sound waves return to your ears, which due to their travel through the air, will be slightly delayed compared to the direct sound of the guitar.

This sound resulting from all these reflections is extremely complex and is called reverb. As sound waves bounce off objects, they lose energy and their level and pitch change. If a sound wave hits a pillow or curtain, it will be absorbed more than if it hit a hard surface. High frequencies tend to be absorbed more easily than lower frequencies, so the longer a sound wave travels, the more “muffled” its sound is. This is called damping. As another example, a crowded concert hall will sound different than if the hall is empty, because the people (and their clothing) will absorb the sound.

Reverb is important because it gives a sense of space. For live recordings, there are often two or more microphones configured to pick up the sound from the room, which can be mixed with the sounds of the instrument. In recording studios, some have “live” rooms that allow a lot of reflections, while others have “dead” rooms that have been acoustically treated to minimize reflections, or “live / dead” rooms that may have materials. sound absorbent on one end, and hard surfaces on the other. Drummers often prefer to record in large live rooms, so there are many natural reflections; vocalists frequently record in dead rooms, such as vocal booths, then add artificial reverb during mixing to create a sense of acoustic space.

Whether generated naturally or artificially, reverb has become an essential part of today’s recordings. This article covers artificial reverb: what it offers and how it works. A companion article includes tips and tricks on how to get the most out of reverb.

Different types of reverb

There are two main types of artificial reverb: synthesized and convolution-based. Synthesized reverb “shapes” the sound of a room using various algorithms. For example, a “Hall” algorithm will take into account that the waves travel more in a concert hall than in a small room, so it will take longer for the reverb to decay. A “room” algorithm could model a small room, such as a club or practice space. Other algorithms model artificial reverbs, such as the “Spring” reverbs found in guitar amps or the “Plate” reverbs that were widely used in the 1960s. Each algorithm has a different sound quality, but they all work the same. Basic way: a signal enters the reverb, is analyzed, and the reverb algorithm generates echoes and reflections that mimic what happens in the chosen acoustic space.

Convolution reverb is a relatively new type of technology that “samples” the sound of a room. Typically, a device like a sports starting pistol will create impulse that will create reflections in a room. These reflections are recorded, analyzed and converted into a very accurate model of that specific room. A good analogy is that the impulse of a convolution reverb is like a “mold” in which you look for the sound, and the sound takes on the characteristics of being in that room.

You can think of the difference between synthesized reverb and convolution as the difference between a synthesizer and a sampler. The synthesizer will give more control over the sound but will have a more “impressionistic” character, while a sampler will provide an extremely accurate sound, but generally less editable.

Another consideration is that convolution reverb is a processor-intensive operation. It is only recently that computers have become powerful enough to allow real-time operation, and even then, you may experience some audible delays due to processing. Fortunately, since reverbs are delay-based anyway, with fast computers you might not notice anything objectionable.

Reverb elements

A sophisticated reverb will have many parameters, but few people know how to optimize these parameters for specific recording situations. So, let’s take a look at how the various parameters affect its sound.

Reverb has two main elements:

Tea early reflections (also called early reflections) consist of the first group of echoes that occur when sound waves hit walls, ceilings, etc. These tend to be more defined and sound more like “echo” than “reverb”. You can often adjust the level of the first reflections.

Decay, which is the sound created by these waves as they continue to bounce around a space. This “washout” of sound is what most people associate with reverb and is often called a reverb tail.

Other parameter, Pre-delay, sets the time for the first sounds to travel from the source to the first set of reflections. The larger the gap, the longer the pre-delay because it takes longer for the signal to reach a wall or ceiling and start bouncing.

Advanced parameters I

Here are some of the parameters found in high-end synthesis-based reverbs; less expensive reverbs will have a subset of these parameters. Convolution reverbs generally have fewer parameters, but in recent years, engineers have figured out how to make convolution reverbs more editable.

Algorithm. We have already mentioned room and room algorithms, as well as algorithms that emulate “classic” synthetic reverbs. But you can also find algorithms like cathedral, gym, small room, closet, everything is possible! There are even “inverse” algorithms in which decay builds from zero to full volume rather than decaying from full volume to nothing, and “closed” algorithms that abruptly cut the reverb tail below a certain level (this effect was very popular in the ’80s, particularly with the Phil Collins albums).

With convolution reverbs, the equivalent concept is called impulse. Impulses can capture the sound of specific rooms (such as private concert halls), or even the sound of spaces such as guitar cabinets. It’s even possible to create pulses from older reverbs, so there might be a boost that sounds like an old Lexicon PCM-70.

The size of the room. This affects whether the paths that the waves take while bouncing in the “virtual room” are long or short. Like real rooms, artificial rooms can have “standing waves” and resonances. If the reverb sound has a flutter (a periodic chirping effect), vary this parameter along with the decay time (described below) for the softest sound.

Decay time. This determines how long it takes for the reflexes to run out of power. Remember that long reverb times can sound impressive on solo instruments, but they rarely work in an ensemble context (unless the arrangement is very tight). The decay time specification is called RT60, which means the time it takes for a signal to decay to -60 dB from its original amplitude. For example, if RT60 = 1.5, it takes 1.5 seconds for the signal to drop to -60 dB or its original level.

Dipping. If sounds bounce in a room with hard surfaces, the reverb decay tails will be bright and “harsh.” With softer surfaces (for example, wood instead of concrete), the reverb tails will lose high frequencies as they bounce, producing a warmer sound. If your reverb can’t create a smooth-sounding high end, introduce a bit of damping to put the focus more on the mid and low frequencies. Listen to these two audio examples to hear the difference.

Advanced parameters II

High and low frequency attenuation. These parameters restrict the frequencies that go into the reverb. If your reverb sounds metallic, try reducing the treble from 4 – 8 kHz. Note that many of the great-sounding board reverbs didn’t have much response above 5 kHz, so don’t worry if your reverb doesn’t provide high-frequency brightness, it’s not crucial.

Reducing the low frequencies that go into reverb reduces clouding; try attenuating from 100 to 200 Hz down.

Diffusion of early reflections (sometimes just called broadcast). Increased diffusion pushes the early reflections closer together, thickening the sound. Diffusion reduction produces a sound that tends more towards individual echoes than sound washing. For voices or sustained keyboard sounds (organ, synthesizer), reduced diffusion can give a beautiful reverberant effect that does not overpower the source sound. On the other hand, percussion instruments like drums perform better with more diffusion, so there’s a smooth, even decay rather than what may sound like marbles bouncing off a steel plate (at least with cheap reverbs). You will hear the difference in the next two audio examples.

The reverb tail itself can have a separate diffusion control (the same general guidelines apply for setting this), or both diffusion parameters can be combined into a single control.

Early reflections before the delay. It takes a few milliseconds before sounds reach room surfaces and begin to reflect. This parameter, normally variable from 0 to about 100 ms, simulates this effect. Increase the duration of the parameter to give the feeling of a larger space; for example, if you’ve dialed in a large room, you probably want to add a reasonable amount of pre-delay as well.

Reverb density. Lower densities give more space between the first reflection of the reverb and the subsequent reflections. Higher densities bring them closer together. Generally, I prefer higher densities for percussion content and lower densities for voices and sustained sounds.

Early reflections level. This sets the level of early reflections compared to the overall reverb decay; Balance them so that the first reflections are not obvious and unobtrusive echoes, nor are they masked by decay. Lowering the early reflections level also places the listener further back in the room and more towards the middle.

High frequency decay and low frequency decay. Some reverbs have separate decay times for high and low frequencies. These frequencies can be fixed or there can be an additional crossover parameter that establishes the dividing line between the low and high frequencies.

These controls have a huge effect on the overall character of the reverb. Increasing the decay of the low frequencies creates a bigger, “massive” sound. Increasing the high frequency decay gives a more “ethereal” type of effect. With few exceptions, this is not the way sound works in nature, but it can sound great on vocals, adding more reverb to sibilants and fricatives, while minimizing reverb at stops and ranges. lower vowels. This avoids a “muddy” reverb effect that does not compete with the vocals.

THE NEXT STEP: APPLY REVERB

Now that we know how reverb works, we can think about how to apply it to our music, but that requires its own article! Therefore, see the article “Reverb Application” for more information.