When we hear sounds in the “real world,” they are in an acoustic space. For example, suppose you are playing acoustic guitar in your living room. You hear not only the guitar’s sound, but because the guitar generates sound waves, they bounce off walls, the ceiling, and the floor. Some of these sound waves return to your ears, which due to their travel through the air, will be somewhat delayed compared to the direct sound of the guitar.
This resulting sound from all these reflections is extremely complex and called reverberation. As the sound waves bounce off objects, they lose energy and their level and tone changes. If a sound wave hits a pillow or curtain, it will be absorbed more than if it hits a hard surface. High frequencies tend to be absorbed more easily than lower frequencies, so the longer a sound wave travels around, the “duller” its sound. This is called damping. As another example, a concert hall filled with people will sound different than if the hall is empty, because the people (and their clothing) will absorb sound.
Reverberation is important because it gives a sense of space. For live recordings, there are often two or more mics set up to pick up the room sound, which can be mixed in with the instrument sounds. In recording studios, some have “live” rooms that allow lots of reflections, while others have “dead” rooms which have been acoustically treated to reduce reflections to a minimum – or “live/dead” rooms which may have sound absorbing materials at one end, and hard surfaces at the other. Drummers often prefer to record in large, live rooms so there are lots of natural reflections; vocalists frequently record in dead rooms, like vocal booths, then add artificial reverb during mixdown 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 covers tips and tricks on how to make the best use of reverb.
Now let’s take a sneak peak into the nitty gritty of reverb…
High and low frequency attenuation. These parameters restrict the frequencies going into the reverb. If your reverb sounds metallic, try reducing the highs starting at 4 – 8kHz. Note that many of the great-sounding plate reverbs didn’t have much response above 5 kHz, so don’t worry if your reverb doesn’t provide a high frequency brilliance – it’s not crucial.
Reducing low frequencies going into reverb reduces muddiness; try attenuating from 100 – 200Hz on down.
Early reflections diffusion (sometimes just called diffusion). Increasing diffusion pushes the early reflections closer together, which thickens the sound. Reducing diffusion produces a sound that tends more toward individual echoes than a wash of sound. For vocals or sustained keyboard sounds (organ, synth), reduced diffusion can give a beautiful reverberant effect that doesn’t overpower the source sound. On the other hand, percussive instruments like drums work better with more diffusion, so there’s a smooth, even decay instead of what can sound like marbles bouncing on a steel plate (at least with inexpensive reverbs). You’ll hear the difference in the following two audio examples.
The reverb tail itself may have a separate diffusion control (the same general guidelines apply about setting this), or both diffusion parameters may be combined into a single control.
Early reflections predelay. It takes a few milliseconds before sounds hit the room surfaces and start to produce reflections. This parameter, usually variable from 0 to around 100ms, simulates this effect. Increase the parameter’s duration to give the feeling of a bigger space; for example, if you’ve dialed in a large room size, you’ll probably want to add a reasonable amount of pre-delay as well.
Reverb density. Lower densities give more space between the reverb’s first reflection and subsequent reflections. Higher densities place these closer together. Generally, I prefer higher densities on percussive content, and lower densities for vocals and sustained sounds.
Early reflections level. This sets the early reflections level compared to the overall reverb decay; balance them so that the early reflections are neither obvious, discrete echoes, nor masked by the decay. Lowering the early reflections level also places the listener further back in the hall, and more toward the middle.
High frequency decay and low frequency decay. Some reverbs have separate decay times for high and low frequencies. These frequencies may be fixed, or there may be an additional crossover parameter that sets the dividing line between low and high frequencies.
These controls have a huge effect on the overall reverb character. Increasing the low frequency decay creates a bigger, more “massive” sound. Increasing 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 very good on vocals as it adds more reverb to sibilants and fricatives, while minimizing reverb on plosives and lower vocal ranges. This avoids a “muddy” reverberation effect that doesn’t compete with the vocals.
THE NEXT STEP: APPLYING 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! So, see the article “Applying Reverb” for more information.
To read the full detailed article see: Understanding Reverb