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This installment of Foundations of Audio explains one of the most essential ingredients in audio mixing, reverb—the time it takes for sound to bounce, echo, and decay during a live performance or recording. Reverb gives a natural richness to your recordings, which is possible to reproduce. Producer and audio engineer Alex U. Case covers the acoustic, mechanical, and digital means for creating reverb, and charts the parameters (room size, density, etc.) you'll need to know to take advantage of the original recording space and enhance it in post. He then shows how to simulate reverb digitally with effects, adding timbre, texture, and contrast, and improve the sound of your mixes with a sense of space and depth.
These techniques can be practiced with the free Get in the Mix sessions, currently available for Pro Tools and Logic Pro.
It's important to understand the tools and technologies we use for creating reverb. There are many of them so we break them down into three families, acoustic, mechanical, and digital. As the very idea of reverberation is born from room acoustics, we'll start there. Allow me to hit you with a bit of math. Here is Sabine's Equation for Reverb Time, which applies to all large sonically diffuse spaces. This equation quantifies how long it takes the room to decay to silence, or more specifically, how long it takes the level to fall by 60 dB, a significant reduction in amplitude and a decent proxy for silence.
Let's listen to a snare drum with 3 seconds of decay. (music playing) The Reverb Time is this constant, 0.05, times the cubic volume of the room, divided by the total sound absorptivity in the room. So we have two principal acoustic properties that we can adjust to drive Reverb Time. For a longer Reverb Time, we can work the top of this equation and seek out a larger room volume.
Big spaces are typically more reverberant than smaller ones. We can also work the bottom of this equation and make our space less sound absorptive. Removing sound absorptive materials or adding the opposite sound reflective structures will also stretch the Reverb Time. Concert halls for classical music are consistently large spaces, some 50 feet wide or more, over 100 feet long, and with a ceiling height of several stories above the floor. They're constructed entirely of materials that reflect rather than absorb sound.
In fact, only two types of sound absorption are found in a typical concert hall. The first is the air. Air absorbs sounds, however slightly, but it's hard to sell tickets to halls that have no air. The second absorber-- speaking of tickets--is the audience. So people and air are the necessary absorbers in a concert hall. The rest of the materials are stone, plaster, concrete, wood, steel, glass, and other similar hard nonporous materials offering high sound reflectivity.
Sound is reflected off these materials like light is reflected off of a white wall or a mirror. You won't find many sound absorptive materials such as carpets, curtains, and glass fiber panels in most concert halls, though they can tame the acoustics of the lobby and the stairs. When classical music is the goal for the space, the architecture also includes bumpy stuff on the walls and on the ceiling that help diffuse the sound. Sound is preserved, sustained, and scattered for a beautiful reverb quality.
(music playing) In pop recording, large reverberant spaces typical of classical music are the exception rather than the norm. In the studio we usually work in smallish rooms, with a tight acoustic quality that deliberately absorbs much of the sound energy in the room. (music playing) There are a few exceptions. If you happen to be working in a large room that's say big enough to park three or four cars, with more than a story in ceiling height, then you might be working in a room whose acoustic qualities are worth capturing.
When you're lucky enough to be recording in one of those grand studios whose great acoustics suite your tracks, plan to record the room. The typical approach these days is to still use close microphones on your instruments, getting all the benefits of close miking craft, but to augment those tracks with other more distant microphones. Send these faraway microphones to their own tracks, label these tracks room, and plan to mix them in with your close-miked tracks later. Most of the biggest recording studios still aren't quite as reverberant as a concert hall.
We utilize studio room tracks less for the long decay and more for the early part of the signal, when the sound is bouncing off all of the carefully designed sound reflectors and diffusers in the room. The best studio room tracks possess wonderfully complex, ear pleasing, early reflections that add excitement, energy, and size to the tracks in our mix. While our Digital reverb Effects Units can also synthesize an approximation of these spaces with their early reflections, there's nothing like the real thing.
Attentive listeners will hear the difference. In fact, medium and smaller studio spaces are often carefully designed with sound diffusive treatments offering up reflected energy that complements the audio, even though they aren't large enough spaces to ring on like a cathedral or an opera house. So when you notice you're in a great sounding room, it may not be because of the obvious long wash of reverb. Studio spaces often have very short decay times. Instead, listen for early reflections that are enhancing the sounds you hear.
In those cases you should mike up the room, capture that natural acoustic liveness, and take advantage of the many signal processing capabilities that they offer, and which we cover in the movie, Getting the most out of Room Tracks later in this course. Natural acoustic reverb makes it into our production through room tracks whenever we get the chance to work in a great sounding room. And there's another acoustic signal processor we can use in our projects, the reverb Chamber, which we discuss next.
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