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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.
All Reverbs come with presets, sometimes hundreds of them. It can be fairly daunting to know which reverb to use, and in turn, the desired values for each of the parameters. With so many possibilities, where do we start? I find it helpful to compare parameters to a good icon of acoustics, an orchestra hall. Acquisitionists have researched were it considered to be the best-sounding halls for romantic orchestral music, and these three rise to the top. Work done by Leo Branic's found a statistically significant number of conductors, orchestras, audience members, and music critics prefer the sound of these three halls over most other halls for romantic orchestral music.
Over time, more and more great halls are built, but the best ones were all highly influenced by the acoustic qualities of these three. So it's useful to look at the Reverb Time, pre-delay, and spectral multipliers for these halls as a touchstone even if our music isn't symphonic music. Looking first at the middle frequency here, we see that these three concert halls have a mid-frequency Reverb Time of around two seconds or just under. So two seconds is a really useful reference number.
When you set your Reverb Time to 2 seconds, you have a Reverb Time associated with a space that's quite large, the size of an orchestral hall. A hall that may seat 2,000 or more people, a hall that adds lush reverberation to a symphony. No reason not to put it on this acoustic guitar track transporting it to a concert hall. (music playing) It sounds smaller than the stadium or the Taj Mahal or a Cathedral, but it sounds larger than a club and probably much larger than even the largest recording studios.
In addition to that approximately 2 second middle frequency Reverb Time, there's another interesting trend to the successful halls. The downward slope left to right of these curve shows that for the great halls the low frequencies resonate longer than the mids while the high frequency reverb times are shorter. Now this isn't the universal right answer, perfect reverb for all forms of music, but this sort of spectral contour seems to be a nice way to support the music of Mahler, Beethoven, and Wagner.
In fact, the low frequencies have a base ratio of about 1.1, that means that the low frequencies last about 10% longer than the mid-frequencies. The shorter decay up high mean time is most likely attributable to air absorption. Air absorbs high frequencies more effectively than middle or low frequencies. Because these halls are real halls, not digital algorithms, and because these halls have, well, air in them, the high frequencies decay more quickly than the mid-frequencies.
So when you dial in a Reverb Time greater than or less than two seconds, you're essentially starting off with a space that might sound larger than or smaller than an orchestral hall. That two second mid-frequency Reverb Time is a useful landmark. In addition when you coaxed the Reverb Time of low frequencies with the multipliers slightly greater than 1 you're emulating what, in fact, goes on in the warm, rich, enveloping, lush, orchestral halls that are thought to be the greatest in the world.
You're free to introduce still more low-end thunder by raising the base multiplied values even greater than 1.1 or tighten up the low end of the reverb and avoid potential pop rock muddiness through base ratio of less than 1. When you specify a high frequency ratio less than 1 you're emulating or beginning to emulate qualities of air absorption, which could lead to reverberation that's very evocative of a real actual space. Of course, there's no reason you can't push the high frequency Reverb Time up, maybe even make it last longer than the mids which will effectively make your reverb sound unnatural.
Maybe better than the real thing or maybe like a reverb from another world. Pre-delay is another essential parameter. The pre-delay of these orchestral halls all hover very close to 20 milliseconds. Less than 15 milliseconds or greater than 30 and the halls are not considered successful. The halls that are most loved for romantic orchestral music have a pre-delay of about 20 milliseconds. Again, for our mixes, the pre-delay parameter can be set to values different from 20, and in fact we almost always do as you'll see in later videos in this course.
But 20 milliseconds is a good reference. That's the amount of time it takes a large, great sounding orchestra hall to go from direct sound to that full dense beginning of reverberation. A two second mid-frequency Reverb Time, a low-frequency multipliers slightly above one and a high frequency multiplier slightly less than 1 with a pre-delay of about 20 milliseconds. These are reference values from the best orchestral halls. We deviate from these freely in our productions, but we should know when were pushing beyond them or holding back, and make sure we do that on purpose to suit our music.
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