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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.
So far, our discussions of Reverb Time have been with full bandwidth signals, which is to say we found the time it took for the entire signal to decay by 60 decibels. Reverb time, however, is rarely the same at mid, low, and high frequencies. So for instance, the reverb Time at 100 Hz might be longer than the Reverb Time at a 1000 Hz. The Reverb Time up high at 10,000 Hz might be shorter. This is the frequency dependence of reverb. We take the concept of reverb and divide it up along the frequency axis to observe reverb times from low to high.
Most physical spaces have decay time that is highly dependent on frequency. In fact, I've never heard a room that decayed evenly across the entire audible band, so knowing the decay time as a function of frequency is a great goal. Or is it? Wait a second, we humans can hear across 10 octaves, so this approach would have us trying to keep track of and dial in 10 different time values in order to specify one frequency dependent reverb program for perhaps one track in our mix.
Now my head is full of Beatles lyrics, Stevie Ray Vaughan guitar solos, and eBay prices for vintage microphones, so I don't really have the mental capacity to keep track of 10 numbers for every reverb I want to use. So instead we simplify. Most reverb units give you some way to globally reshape the decay time across frequency bands with just one or two parameters. The most common approach is to use a Bass Multiply or Bass Ratio parameter. When set to a value of exactly 1.0 the Bass Ratio will cause low frequencies to decay with the exact same Reverb Time as mid-frequencies.
A value less than one causes the low end to die off more quickly than the mids, perhaps preventing unwanted mix muddiness. A Bass Multiplier parameter greater than one, coaxes the Reverb Time into a condition where the low frequencies linger on a little bit longer than the mids possibly adding to the warmth and low end envelopment of the track. The way we push low frequencies to resonate longer or shorter than mid-frequencies varies by reverb unit and plug-in. Look for some sort of parameter labeled ratio or multiplier often accompanied by a selectable low frequency so that you can specify the starting point below which the low frequencies will be affected and the amount by which they are adjusted.
You might reduce everything below 100 Hz one time, but find reason to pullback everything below 400 Hz in another situation, and a similar parameter for high frequencies might be available on your devices though this is less common. Again, it's a ratio, set to one the high frequencies will decay at the exact same rate as the mid-frequencies. Set to a value greater than one, the high frequencies linger on a little bit longer, perhaps adding polish and some sort of sparkle to the track. The specifying a value less than one might soften the reverb a little bit getting it out of the way of the top of your mix, so you can better enjoy the cymbals, the breathiness of the vocal, and the metallic shimmer of the 12 string acoustic guitar.
I don't know if you've noticed, but I'm not telling you what the right answer is for these bass ratios and high frequency multipliers, that's because there's not a single right answer. These are simply parameters were given to adjust the frequency dependence of the complex decay time of our reverbs, it's up to us to use the parameter effectively. We'll hear them in action later in this course. While these ratios give us control over the spectral content of the reverb resonance there's nothing stopping us from using equalizers to directly affect the spectral content of the signal going to the reverb.
While we can always patch in our own equalizers are often provided as an additional parameter within the reverb itself. This gives us two different ways to affect the frequency content of our reverb effects, Ratios and EQ. EQing the send or the return of the reverb lets us emphasize or minimize certain frequency ranges by boosting or attenuating the level within that specified frequency range. EQ is all about level and is divorced entirely from decay time.
Ratios and multipliers on the other hand don't adjust the level of the frequency band, instead they modify the reverb program to make the reverb time longer or shorter at the frequency areas specified. The idea of Bass Multiplier, for example is that it can make the lows easier to hear by making them last a little longer instead of just boosting their level as EQ does. Think of EQ as the way to determine the loudness or softness of the signal from low to high within your reverb, while ratios determine how long the reverb then last at those frequencies.
Use both to sculpt the spectral content of your reverb.
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