Using EQ on your effects sends and returns can dramatically reduce mud in your mixes
By Phil O'Keefe
Modern music production tends to rely less on actual room ambience and more on "in the box" software solutions. This is especially true in the typical home studio, where large, great sounding "live" rooms are typically unavailable, and close miking when tracking and later creating an acoustical "space" with plugin processors during mixdown is common. Today there are a lot of reverb and ambience plugins designed to fill this need, including traditional algorithmic reverbs such as the plugins from Lexicon and SSL, as well as convolution reverbs like TL Space and Altiverb that use impulse response recordings made in a real physical space and use them to reproduce the sound of that physical space within the plugin. Most DAW software comes with at least some sort of basic reverb plugin, and their quality keeps getting better and better. With most modern computers, there's more than enough CPU power available to use several different reverb plugins per mix, and while this can offer lots of options, it can also lead to muddy mixes.
Rather than inserting a reverb plugin inline as an insert on each individual track, many engineers will instead use an aux send and return type of setup. An aux send is inserted on each mixer channel you want to use that particular reverb on, and either a stereo or mono aux return is used to feed the reverb's output back into the mix. (Figure 1) Since you can use that same reverb with as many sound sources as you wish, this allows for more efficient processing than using multiple reverbs on individual tracks does.
Figure 1: Aux sends and returns allow multiple source tracks to feed a single reverb unit
It's not uncommon for engineers to set up three or more different types of reverbs for their mix processing. For example, a small room ambience, a medium decay plate and a longer hall reverb will generally give you the basics for processing a wide variety of different sounds. However, having several reverbs going at once can lead to mud. Much of this is due to the material that is feeding the reverb itself - low and low-midrange frequencies tend to cause mud in a reverb, and don't really add to what we perceive as spaciousness.
EQ AS A SOLUTION
An EQ plugin placed in the signal chain directly in front of the reverb (or on the reverb "send") can be used to filter out the low and low-mid frequencies that contribute to the muddiness before they ever hit the reverb plugin. A basic high-pass filter is commonly used for this task. Try setting it at 100Hz and slowly sweeping it upwards until you hear the sound of the reverb "thin out." You can always back it down if you go too far, but some thinning is usually beneficial, and depending on the source material, it's not at all unheard of to have the high-pass filter set as high as 400 or 500Hz. EQ in front of the reverb shapes the signal that "drives" the reverb. By shaping it, you can have influence what sounds will trigger the reverb. By boosting the mids at 2.5kHz, I was able to bring out the "crack" of the snare in the reverb and accentuate it, giving the snare and verb combo a brighter sound and "more" apparent reverberation. (Figure 2)
Figure 2: Pre-reverb EQ filtering. By rolling off the lows and boosting at 2.5kHz, I was able to make the snare hit the reverb harder, while stopping the kick bleed from triggering it as much
Another approach is to use the EQ after the reverb plugin to filter out the low frequencies post-processing. (Figure 3) Remember, reverbs are designed to mimic the characteristics of real rooms and acoustical spaces, and in real rooms, the low frequencies tend to build up. Even with careful pre-filtering, sometimes you can wind up with too much low and low-midrange signal in the reverb return. Post-reverb processing (on the reverb "return") lets you shape the sound of the return and adjust the final, overall tonality. Anything's fair game here, and if you find that you like the overall sound and all your balances are working great, but you wish that the reverb was a little brighter overall, don't hesitate to grab an EQ plugin, insert it after the reverb and make some adjustments.
Figure 3: Putting EQ after your reverb allows you to tailor the final sound and overall frequency response of the processor itself
Some reverb plugins include their own onboard EQ that is built right into the plugin itself. The Eventide Reverb plugin shown in Figure 4 is a great example; it actually has multiple EQ controls to tailor the sound going into the reverb and coming out of it. While this can be very convenient, you may prefer the sound of a dedicated EQ plugin over the sound of the reverb's onboard EQ, so make sure you experiment with both to see what sounds best to you.
Figure 4: Many reverb plugins, such as the Eventide Reverb from the Anthology II bundle, have EQ capabilities built-in
DIFFERENT EQ, SAME REVERB
You can separately EQ the signal that is being sent to the reverb from individual tracks if you feel the need. For example, you can use a low pass filter to pull some of the brightness out of the lead guitar while leaving the brightness of the rhythm guitar intact, or EQ the signal from the snare and toms that feeds the reverb differently than the overheads and room mikes (Figure 5). To do this, you may need to get creative with your aux sends. In Pro Tools, one way to do it is to create a stereo aux send bus (Bus 1-2) and stereo aux return channel with a reverb plugin assigned to it as outlined previously; we'll call this return Aux 1-2. Then create a second aux send and return; we'll call this Bus 3-4 / Aux 3-4. On the return for Aux 3-4, insert an EQ and adjust to taste. Assign the output of Aux 3-4 to the input of the Aux 1-2 by selecting Bus 1-2 as its output assignment. Any signal you want to EQ separately should be routed to Bus 3-4, and anything you don't want to pass through the EQ would be sent directly to Bus 1-2.
Figure 5: Separate EQ on the sends can be useful for adjusting the reverb tonality of separate instruments feeding a single reverb processor
TIPS AND SUGGESTIONS
Every situation is different, and the specific frequencies to use will depend on the individual mix. Forget formulas - use your ears!
What you send to the reverb can also make a big difference. Kick drums and bass guitars are often used without additional reverb since their low frequency characteristics are generally not terribly compatible with reverb. Again referring to Figure 1 (above), notice how the two kick drum tracks do not have Bus 1-2 assignments, and they are not being sent to the reverb directly, although some kick is hitting the reverb anyway due to it "bleeding" into (being picked up by) other mikes, such as the snare and overhead microphones.
EQ placed before the reverb is good for accentuating frequencies and instruments you want to have "stand out" or to be processed more heavily by the reverb, while EQ after the reverb is good for filtering out whatever there is "too much of", such as excessive and harsh high frequencies or overabundant lows and low-mids.
Automating the EQ can sometimes be useful. For example, suppose you have a sparse string section introduction, and prefer the fuller sound of the reverb when the high pass filter is set to a lower frequency, but once the whole band kicks into the heavy rock section, it's "too much" and you prefer the sound with the high-pass filter set to a higher frequency. By using your DAW's plugin automation to adjust the frequency of the high-pass filter, you can use that same EQ for both sections of the song, while optimizing the sound of it for each part.
Sometimes adding a compressor in front of the reverb can smooth things out and prevent heavy transients from really hitting the input hard and causing huge fluctuations in the amount of reverb.
Sometimes I run into people who believe that the reverb "is what it is" - you set up your sends and insert a reverb, and if you don't like the sound, you load a different reverb or call up a different preset until you do, and that's where it all ends. Frankly, I disagree. Sure, go ahead and start from a preset if you'd like, but don't be afraid to get your hands dirty and adjust things until you have the exact sound you're looking for. Nothing says you can't EQ and even compress your EQ sends and returns, and as we've seen, there can be benefits to doing so in terms of overall reverb tonality and reduction of mud in your mixes. Reverb coming back into the mix via an aux return is just another sound source, so adjust it, pan it, EQ it and get creative with it!
Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Associate Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.