Mixing : Reverb

Reverb is hundreds and hundreds of delays. When a sound first occurs, it travels throughout the room at the snail's pace of around 740 miles per hour. It bounces of the walls, ceiling and floor and comes back to us as hundreds of different delay times. All of these delay times wash together to make the sound we know as reverb.

Reverb is actually like placing hundreds of spheres of sound between the speakers. It takes up a tremendous amount of room in this limited space between the speakers. In a digital reverb, all of these delays are panned to virtually hundreds of different places between the speakers. This is why reverb masks other sounds so much in the mix.

There are certain parameters of control found in units that create reverb. I will explain each setting

Room Types

Modern digital reverbs allow the user to change the "type of room." You can simply imagine different types of rooms between the speakers. There are no strict rules as to the type of room that is used in a mix. Some engineers prefer a plate reverb sound on the snare drum. Some use hall reverbs on saxophones.

It is best to always set the type of reverb while in the mix (with all the sounds on) to make sure it cuts through the mix like you want it to. Different types of sounds will mask the reverb in different ways.

Reverb time

You can also change reverb time: the duration or length of time it lasts.

A common rule is to set the reverb time on a snare drum so that it ends before the next kick lick; this way, the snare reverb does not obscure the attack on the next kick note, which will keep the kick drum sounding clean, punchy and tight. The faster the tempo of a piece, the shorter the reverb time. Again though, rules are made to be broken. (You won't go to jail for this one.)

Pre delay time

When a sound occurs, it takes awhile for the sound to reach the walls and come back. The time of silence before the reverb begins is called the predelay time. On many units it is just called delay.

Different-sized rooms will naturally have different pre delay times. A medium-sized auditorium has around 30ms of pre delay time, while a coliseum might have as much as 100ms of pre delay time. Therefore, it is important to have a bit of pre delay time if you are looking for a truly natural reverb sound. Most times, when you call up a preset in a reverb unit, someone has already programmed a pre delay time. You can adjust this as desired.

The cool thing about longer predelay times (over 60ms or so) is that they help to separate the reverb from the dry sound. With shorter predelay times, reverb will very quickly "mush up" the original dry sound, making it unclear. With longer predelay times, a vocal, for example, will remain clean and clear even with a good amount of reverb. When using extremely long predelay times, it is important to set the delay time to the tempo of the song (as was covered when we discussed delays).

Diffusion

In most effect units, diffusion is the destiny of echoes that makes up the reverb. Low diffusion has fewer echoes.

You can actually hear the individual echoes in a low-difussion setting. It sounds kind of like "Will, il, il, il, il, bur, bur, bur, bur, bur, bur." A hall reverb setting is preset with a very low-diffusion setting. High-diffusion has more echoes - so many that they meld together into an extremely smooth wash of reverb. Plate reverbs often have a very high-diffusion preset.

There are no strict rules of rat use of high- or low-diffusion settings. High-diffusion settings tend to be sweeter, smoother, and more silky, Low-Diffusion tends to be more intense. Some engineers prefer a low-diffusion setting on a snare drum to make it sound more raucous for rock and roll. High-diffusion is often used to make vocals sound smoother.

EQ of Reverb

You can equalize reverb at various points in the signal path. First you can EQ the reverb after the signal comes back into the board (if you are using channels for your reverb returns that have EQ on them). It is usually better to use the EQ in the reverb unit itself.

Not because it is necessarily a better EQ, but because in some units you can place the EQ before or after the reverb. Ideally it is best to EQ the signal going to the reverb. If your reverb unit does not have this capability, you can actually patch in an EQ, after the master auxiliary send, on the way to the reverb unit. The truth is I don't like to EQ reverb because it screws up the natural sounds (which is just fine) should you use EQ on a sound. Sometimes EQ might be used to simply roll off some low-frequency rumble. Normally, if your reverb sounds like it needs EQ, it is often better to go back and EQ the original sound that is going to the reverb.

High- and Low-Frequency Reverb Time

Even better than using EQ on your reverb is to set the duration of the highs and lows. Many reverb units have this setting these days. This is a bit different than EQ, which changes the volume of the frequencies. High- and low-frequency reverb time changes the time that each frequency range lasts. Using these settings will generally make the reverb sound more natural than any type of EQ.

Regardless of whether you EQ your reverb or set the duration, there is a huge difference as to how much space it takes up in the mix-and the resulting masking it creates. Remember that low-frequency sounds take up way more space than high-frequency sounds. And because reverb is also hundreds of sounds, reverb with more low frequencies will take up an enormous amount of space in a mix.

Reverb with more high frequencies still takes up a lot of space, but not nearly as much as when lows are present.

Reverb Envelope

Another setting of reverb is the "envelope"; that is, how the reverb changes its volume over time: Normal reverb has an envelope where the volume fades out smoothly over time.

Engineers (being the bored people they are) thought to put a noise gate on this natural reverb, which chops it off before the volume has a chance to fade out. Therefore, volume stays even, then stops abruptly.

You can put a noise gate on your reverb, but it's much simpler to use the gated reverb settings on your effects unit. If we were to reverse the envelop of normal reverb, the volume would rise then stop abruptly.

If you take the tape, play it backward, add normal reverb, record it on open tracks on the multitrack, and turn the tape around to run forward, you'd get an effect commonly called preverb.

This effect is the most evil one that can be created in the studio; only the devil could put an effect on something before it happens. Furthermore, it has been used in every scary movie made, including The Exorcist and Poltergeist. And of course, it is one of Ozzy Osbourne's favorite effects.

One of reverb's main functions is to connect sounds in a mix and fill in the space between the speakers. 

Like any sound, reverb can be panned in various ways.

Reverb can be spread to any width by how far left and right you pan the reverb return channels on your mixing board. Depending on how you have your effects patched back into your console, or how your computer plug-ins work, you may not have this option.

Final notes:

Reverb can also be brought out front with volume...

....placed in the background by turning down the volume...

....or raised or lowered a bit with EQ.

You can read more about the most important things you need to know about mixing, and how to setup the compressors properly depending on the task you want to realize in Dave Gibson's book "The Art of Mixing 2nd Edition" A visual guide to recording engineer and production.