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Reverb Units in PA Systems

Reverberation is a natural effect that occurs when some of the original sound is reflected from the surfaces in an enclosed space. Some of the reflected sound is in turn reflected, as a result of which the sound does not end abruptly, but gradually dies away. A reverb unit (or the Reverb function in a multi-effects unit) is designed to simulate this effect digitally or electronically.

What it is

Usually a reverb unit - which may also incorporate other effects - is a 19″ 1U box with knobs or buttons (and more often than not some kind of menu display) on the front.

What it does

It simulates the way sounds naturally reflect and gradually die away in enclosed spaces.

How it works

Historically there have been three basic methods, of which only the third is now in widespread use (although you might still find spring reverbs in classic guitar combos):

  1. Mechanical. A copy of the original electrical signal drives a transducer. The transducer agitates a spring. At the other end of the spring, another transducer converts the movement of the spring back into an electrical signal. The resulting signal is mixed with original signal. A ‘plate’ reverb uses a metal plate instead of a spring, but the principle is the same.
  2. Electronic. A copy of the original signal feeds a ‘bucket brigade’ delay line, in which diminishing delayed copies of the original signal feed further stages. The output of some or all of the stages are mixed with the original signal, as well as feeding earlier stages. This usually means some signal degradation (particularly loss of high frequency detail) towards the end of the decaying sound. This is not necessarily a problem, as the same thing happens in real reverberant spaces. Electronic reverbs are typically much noisier than digital units, however.
  3. Digital. Delayed copies of the original signal are diminished and mixed together using complex algorithms to determine delay intervals and feedback intensity. The summed output of the digital processing is mixed with the original signal.

How do you use it?

If all else fails, read the manual!

For connection details, refer to the unit's manual and/or to the section on Mixers. Usually only a single send is required (even where the reverb unit has stereo inputs), and this can be taken from any postfade auxiliary. Reverb is a stereo effect, however, so a stereo return is generally required (either to dedicated auxiliary returns, or to a stereo channel or pair of mono channels). If you are using reverb with a mono system you will generally need much higher levels for it to be noticeable, and it will not sound as natural as it would in stereo.

Generally, reverb is best used in moderation: if the sound of the reverb is too obvious, you are probably overdoing it.

The controls for spring, plate & older analogue delay units are generally limited to How Much Reverb you want. Modern units (apart from a few esoteric ‘retro’ models) are all digital. Most will have presets with descriptions (e.g. ‘Stadium’, ‘Small Hall’), which will enable you to find the kind of reverb you are looking for fairly quickly. Because of their historical common use in commercial recording studios, most will also have at least one ‘Plate’ preset (and some will have several, e.g. ‘Vocal Plate’).

As far as application goes, you need shorter reverbs on faster numbers, longer reverbs on ballads. Give the sounds that need to be ‘forward’ in the mix (e.g. lead vocal) less reverb than sounds that need to be further back in the mix (e.g. backing vocals & ‘fill’ instruments). Avoid reverb on low frequency instruments (primarily kick drum & bass guitar), as this will tend to muddy the sound overall. A lot of guitarists use their own reverb sounds (and many keyboards have inbuilt reverb effects), and adding another layer of reverb may not improve the sound of these. Indeed, too much reverb from the lead guitarist's pedal board can make it impossible to bring the lead solo as far forward in the mix as it needs to be (it may still sound ‘distant’ no matter how loud it is).

When it comes to fine-tuning, you may find any or all of a number of parameters. Often these can include the following approximate descriptions:

  1. Pre-delay. In a real environment this corresponds with how long it takes the sound to get to the first reflective surface before the first reflection takes place. As far as simulated spaces go, longer = bigger, shorter = smaller.
  2. Reverberation Time. Usually this corresponds with the time it would take in a real room for the signal to be reduced by 60dB. Here, longer = livelier, shorter = deader.
  3. Diffusion. In rooms with a few big flat surfaces, the reverberation more closely resembles closely-packed discrete echoes. In rooms with more elements to break up the reflected sound (pillars, shelves, etc.), the reverberation is more complex.
  4. EQ. In real life, high frequencies are broken up and die away faster than low frequencies. If ‘this’ has started to sound like ‘thissssssss’, some units give you at least a token chance to clean it up. If you are using mixer channels for returns, you can also use the channel EQ for this.

Do you need one?

Live venues often provide more than enough reverberation without any help from the sound system. However, a little reverb won't be noticed much at the back, and might improve the sound in the front few rows. Outdoors the sound will probably be a bit too dry without some help. Generally, at least one basic reverb unit is very desirable, if not absolutely necessary.

What sort do you need?

For live work, you want a unit that gives you a few good reverbs (a range of pre-delay and decay times is useful) and which is easy to operate. Highly complex menus and total user control over hundreds of subtle parameters are not much use without total familiarity and/or pre-programming and/or midi control. If you mean to program your reverbs before the show goes on the road, go for a unit that allows you to set up and recall user patches easily.