Reverb
Reverberation is probably one of the most widely used effects because it allows you to simulate the sound reflection characteristics of almost any kind of room. In a real room, reverb is a result of sound reflecting off room surfaces such as the walls, floor, ceiling and objects. The materials, size and shape of the room determine how long these reflections echo and decay before dying out completely. These factors also help determine the audio characteristics of the room, such as how long the high frequency reverberations ring when compared to the low frequencies or how much initial slapback the room wall produces when a sound hits.
As with most things is sound recording, the control parameters are dependent upon the functionality which is usually inherent of cost. The typical reverb unit will produce a variety of presets which can be tweaked to preference.
Types of Reverb
Plate
Before the digital revolution, plate reverb units were not just rack gear, they were rooms! Most recordings before digital technology used some form of plate reverb because it was very pleasing to the ear. Unlike most units today where parameters can be changes, the plate units didn't really have anything to change except microphones. They normally consist of 2 large steel plates, a speaker and 2 microphones. The sound is then played through this environment and the sound is recorded. With the hardware and software equivalents, all the parameters can be changed with ease.
Spring Reverb
Spring reverb is not usually known for its great sound as they tend to be found in guitar amps. They use the motion of a spring's vibration to transduce the signal back into electrical energy which can then be fed back into the dry signal. They tend to be noisy and inefficient which is why most studio applications tend to use other forms of reverb. However, some are quite sought after for example, Hammand Organs.
Acoustic Chambers
These tend to be rooms rather than units as this is more to do with technique than actual hardware. Creating an acoustic chamber usually costs a lot of money as everything needs to be surface treated and the room itself has to be a minimum of 2000sq./feet so no bass information is lost. However, it is possible to create a similar effect using stairwells or lift shafts. The sound can then be controlled using some form of absorbing materials (curtains, foam padding etc). The basic idea is to record the natural environment using a set of microphones.
Digital Reverb
With digital technology comes a set of reverb types which couldn't have existed before. The parameters to control them will usually control all the different modes.
The different types of digital reverb are:
Standard Reverb - Simple reverb with only the most basic parameters
Dual Reverb - Multi dimensional reverb with flexible frequency band-splitting capabilities. The reverb can be divided into primary and secondary stages using selectable high or low pass crossovers.
Stereo Reverb - A true stereo version of the mono reverb
Stereo Dual Reverb - A true stereo version of the dual reverb.
Gated Reverb - A very linear, high energy reverb that can be set to decay, stay flat, or ramp up the reverb decay, creating many unique ambient effects.
Room Echo - This mode is primarily governed by the delay which in most units can be defined or tapped in (tap tempo). This type of reverb is usually for creating small ambient spaces. The delays are divided into sections of early reflections. These reflections can be placed anywhere in a stereo field and can be as dense or sparse as necessary.
The diagram above shows a typical reverb envelope. After the initial sound has been fed into the reverb unit, the parameters described below affect the rest of what happens.
Parameters
Type
Hall, Room, Plate, Non Linear etc
Time
Usually the time taken for the revert level to decay to -60dB
Pre-Delay
The time taken for the reverb to start after the initial sound
Diffusion/Density
Controls the spacing between reflections
Early Reflection Level
Controls the level of the first reflected sound (usually a distinct echo)
Early Reflection Time
Controls the time taken foe the first reflection to reach the listener (gives the initial clue to the type of environment the listener is in)
High Frequency Damping
Progressively filters out the high frequency content of a reverberant space, high levels of damping will make the environment acoustically dead
Low Frequency Damping
Opposite of above, high levels will give the revert a brighter sound, due to letting only the high frequencies reverberate in the space