Digital Reverb Explained

Digital reverb simulates a natural or unnatural ambient space using a mathematical model aka algorithm. When an impulse signal (IE music) produces sound pressure in an ambient space surrounded by reflective surfaces, the sound propogates in all directions around the space then bounces off each of the surfaces to be returned to a position of listening reference.  These multiple returns are called early reflections.  They continue to bounce all over the room in an accumulative manner while decaying over time, forming diffuse or late reflections.  These reflections mixed with the direct impulse signal defines reverb.

There are many variables that shape the sound of the reverb.  Composition (wood, tile, metal) and flatness of the reflective surfaces affect how impulses are reflected or absorbed.  Whether the ambient space has parallel or non-parallel walls and/or ceilings impacts the behavior of the reflections.  Any absorbant materials or obstructions in the room such as carpets, human bodies, support columns, acoustic treatments, or even air humidity will effectively attenuate the reflections, which alters the reverb sound.   Thus the stereo spatial movement and frequency response of these reflections is dynamically changing.  The time displacement of these densely packed reflections is so small that the ear does not perceive them as separate echoes, and the multiple reflections are so numerous - they can be in the thousands - that our two ears perceives them as a three dimensional psychoacoustic phenomenon we know as "reverb". 

Much research has been done (and continues to this day) on deriving a mathematical model that allows an embedded system to emulate a natural ambient reverb.  These models are not at all simple and the algorithms used on digital reverbs are a highly guarded trade secret.  The study of reverberation is a complex one and few in the industry has a competent grasp on the subject. 

It would be convenient for the rest of us if we could use terms such as non-parallel wood walls, carpetted floor, arched open ceiling to define our reverb.  But these are not input variables that can "plug into" the reverb model of a digital reverb - we are instead given foreign terms such as RT60, diffusion, predelay, etc.  There is a lot of confusion over how these parameters work and how they can be applied, so I have compiled these tables from multiple sources to make the operation of digital reverbs a lot clearer.

Reverb Classes

Reverb Classes
Description Uses
Simulates a concert hall of square or flared structure.  Sounds dark and splashy, lending a sense of space and depth.  Puts the sound of the music behind it rather than with it.  With moderate use, hall reverb adds ambience and does not color or muddy the direct sound.  Clean distinct initial sound due to few early reflections that build in number as time progresses.  With large halls, they can sound cluttered with too many diverse sound sources.  Subclasses include the sound source in the middle of the hall, or at one end of the hall.  The latter is more common.
Traditional, classical, popular music.  When used on a mix, produces the sense of multitracked instruments in the context of a real-sounding acoustic space and belonging to the same performance.
Simulates ambient rooms of square structure and low ceilings.  Usually most effective with short RT60 times and can contribute to the clarity and presence of an individual instrument or a mix.  Effective use can "lift" an instrument out of a dense mix better than simply turning it up.  Variations of small, medium, large refer to the size of the emulated room and are independent of RT60 time.  Best used as a subtle effect.
Snares, marching drums, percussion, drum kits, vocals, piano, acoustic guitar, ethnic ensembles, strings.  Inherent high diffusion may make signals with fast transients (percussion, piano, guitar) sound grainy at long RT60 times.
A chamber is a room specifically designed as an open ambient space with a speaker that reproduces the signal source, movable curtains to alter the reverb response, and a movable microphone to capture the reverb sound.  Many major studios had separate room(s) for this purpose, and there is no reason that a stairwell or a tiled bathroom could serve the same purpose for a different reverb.  Chamber also can be the ambient space of small court spaces of 17th century baroque era mansions of stone or marble construction.  Sound is livelier with low coloration, and decay tail gives little or no sense of a specific space unless predelay and/or pre-echoes is used.  Tends to "fill up" the frequency spectrum of a mix.
Traditional, classical (especially small ensembles), popular music.  Can be effective at high reverb levels.
Mimics the sound of spring-tensioned suspended metal plates that are excited by a transducer.  Plate reverb was a 1960s early attempt to create artificial reverb, not designed to emulate any specific natural ambient space.  They have very dense early returns, high initial diffusion, and a bright colored sound with an emphasis on high frequencies.  The sound becomes more colored and denser with decreasing plate size.  Has an inherent initial high diffusion.  The sound of plate reverb is what most people associate with the word "reverb".
Percussion, vocals, brass, reeds, strings.  Useful for all popular music.  Designed to be heard as part of the music, mellowing and thickening the initial sound itself.
Simulates the perspective of a performer on a stage floor.  Low diffusion, high initial reflections, and slight coloration with some emphasis on midrange frequencies.  Strong pre-echoes from the stage side and rear walls.

Individual instruments or mixes.

Also known as "church".  Simulates a congregation hall constructed of stone floors, walls, ceilings with a high ceiling.  High reflections produce a very colored resonant sound.  Long RT60 times can muddy the direct source.
Choirs, organs, string quartets.  Not recommended for speech.
A 1950s early attempt to simulate reverbrant spaces using a set of two or three suspended springs that are tensioned.  The springs are coupled to transducers at each end.  When a sound source is presented to the drive transducer at one end of the spring assembly it produces a mechanical impulse that is propogated back and forth from one end of the spring to the other, with RT60 decay inherent into the spring.  A receiving transducer at the other end of the spring assembly captures the reverb sound.  RT60 time is fixed and sound quality varies with tension of springs, number of springs, drive/receive circuitry, and alloy composition of spring.  Very common item in guitar amplifiers and electronic organs.
Guitar, jazz/rock organ
Inverse Room
Used for unnatural reverbs.  Allows the envelope of the reverb tail to be continuously varied.  Can produce a reverb for a fraction of a second then abruptly drops off.  Similar to gated reverb but without any threshold control.  With a slope control an inverted reverb can be created.
special effect
Used for unnatural reverbs - the reverb sound is played back in reverse, with the decayed tail gradually increasing in volume.  Best used with a short sound with a percussive attack as an dramatic special effect and with moderation.
special effect
A popular 1980s effect in which a gate is inserted at the output of a reverb, whose threshold is adjusted such that the reverb is prematurely muted.  Can sound explosive and will alter the feel of a track.  Easily overused.
Toms, snares, kicks, bass, guitar, synthesizers, background vocals.

Reverb Parameters

RT60 or Reverb Time or Decay
Sets the time required for 60dB of decay at 1Khz.  Can be impacted by size parameter.
Usually the RT60 time should be inversely proportional to reverb level.  Long RT60 times can work in a sparse mix but  can clutter a busy mix.  Best approach for busy arrangements is short RT60 on individual instruments so that they each have their unique ambient space that do not conflict each other in the final mix.
Delays the input signal to the reverb processor.  Settings of 40ms or less allows the reverb to blend closely with the direct sound.  External analog BBD or tape delay is sometimes substituted for their warm sonic sound.  General guidelines: Up-tempo drums/percussion 25-50ms Ballad drums/percussion 40-80ms Vocals 75-125ms String Section 100-200ms Acoustic instruments 45-90ms Brass 50-100ms

Enhances clarity of direct sound or front-to-back imaging.  Conveys distance and volume within an acoustic space.  At moderate predelay times > 40ms the reverbration is placed behind rather than on top of the direct signal.  Can produce punchier less washed out mixes.  Long predelay times can sound unnatural but may have interesting uses.
RT Contour EQ
Alters the attenuation or gain of the signal source at selected frequency centers before it enters the reverb processor.  External EQ can be used.  Starting points can be 50-120hz and 7-12Khz.
Simulates reflective quality of absorbent or hard surfaces from warm-sounding old wooden concert halls to hard metal walled rooms.  Can be used to correct unwanted resonances or to correct dull/edgy/rumbly reverb sounds.
Rolloff or HF  Cutoff
Uses low pass and/or high pass filters to roll off high frequencies and/or low frequencies before the reverb processor.    EQs affect only those harmonics within a bandwidth surrounding the frequency center, while filters affects all harmonics above (low pass) or below (high pass) a cutoff frequency at a slope of (x)dB/oct.  Typical low pass filters: 3Khz @ 6dB/oct, 7Khz @ 6dB/oct, 10Khz @ 48dB/oct.
Mimics the effects of air absorption due to environmental variances.  Prevents large size reverbs from sounding unnaturally bright.  Natural reverbs contain few harmonics above 10Khz.
Simulates the slap-like echo(s) heard before the reverb which are the first reflections of an ambient space of hard surfaces.  Not the same as predelay.  Number of reflections, timing, and level varies depending on reverb class.  Some digital reverbs offer variations of echo times and level.
Can be used for doubling (50-80ms) and echo effects combined with the reverb.  Can intensify the initial punch of drums (0-40ms).  May interfere with the feel or tempo of the music.  Can simulate the effect of a procenium arch in a concert hall.  Use caution if echo time is tempo based.
Controls the extent to which the initial reflections of a reverbrant space are smoothed out over time.  Higher diffusion emulates rough irregular reflective surfaces or obstructions in free space which creates a smooth more mellow more colored sound while low diffusion emulates a non-obstructive smooth flat reflective surface which creates a clearer brighter less colored sound.  Can be impacted by position, size, and density parameters.
Simulates different reflective properties of materials such as hard flat surfaces (tile, metal, glass) or surfaces with rough irregular cavities (unfinished wood).  High diffusion works well with signals containing fast transients (percussion, piano, guitars) while low to moderate diffusion works well with signals of slower transients (vocals, brass, reeds, strings).
Controls the time displacement and number of the discrete closely spaced reflections in an ambient space.  Higher density settings creates richer sounding reverbs with more even natural reverb tails.  As density is lowered the reverb sound thins out and the reverb tail may "flutter".
Decreasing density may not be pleasant on fast transient sources such as percussion.
Varies the envelope properties of a reverb.  The time it takes for the reverb sound to build-up and decay 15dB determines the perceived reverb time regardless of RT60 setting.  At minimum settings, the reverberation envelope builds up very quickly to maximum amplitude then decays quickly at a smooth rate set by RT60.  Increasing this parameter slows the build-up and introduces a sustained stage (see Spread parameter) before the decay stage.  High settings introduces a secondary lower level sustain portion to simulate the very diffused reflection off the back wall of a hall. Extreme high settings results in an inverse sound.  Predelay is not an effective substitute as it does not create the same effect.  Can be impacted by size parameter.
Effective for creating a sense of depth and space without using long RT60 times.  Echograms of some very good recording halls reveal a rather uneven initial build-up and decay, giving a much longer effective reverb time than the RT60 setting would suggest.
Controls the attack time and length of sustain at increasing settings of the Shape parameter.  Can be impacted by size parameter.
Combined with Shape parameter, can provide the sonic impression of room size.
Simulates the listener's perspective of any position in an ambient space relative to the sound source.  Impacts arrival time, energy, frequency response, and diffusion of the early reflections.
Creates sonic image of any position in a large ambient space - close to the sound source, the rear of the ambient space, or anywhere in between.
Simulates the size of a room which impacts the sonic character of a reverb class.  Diffusion, RT60, shape, and spread are usually scaled with this parameter.  This parameter refers to the longest dimension of the room and is usually in units of meters.
Simulates any room size from a closet to a living room to a recital hall to a concert hall to the Houston Astrodome.  As size increases, diffusion should be increased.
Shortens the RT60 times at selected frequencies.
Decreases reverb resonance at desired frequencies of a signal source.