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Reverberation Effect: Theory & Practice

Reverberation Effect used in professional applications

In this article meant mostly for beginning musicians we will examine all aspects of the most popular effect in course of music composing.

We will start from scratch of the reverberation theory, then turn to its brief history before the digital era, take a look at types of reverberators, examine types and adjustable parameters of reverberation and finally give some practical recommendations of its utilization.

Idea of the reverberation effect

Reverberation accompanies any sound emerged in a natural acoustic environment. It results from reflection of any sound wave from obstacles and its return to the point of listening. That is why we perceive a direct source of an acoustic sound and its multiple reflections from closest surfaces - obstacles. Here is how it looks graphically: let it be a source of signal (2) and a listener (3) in a certain room (1). 
When a short sound impulse is applied we will have the following picture in the point of listening: 

The direct signal which has the highest intensity comes first. It is followed by early, or primary, reflections from walls, floor, ceiling which have lower intensity which depends on a distance covered and material properties. They are followed by secondary and multiple subsequent reflections with rapidly decreasing intensity. In a real situation sound impulses are lengthier than the time of arrival of first reflections, that is why reverberation is superimposed on a source sound.

Secondary reverberations, being behind the source signal by not more than 60 ms, are also often considered as early reflections.

The reverberation effect shows up in a richer booming volume sounding which is usually more pleasant for perception than a source "dry" sound.

In audio recording reverberation adds a sense of depth of space. A sound source with a brighter reverberation effect seems to be located at a longer distance from a listener. 

Reverberation is perceived wholly if intervals between reflected signals are less than 100 ms. If coming signals are separated by over 100 ms they are perceived by a human as separate echoes.

The smaller the room and lower absorbing capacity of surfaces, the longer the reverberation. Time of reverberation is duration of signal damping by 60 dB from the starting value.

According to time of reverberation and its depth in a natural acoustic environment it's possible to estimate dimensions of the room and its acoustic properties. Sound of a voice on a scene of a concert hall, in an empty room, in a room with a great deal of soft things differs considerably in a perceived timbre.

In a course of a natural reverberation an acoustic frequency spectrum changes. High frequencies fade faster than lower ones, that is why a timbre of the reflected sound is softer and more muted than of the original one. Amount of lost high-frequency components of the spectrum depends on a distance covered by the acoustic wave and properties of materials of reflecting surfaces.

History of artificial reverberation

The most natural and quality way to deliver reverberation in recording is to record in a good concert hall. It is obvious that this approach is not affordable in all cases that is why the need in a reverberation simulation gave birth several semiacoustic and semimechanic ways to get it yet before the digital electronic epoch.

An echo chamber was created first for simulation of reverberation. This is a small room with a sound-reproducing system and a microphone for recording. To make the effect stronger the walls of the room are covered with sound-reflecting discs or other similar materials. By changing a location of the speaker and the microphone it is possible to get some variations in a recorded sound.

A relatively popular method of reverb simulation was realized with big hanging metallic plates under tension with electromagnetic transducers attached. It was possible to control time of oscillations of such plate using damping. Vibrations of the plate simulate a real reverberation quite conditionally, nevertheless, this approach was applied in all modern digital reverberators.

A lower-quality variation of this approach was a spring reverberation used earlier in guitar amplifiers. One end of the spring had an electromagnetic transducer that made it oscillate and the other end had a sound-receiver that got all its useful and parasitic oscillations.

Digital reverberators

All modern digital devices can be divided into several types according to a technology, quality and a field of application.

The simplest group consists of reverberation devices used in multimedia sound cards like Diamond Monster MX300 based on the Vortex2 chip or unsophisticated electronic synthesizers with autoaccompaniment from Casio and Yamaha. Usually, it is possible to control an effect depth; no other adjustable parameters are provided.

Professional and semiprofessional sound cards, musical instruments and sound modules have quite good effect processors onboard where it is possible to choose a reverberation type and change parameters. Apart from general system effects (reverberation, chorus) there is at least one more assigned unit of effects which provides a couple of additional reverberation types.

It is considered that separate modular effect processors are the most flexible and of the highest quality. You can find among them good universal devices for which reverberation is one of a great number of available effects (Boss VF-1, Zoom RFX1000) and special studio reverberators (Sony DRE-S77, Yamaha REV500).

Software effect simulators have become very popular not so long ago. Software samplers and synthesizers support real-time reverberation. Contrary to hardware analogs which work in any conditions, when using software effects one should remember about scare resources (power) of a computer.

Simulators working not in a real-time mode have excellent flexibility and top quality. For example, the popular Cool Edit Pro contains a reverberator which models all possible rooms with a great heap of respective settings:

Reverberation types

Any modern digital reverberator offers several programs which simulate various real conditions for listening or synthesize some fantastic situations for special effects. Below the most popular examples are described.

Hall - simulation of acoustics of a concert hall. Deep reverberation with a great dying-down time. Subjectively, it separates a sound source from a listener.

Room - reverberation of a small room. It suits for acoustic instruments in a chamber atmosphere.

Live (Stage) - simulation of live stage performance; it is considered that this type suits well for soloing instruments.

Plate - simulation of a flat electromechanical reverberation of a metallic plate described above. It is used for vocal and percussion instruments.

Spring - lo-fi reverberation - it emulates the above mentioned spring electromechanical construction.

Chamber - simulation of the above mentioned room for recording of reverberation.

Gate - reverberation with cutting off the final phase of damping. It imparts a dynamic character to sound and is used for percussion instruments, in particular, for drums.

Reverse - artificial reverberation with an inverted envelope, i.e. first it gradually increases then suddenly breaks. Such effect can be obtained by inverting sound in the editor, applying a usual type of reverberation and reverse inversion. In this case reverberation starts even before the original sound. This is a specific effect, sometimes it is used for vocal.

One can also come across some more exotic types of reverberation. For example, the XG devices from Yamaha offer White Room, Tunnel, Canyon, Basement - reverberation in artificial conditions, and the Sound Canvas series from Roland provides Panning Delay - a separate stereophonic reverberation.

Parameters of reverberation

Here we will study adjustable parameters which are used in modern digital reverberators.

Balance (Dry/Wet) adjusts a ratio of a direct sound and an effect-processed one.

Density of early (primary) reflections characterizes geometry of an emulated room.

Diffusion of reverberation - when this parameter is low, the effect becomes discrete and sounds like echo.

Early Reflection Level corresponds to reflective properties of materials of the room.

Er/Rev Balance - ratio of levels of early reflections and a remainder of the reverberation.

Feedback Level.

High Cut - low-frequency filter parameters (equalizer). It makes a timbre of reverberation softer.

High Damp (LPF) - parameters of damping of high-frequency components of the reverb spectrum (sometimes level and frequency are controlled separately). It is based on a natural effect of faster damping of a high-frequency sound spectrum in course of an acoustic reverberation. To some extent it simulates properties of materials of reflecting surfaces of a room. 

Low Cut - HF filter parameters (equalizer).

Low Damp (HPF) - parameters of damping of low-frequency components of the reverb spectrum (sometimes level and frequency are controlled separately).

Pre Delay (Initial Delay) - an interval between the direct sound and early (primary) reflections (simulates dimensions of a room with a listener's location accounted for).

Release Density defines density of reflections of the final reverb phase.

Reverb Delay - an interval between early reflections and a remainder of reverberation.

Reverb Send Level (Depth, Volume). This is a main parameter that controls depth of the effect.

Reverb Time - time of sound damping by approximately 60 dB.

Size (Room Size, Hall Size, Height, Width, Depth).

Wall Vary defines geometry (imperfections) of reflecting surfaces. The greater the value, the more dissipated the reverberation.

MIDI reverberation management

The General MIDI (GM) standard provides only one parameter for management of a reverberation depth (level) - control message 91 (5BH).

In the GS and XG the management is extended. First off, with the NRPN it's possible to change a reverberation level separately for different percussion instruments. For example, here is how it's possible to reduce reverberation for a Kick of the standard GM Drum set:

# CC Value Comment
63H 1DH Set MSB
62H 24H Set LSB
06H 10H Enter a new value of reverberation level

The above values can be entered manually in the editing window of MIDI events of any sequencer (Events in the Cakewalk, Sonar).

Secondly, it's possible for the GS, XG, GM2 to change a reverb type on-the-fly. It is carried out by sending a special SysEx message. For example, the following message sets Room3 for a Roland Sound Canvas synthesizer (SC-8820):

F0 41 10 42 12 40 01 30 02 0D F7


F0 41 10 42 12 - head of the SysEx message;
40 01 30 - three bytes determine a character of a MIDI message - change of a reverb type;
02 - Room3 reverb type;
0D - checksum;
F7 - end of the SysEx message.

For the XG it looks like that:

F0 43 10 4C 02 01 00 02 02 F7


F0 43 10 4C - head of the SysEx message;
02 01 00 - three bytes determine an address for changing a reverb type;
02 02 - two bytes determine Room3 reverb type;
F7 - end of the SysEx message.

Thirdly, it's possible to change a lot of reverberation parameters. The table below shows adjustable parameters for the GS and XG compatible synthesizers:

Parameter GS (SC-8820) XG (SW1000XG)
Level yes yes
Time yes yes
Diffusion no yes
Pre-Delay Time (Initial Delay) yes yes
LPF yes yes
HPF no yes
Reverb Delay no yes
Density no yes
Er/Rev Balance no yes
Feedback Level no yes
Wall Vary no yes

They are realized by sending SysEx messages as well. For example, for an XG device the following message sets the maximum reverb time:

F0 43 10 4C 02 01 02 7F F7

In this case three bytes 02 01 02 determine changing of the reverb parameter - Reverb Time, and the last but one byte 7F sets the maximum duration.

To control the above parameters it's better to use special editors (XG Edit, GS Advan˝ed Editor etc.), which allow avoiding programming in hexadecimal codes.

Practical usage

Well, recommendations given below can be neglected; experiments and imagination are always welcome.

Usually, to reach the sense of integrity of space a reverberation type like Hall (Room, Live) is used for the mix on the whole; and for separate instruments of groups of instruments it's possible to use additional processing by the reverb processor to get special effects.

This effect can be used for modeling a scene's depth. Instruments with deeper reverberation are perceived as though they are located farther. And vice versa, an instrument or a voice without reverberation seems to be near.

One can get a lot of sound tints using effects in the stereo mode. For example, one can set the original sound in the center, short reverberation with low time of primary reflections to the left channel and with greater time to the right one.

Reverse reverberation creates an interesting effect for a vocal part. Besides, double reverberation - with short and long damping time - brightens up a voice.

Deep reverberation with long damping time suits well synch pads.

To get rougher dynamic feeling of rhythm in the mix for percussion instruments (drums) it's possible to use Gate reverberation.

Big drums and basses sound excellently with a small amount of reverberation or even without it.

And the main point is that you should never go to far with reverberation depth as it's easy to turn the mix into a mess with sounds overlapping each other. The faster the time of a composition, the lower the reverb should be.

Sergei Kotov (kotov@rosagrosnab.ru)

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