Reverberation or reverb, in music is a persistence of sound, or a very short echo after a sound is produced. Reverb is created when a sound or signal is made inside an enclosed or semi enclosed area, the reflected sounds from the enclosing walls causing complex sound reflections to build up and then decay as the sound is absorbed by the objects in the space – which could include furniture, people, walls etc. This is most noticeable when the sound source stops and the reverberations continue, their volume decreasing until zero is reached. Reverb is pitch dependent, as sounds of different pitches will arrive back to the listener at slightly different times, and unless the room is very carefully designed it will have a resonant frequency at which sounds will be emphasized.
In comparison to a distinct echo, which usually 50 to 100 ms after the previous sound, reverb is made up of reflections of sounds that arrive in less than about 50 ms. As time passes, the level of the reflections gradually reduces to non-noticeable levels. Anywhere there are surfaces to reflect sounds from you get reverb. The more complex the shape of the room and the more objects in it the more complex the reverb is. Certain frequencies can be boosted, and others cut due to the wavelength of the sounds interacting with the size of the room (resonance). It may be created through physical means, such as echo chambers, or electronically through audio signal processing. There are various means of achieving a reverb effect listed below.
Echo chambers
The first reverb effects, introduced in the 1930s, were created by playing recordings through loudspeakers in medium to large spaces (various spaces such as empty rooms, bathrooms and even stairwells have been used) and mixing the sound with the original using strategically placed microphones.
The American producer Bill Putnam is credited for the first artistic use of artificial reverb in music, on the 1947 song “Peg o’ My Heart” by the Harmonicats. Putnam placed a microphone and loudspeaker in the studio bathroom to create an echo chamber, adding an “eerie dimension”.
The first two examples of reverb (Spring and Plate) are included just to give some historical background as we are going to be using digital sound processing to achieve our reverb effects.
Plate reverb
A plate reverb system uses an electromechanical transducer, similar to the driver in a loudspeaker, to create vibrations in a large plate of sheet metal. The plate’s motion is picked up by one or more contact microphones. The audio signal from these is then mixed with the original “dry” signal. Plate reverb was introduced in the late 1950s by Elektromesstechnik with their EMT 140 design. The greatest problem with plate reverb units is their size and weight, which limits their use to recording studios.
A spring reverb.
Spring reverbs, were introduced by the company Bell Labs, using a set of springs mounted inside a box. They work in a similar way to plate reverb, with a transducer at one end of the spring, and a pickup placed at the far end of the spring, to reproduce more realistic reverb different length springs could be mixed together (longer spring = longer delay time) .
They can have a very distinctive “twangy” sound with loud percussive sounds due to the springs. One major drawback is sensitivity to outside vibration.
They were popular in the 1960s, and were first used by the Hammond company to add reverb to Hammond organs.
They became popular with guitarists, including surf musicians such as the Beach Boys, and spring reverb could easily be built into guitar amplifiers.
They were also used by dub reggae musicians such as King Tubby. The American engineer Laurens Hammond of the Hammond company was granted a patent on a spring reverb system in 1939.
Digital reverb
Digital reverb units simulate reverb by using multiple delay lines that have different delay times and variable feedback, giving the impression of sound bouncing off of multiple surfaces. Some digital effects allow users to independently adjust early and late reflections. Digital reverb was introduced in 1976 by EMT with the EMT 250, and became increasingly popular with many groups and studios in the 1980s.
Gated reverb
Gated reverb combines reverb with a noise gate, creating a “large” reverb sound with a short tail (the tail is cut short by the noise gate).
It was pioneered by the English recording engineer Hugh Padgham and the drummer Phil Collins, and became a staple of 1980s pop music.
Convolution reverb.
Convolution uses impulse responses to record the reverberation of physical spaces and recreate them digitally. The first real-time convolution reverb processor, the DRE S777, was announced by Sony in 1999.
Convolution reverb is often used in film production, with sound engineers recording impulse responses of sets and locations so sounds can be added in post-production with realistic sounding reverberation.
Basically, a convolution reverb takes an input signal (the sound to be reverberated) and processes it with the sound of an actual or virtual acoustic space to create the illusion that the input was recorded in that space. The sound of the acoustic space is captured in what is called an impulse response (IR), which often starts as a recording of a short, sharp sound, such as the firing of a starter pistol or the bursting of an inflated balloon (the impulse), in the acoustic space in question. As you can imagine, such a sound excites the reverberation (the response) in the space, and so the impulse response (or at least its initial recording) sounds like an explosion followed by the reverberating reflections created by the recording space.
Once you have the IR for the space as a file, you then load it into the convolution reverb and input your sound to be processed. At that point, the software convolves the two digital audio signals together to create the output. Convolution itself is a mathematical process that has many applications including statistics, image processing, and electrical engineering as well as audio processing (It’s a very complex subject, understanding the precise details of how it works is not easy!). If you like, you can think of convolution as a kind of multiplication of each sample of the input with each sample of the IR, with the result being that the audio input takes on the sonic characteristics of the space in which the original IR was recorded.
Shimmer Reverb.
Shimmer reverb alters the pitch of the reverberated sound up or down in frequency by placing a pitch shifter in the delay lines feedback loop.
Once we apply any feedback in the reverb the pitch of the reverberated sound will steadily change up or down in pitch depending on the amount of pitch change and the feedback level. Shimmer reverb is often used in ambient music. Brian Eno has been one of the earliest adopters of this sound.
