Reverb is the sound that remains in a certain environment when the direct signal has died out. To get a better picture lets take a macroscopic example: a gun-shot in a cathedral. It is admittedly a rather rare event to hear a gun-shot in a cathedral, but nevertheless it is easy to imagine, sound-wise anyhow! After the gun shot (the sound coming from the gun is a short blast) the sound echoes for a few seconds, slowly dying out. This fading echo is in fact the original sound reflecting on the surfaces it meets. We have seen how, in the phenomenon of reflection, a portion of the energy is absorbed and another portion is reflected. So, at every reflection the wave loses part of its energy until it dies out completely. Reverb follows very specific rules, summed up in the following diagram:
The diagram shows the amplitude of the various reflections and the instants in time in which they take place. The first sound that reaches the listener is still the direct signal (Direct Signal), the latter being the shortest path taken by the sound. After a brief pause called pre-delay, the early reflections arrive which are those that have met one surface only on its journey to the listener's ear. Finally the late reflections arrive which are those that have met more than one surface. They arrive overlapping one over another, thus generating a sound that is quite continuous. These late reflections, by the fact that they are very close to one another, are called reverb cluster. To reproduce such behaviour, very ingenious techniques have been implemented. We will now look at these techniques, whose importance lies in them being simulated on modern digital reverb-modules.
Spring Reverb: In order to simulate the reverb-effect a spring is used inside a cavity to whose far ends two transducers are applied which, in turn, apply the sound signal to the spring. A microphone is used to pick up the sound that is in this way generated. It's not a particularly realistic simulation; the spring is used in some guitar amplifiers, whereas it is rarely used with vocals. A digital module which uses this kind of reverb may use it to give a guitar a 1960's sound.
Plate Reverb Vertically suspended on a plate, two transducers are applied, one transmitting an input signal, and the other that picks up the vibrations that are generated. If the two transducers are placed at different spots the effects' characteristics can be varied. This technique offers a more realistic effect than that created by the spring delay, especially at high-frequency levels
Chamber Reverb the sound source is placed inside a room whose walls have particular reflective qualities. Here too, the room's sound is picked up by a microphone
This last generation's reverb machinery is practically all digital and employ highly sophisticated simulation-algorithms (whose creation was possible thanks to a continuous increase in available calculation power). Here is a list of the main controls:
Pre Delay: allows pre-delay-time to be controlled
Early Reflections: Length of the early reflections
Decay: Decay-duration of the late reflections
Mix: the percentage-levels of dry and wet (reverberated) signals
Room dimensions: often values are referred to the shape-dimensions of environments (hall, room, chamber, cathedral, spring/plate)
HF Ratio: high frequencies are the first to be attenuated during reflection. This control allows us to simulate the absorbing-capacities of surfaces
Stereo width: widens or tightens the stereo image of reverb
Some units allow different decay-times to be defined for different frequencies.
The following is the sound of a dry instrument followed by the same instrument to which reverb has been added in different modalities.
Dry sound [Track 19]
![Effects and signal processors - Dry sound [Track 19]](/images/en/figures/layout/icona_suono.jpg "Effects and signal processors - Dry sound [Track 19]")
To the same sound we will now add different kinds of reverb:
Table 7.1. Different kinds of reverb
|
Related topics on Wikipedia
Behaviour of reverberated sound