Reverberation relates to the property of sound that was introduced in week one of this course: the property of propagation. Real spaces contain reflective surfaces, and when a sound meets a surface some of the sound energy gets absorbed by the surface material, but the rest of the energy is reflected, which is heard as a copy of the sound that is separated by a small delay. The further away the surface, the longer the delay. Since each surface absorbs some of the energy, each reflected copy of the sound will have lost some amplitude; it will be attenuated. That means the reflected copy (the wet) will not sound as loud as the original sound (the dry). Different surface materials will also absorb sound energy at different frequency ranges, though these are typically in the high end of the sound spectrum.

Reverberation is defined mathematically and is controlled by changing the values of various parameters relating to sound propagation, such as delay, room size and shape, etc.

There can be quite a variety of parameters that can be included by plugin developers, but we can mention what is typical.

These settings allow you to model the number of reflective surfaces (room shape) and the distance those surfaces are from you (room size). These, along with predelay are parameters associated with the early reflection part of the reverb.

The main settings typically provided with the diffuse reverb (aka late reverb) portion of an algorithmic reverb plugin are:

The reverb time controls the volume or size of the space being modelled. In most cases we want to keep this below two seconds. Few large spaces have greater than two seconds natural reverb time, so anything over two seconds will tend to sound unnatural.

These parameters model different attributes of surface materials. The sound absorbing effect of soft materials, like the fabric in curtains, can be simulated by filtering the high end frequencies using a high end filter. Hard solid materials like stone, that tend to reflect sound, can also be simulated by adjusting the density. The higher the density of surfaces, the more sound they will reflect.

Where the delay time tends to control the overall sense of the volume of a space, the spread allows us to adjust the apparent width of the space by adjusting the stereo spread of the sound. This parameter is the diffuse reverb equivalent to the stereo base setting for the early reflection settings in the above reverb plugin (GoldVerb). The way we process stereo sound is one of the key ways we build our natural subjective experience of space and direction.

Convolution reverb uses an Impulse Response (IR) to create a reverb.  This impulse response represents how the sound changes as it travels through an environment. Typically, these impulse responses are sampled from measurements of real physical acoustic environments.

The reverb plugin typically has a method for selecting an impulse response sample for a particular space or type of space. A limited range of additional settings like EQ, delay, etc are also often provided. However, the range of controls are far fewer than what is found in algorithmic reverb plugins.