The previously released reverb effect is commonly known as Level 1.0 reverb in three-dimensional audio technology. In 1999 a new method for describing the reverberations has been standardized to account for the basics of human hearing, and to provide knowledge for evaluating performance of 3D audio and called the Level 2.0 specifications, which is also known as the I3DL2 reverb. The new standard adds more advanced models for environment reverberation, distance, occlusion, and obstruction. These guidelines define the minimal rendering requirements, lexicon of terms, and evaluation guidelines for the 3D audio developers, renderers, and vendors.
When a sound occurs in an enclosed environment, such as a room or a cave, the sound waves travel through the air and reflect off the surfaces of the enclosure, creating the sound of a short echo. Each of these first reflections travels through the air and reflects off all the other surfaces, creating even more echoes. At each reflection some energy is absorbed by the surface making the echo slightly quieter. This process continues, creating the overall effect of an increasing number of echoes as time progresses, while the level of the echoes decreases over time. This effect is known as reverberation.
What we call the reverb effect, i.e., the artificial reverberations, is designed to emulate the sound of reverberation created by enclosed environments. It can be used to create a sense of space, and the reverb parameter settings can be changed to create different types of space. For example, if a game character is inside a large stone room you would want a reverb where the first reflections are spread far apart (due to the time it takes the sound to travel across the large room), and where the sounds take a long time to decay (due to the high reflectivity of the stone walls and floor.) If the character then enters a small carpeted room with lots of furniture, the reflections would be quicker and would decay more quickly (due to the sound being absorbed by the surfaces.)
In describing the effect of reverb we divide the sound into three parts. The direct sound travels directly from the sound source to the listener and arrives first. The direct sound is created by the signal which travels through the mixer without going through any effects.
A short time after the direct sound the early reflections begin to arrive. These first reflections are few and relatively spread out in time. As the reflections multiply they eventually become so close together that they are indistinguishable. We are now in the late reverb, and the levels of the reflections decay until they are inaudible.