ValhallaRoom: Early Reflections versus Early Energy

The reverberation algorithms in ValhallaRoom are separated into 2 sections, Early and Late. This is a fairly common division of tasks in algorithmic reverbs. However, while most reverberation algorithms describe the early part of the reverberation as Early Reflections, ValhallaRoom’s early section would be better described as “Early Energy.” The important distinctions:

  • Most reverberators that feature “Early Reflections” use up to a few dozen discrete delay taps, or echos. These delays are often based upon a ray-traced model of a room, but fail to take into account the diffusion found with real acoustic surfaces, and the huge amount of complexity that is found in a real performance space. Things like furniture, seats, columns, and other performers and musical instruments, all combine to create a reflection pattern that is far more complex than what a few dozen delay taps can reproduce.
  • In order to add the complexity that is missing from the tapped delay early echo generation, some manufacturers have resorted to allpass delays in order to increase the echo density. This can work well for many signals, but can result in metallic artifacts on vocals and impulsive signals such as drums. I’ve described this phenomenon extensively in an earlier post. The “Diffusion” control in most reverberators can be seen as a way of dealing with the unpleasant side effects of allpass filters, by allowing the user to control the allpass delay coefficients to suit particular input signals.
  • ValhallaRoom doesn’t attempt to model any particular room geometry. Instead, it takes the approach that the early energy should be as dense, colorless, and spacious as possible, in order to emulate an “impossible” room. The user can dial in the duration of the early energy (using the Early Size control), to emulate smaller or larger spaces. A range of 10 to 50 milliseconds is common for smaller spaces, while larger durations are useful for emulating concert halls and cathedrals, where the initial reverberation tends to take some time to build up intensity.
  • The crossfeed of energy between spatial channels in ValhallaRoom can also be controlled with the Early Cross control, to create reverbs that have a realistic spread of energy between left and right channels, or an unnatural separation of the early energy that can be useful in preserving the initial spatial image of a stereo mix.
  • The Early section of ValhallaRoom has no allpass delays in it, which allows it to avoid the coloration caused by short allpasses. The Early Diffusion control adjusts the overall echo density, but it is mainly there for special effects – you can turn Diffusion all the way up, without getting the “ringing” sounds associated with high diffusion settings in some algorithmic reverberators.

ValhallaRoom was designed from a pyschoacoustic perspective, as opposed to a physical perspective. The goal was to create a sound that produces the impression of an “ideal” room, versus a specific room that exists in the real world.

About the author:

Sean Costello is the "algorithmic reverb plugin wizard" [citation needed] at Valhalla DSP.

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