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While reverb Time, Pre-Delay, Wet/Dry Mix, and the spectral manipulation of Reverb Time are common, nearly universal parameters, you'll see other terms that are less well defined and aren't particularly consistent from one make and model of reverb to another. Terms like Diffusion, Density, Room Size, and similar words sometimes appear as adjustable parameters on our reverbs. Remember, all of the parameters are there to give us some ways recording engineers to interact with qualities of the reverb, but reverb is itself such a complex and musical sound, and we savored it in part by listening to fine details and attributes not captured by the core reverb parameters.
The number of individual contributing reflections within the impulse response is sometimes an audible trait. Surfaces, like the wall behind me, are common design features in concert halls and recording studios. These highly articulated, geometrically complex surfaces come in many forms, all with the goal of breaking up the sound. In architecture, bumpy surfaces like this diffuse sound creating a more complex impulse response. So parameters like Diffusion and Density drive the digital algorithm to emulate this feature of the architecture.
They don't change the Reverb Time or pre-delay, they just drive the build up of complexity within the reverb sound. I hear the effect of these parameters is changing the sonic texture of the reverb from open and gauzy to thick and smooth. While Reverb Time and pre-delay are easily defined and quantified, parameters like Diffusion and Density aren't. One plug-in might have a slightly different interpretation than another. These are simply adjustable parameters that go from some low value to some high value.
The numbers have no universal meaning across our industry. The approach is simply to fine tune and listen. A Room Size parameter likely tries to approximate the qualities associated with the big room versus a small one. Reverb time itself is of course intimately tied to room size, so adjusting the Room Size parameter often changes your Reverb Time setting as well, but it may also adjust the timing of the early reflections, close together in a small room and spread out in a large hall.
Depending on the particular make and model of your reverb a room size parameter might also be available influencing how the density of the impulse response builds over time. Again, Room Size is a concept with no universal definition for how it changes a reverb processor's algorithm, we can only fine-tune and listen for these types of changes. Some reverbs provide an image of a room, sort of an architectural blueprint of the space and allow you to adjust qualities in the architecture.
You can change qualities of the space like the number of walls and the dimensions of the room. Maybe there are parameters for changing the acoustic reflectivity of the materials that make up the floor and the ceiling and the walls. This is all an interesting way to interact with a concept of artificial reverb, but it's important to realize that this is just an alternative user interface for driving subtle sonic qualities of your reverb processor. You're not hearing the actual sound of the variable room you've just envisioned.
The reverb output is influenced by the science of acoustics for sure, but it isn't actually calculating the precise reverberant qualities of the space you modified with these parameters. While Reverb Time, even as it varies with frequency and pre-delay, are crisp concepts that are straightforward to understand and relatively easy for the reverb unit to generate, these other parameters are more abstract. You don't need to worry about hearing the precise audible changes of a small tweak to a parameter labeled Density or Diffusion, instead we view them as ways to coax the sound in different directions.
The typical approach is to listen to it at its preset value, then crank it up to an extreme setting and listen for sonic differences. Then crank it down to a value near the other extreme and listen again. Then return to the middle and listen. If global trends are revealed, and you like what you're hearing then adjust it to taste. If you don't hear much change, don't sweat it, it may be too subtle to bother with given the spectral and temporal qualities of the tracks you're mixing today.
In those cases, I just leave those other parameters at their default values and move on.
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