Soak it in
To understand what damping is all about you first need take a look at what happens in real life. When a sound bumps into an obstacle, part of it is reflected off the surface while another part is absorbed by it. The reflections produced give rise to the reverberation phenomenon, which has a natural tendency to gradually engulf certain frequencies as time goes by. Even though the obstacle itself directly affects the frequencies that are reflected or not, providing useful information about the environment, most materials tend to absorb mainly high frequencies. What's more, these same frequencies are the ones that have a hardest time traveling in the air around us.
In this sense, it would be useless to try to EQ the treble before it goes into the reverb or after it has come out with the intention of reproducing this natural behavior, since the decline in high frequencies would be static through time. That's why reverb plug-in algorithms feature a damping option, to replicate this natural decline of frequencies depending on time. Generally speaking, damping is expressed as a ratio. For example, a damping of 1/2 on a two second reverb means the high frequencies disappear after just one second, while the rest of the signal extends for another second.
That's all very nice, but what use does it have for you? Besides making the reverb seem more real, it might come in handy to get rid of unwanted noise that would otherwise have a lot of presence in the reverb's diffused sound field. If that's the case, why not remove these noises directly from the source signal or right before they go into the reverb, you ask? Quite simply because as unwanted as they are, these noises are necessary for a correct and realistic interpretation of a sound by the brain. So, what are these dreadful noises that seem to just want to make your life more complicated? There are lots of them actually, but the most common are sibilants, breathing noises of wind instruments, mechanical noises produced by the instruments, etc. Although these types of sounds aren't very pleasant to human ears, you can't do away with them completely or you risk not being able to tell what's actually going on. That said, there's no need to have them in the reverb tail and and the damping feature is useful for reducing their presence in it. There's no need to go too wild with damping, however, subtlety is the keyword again.
Finally, some damping controls feature a crossover-frequency setting, which allows you to control the part of the spectrum that's affected by the phenomenon. The fanciest plug-ins also come with a damping option for the low end, which can be helpful to avoid ending up with a reverb that's too intrusive.
In the next installment we'll discuss post-reverb EQing.