The parameter "Reverb density" usually controls the "thickness" of the reverb tails. A high-density setting will result in a much smoother effect due to thickness of the reverb.
On the other hand, a low-density setting makes the reverb tail to sound thin and grainy. Although with low settings of reverb density discrete echoes can be clearly discerned
However, on some reverb plugins, there might also be a density parameter specifically for "early reflections" and another one for "late reflections."
So in a scenario where you have separate density parameters. It means you will be able to adjust the thickness of early reflections and subsequently adjust the thickness of late reflections.
Basic settings
In order to set the right amount of reverb density, you need to have a vision of a particular sound you are aiming for, or whatever sound design idea you have in mind.
For example, to replicate large rooms you may need to set the density control fairly low. That's because a large room typically has very spaced-out reflections and will also have a much longer reverberation duration e.g. reverb decay time.
As for emulating smaller rooms your best bet is to have a fairly high amount of reverb density. You want your reverb reflections to be quick and not only that but also thick.
But high reverb density is more suitable for percussive instruments to reduce the unwanted metallic effect similar to flutter echo.
It's also important to remember that the higher the reverb density the more space will be taken in the mix. Although the result is a smoother and even more natural sound.
Less dense reverbs don't have so much space in the mix but may sound unreal. Another thing is that the sound will not be thick and smooth.
On the other hand, a low-density setting makes the reverb tail to sound thin and grainy. Although with low settings of reverb density discrete echoes can be clearly discerned
However, on some reverb plugins, there might also be a density parameter specifically for "early reflections" and another one for "late reflections."
So in a scenario where you have separate density parameters. It means you will be able to adjust the thickness of early reflections and subsequently adjust the thickness of late reflections.
Basic settings
In order to set the right amount of reverb density, you need to have a vision of a particular sound you are aiming for, or whatever sound design idea you have in mind.
For example, to replicate large rooms you may need to set the density control fairly low. That's because a large room typically has very spaced-out reflections and will also have a much longer reverberation duration e.g. reverb decay time.
As for emulating smaller rooms your best bet is to have a fairly high amount of reverb density. You want your reverb reflections to be quick and not only that but also thick.
Summary
In a nutshell, low-density settings can retain clarity when applied to sustained sounds such as pads or vocals.But high reverb density is more suitable for percussive instruments to reduce the unwanted metallic effect similar to flutter echo.
It's also important to remember that the higher the reverb density the more space will be taken in the mix. Although the result is a smoother and even more natural sound.
Less dense reverbs don't have so much space in the mix but may sound unreal. Another thing is that the sound will not be thick and smooth.
