The second-most powerful sound-shaping tool (after EQ) available to mix engineers is compression. This is most commonly used to reduce the dynamic range of a sound. More extreme compression can be used to reduce the crest factor of a sound. Unlike EQ, excessive amounts of compression might not sound unpleasant. Here, it depends on the style of music. A lot of acoustic folk would sound silly with extreme compression. On the other hand, a lot of modern electronic dance music would sound silly without extreme compression. It’s no coincidence that the more important loudness is for a style of music, the more compression is tolerated or even expected.
When using compression to increase loudness, it’s often useful to start with extreme settings and then back off until it sounds natural or acceptable. Usually, this means starting with fast attack and release times, high ratio and low threshold. First increase the release time until distortion is low enough to be acceptable. Then reduce the ratio and/or raise the threshold if you want to retain some of the original dynamics of the sound. There are many different types of compressors, and you might find that even a modest collection provides a wide variety of sounds and colours. The differences between compressors are most apparent at the kinds of extreme settings described above. It’s worthwhile trying different compressors on particular difficult or sensitive sounds such as kick drums.
While EQ and compression alone are sufficient for many styles of music, sometimes mix engineers need to go further. Saturation can be handy here. While there are many, many different kinds of saturation, they all have one purpose (when used deliberately) – to destroy crest factor. For sounds with a high crest factor, peaks are crushed and made noisier. For sounds with a low crest factor, more steady upper harmonics are generated, which increases the energy in the upper-mids. At extremes, saturation sounds like a kind of mild distortion. Broadly speaking tape-style saturation is often softer and smoother than tube saturation, which itself is softer and smoother than native digital clipping. Which style of saturation you use on a sound will largely depend on the nature of the sound, the behaviour of the specific tool you’re using, and the creative direction of the mix. As with compressors, each saturation tool is different and it’s often worthwhile trying different tools on difficult or sensitive sounds.