Explain the role of the 'knee' parameter on a compressor when processing vocals, and describe how its adjustment affects the perceived sound.

The 'knee' parameter on a compressor determines how gradually the compression is applied as the input signal approaches and exceeds the threshold. The threshold is the level at which the compressor begins to reduce gain. A hard knee means that compression is applied immediately and fully once the signal crosses the threshold. A soft knee, on the other hand, introduces a gradual transition into compression, starting to apply gain reduction before the signal actually reaches the threshold and increasing the amount of compression as the signal moves further above the threshold. The adjustment of the knee parameter significantly affects the perceived sound of the compressed vocal. A hard knee results in a more aggressive and noticeable compression effect. It can be useful for controlling very dynamic vocals or for creating a more assertive and in-your-face sound. However, it can also sound unnatural or harsh if not used carefully, as the sudden gain reduction can be quite apparent. A soft knee creates a more subtle and transparent compression effect. It allows the compressor to gently smooth out the dynamics without sounding as obviously compressed. This is often preferred for vocals where a more natural and less processed sound is desired. A soft knee can help to even out the vocal's performance while preserving its natural expressiveness and dynamics. The choice between a hard and soft knee depends on the desired effect and the characteristics of the vocal performance. A soft knee is generally a safer starting point for vocal compression, while a hard knee can be used more deliberately to achieve a specific sonic result.