If an engineer needs to heavily compress the signal, a hard knee compression will be very perceptible, especially when the signal crosses the threshold, and the audio will sound very unnatural. The same problem happens again when the signal goes below the compressor threshold, as the compression will abruptly stop. A hard knee compression is typical for hard limiters and de-essers.
Soft knee – devices with this mode are common in hip hop mixing and mastering services as well. A soft knee compression can gradually increase or decrease the compression ratio automatically until it reaches either a maximum level or the zero value, depending on how much the signal exceeded the threshold or dropped below it. Simply put, the compressor already starts working when the signal only approaches the threshold. The compression ratio then gradually increases until the signal reaches the threshold (however, the maximum ratio value is usually achieved a little above the threshold). The changes made by such compressors are less perceptible, so these devices are preferable in most cases. But if you want to add a fast attack to the instrument (for example, add a snap to the snare), then you must use a hard knee, because a soft knee won’t be very effective. As you can see, both device types became widely used in professional mixing and it’s impossible to say which of them is preferable – it depends on the particular tasks an engineer has to complete. To hear for yourself the ending result of these tips please check demos of professional mixing mastering services.