Final answer:
The point at which a hearing aid begins to apply compression to louder sounds and ceases linear amplification is known as the compression knee. This helps to protect the user's hearing from potentially uncomfortable or damaging noise levels.
Step-by-step explanation:
The point where a hearing aid departs from linearity and enters into compression is known as the compression knee. This is the level at which the hearing aid begins to apply compression to louder sounds to prevent amplifying them to levels that might be uncomfortable or damaging to the user. Below the compression knee, sounds are amplified linearly, meaning there is a one-to-one increase in decibels between the input and output. Once the input level exceeds the compression knee threshold, the increase in output is less than the increase in input, resulting in a reduced amplification or 'compression' of loud sounds.