Answer:
- Just-noticeable difference.
Explanation:
'Just-noticeable difference'(also known as JND) is demonstrated as the minimum amount of stimulation that needs to be altered or changed in order to create an observable or noticeable difference. The noticeable difference needs to be half of the threshold point.
In the given situation, Carol would have to alter the volume(more than usual) in order to attain a 'JND' which is half than the threshold point('Carol gives the volume control ...half turn'). At her mother's constant yelling, she has to lower the volume even below this point which is known as 'Just-noticeable difference' which her mother could now experience.