Answer:
The phenomena that best explains what Susan is experiencing is the Weber's Law.
Step-by-step explanation:
The Weber's Law states that there's a quantitative relation between the magnitud of a physical stimuli and how this is perceived, this law formulated the mathematical relation that existed between the intensity of the stimuli and the sensation or the perception produced by the stimuli. So in our case when Susan changes the volume and as she perceives "Only slightly" she notices a large difference therefore stablishing a relation between the stimuli and what she perceived, this whole phenomenah could be best explaint by Weber's Law because this relation is what the law studied and explained.