Weber's Law of Just Noticeable Differences

Weber's Law states that the ratio of the increment threshold to the background intensity is a constant. So when you are in a noisy environment you must shout to be heard while a whisper works in a quiet room. And when you measure increment thresholds on various intensity backgrounds, the thresholds increase in proportion to the background. Weber's Law predicts a linear relationship between the increment threshold and the background intensity.

source: https://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_3/ch3p1.html

Weber's Law (pronounced vay-ber) can be applied to variety of sensory modalities (brightness, loudness, mass, line length, etc.).  The size of the Weber fraction varies across modalities but in most cases tends to be a constant within a specific task modality.

source: http://apps.usd.edu/coglab/WebersLaw.html

It has been shown not to hold for extremes of stimulation.

source: https://www.britannica.com/science/Webers-law