Amount of masking is expressed as a decibel difference, not an absolute SPL level

Evidence

When referring to amount of masking, we use dB and not dB SPL. The decibel level of a sound always is in reference to a quantity, and 20 µPa is commonly used and is the typical reference for dB SPL. On the other hand, differences in dB between sounds are referred to using dB. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 460

Categorization

• DEFINE: Clarifies the crucial distinction between dB as an absolute reference level and dB as a difference measure — a common point of confusion.
• XREF: connects to: A decibel value is meaningless without an attached reference level — Almo Farina, Soundscape Ecology: Principles,…, 1943