Audibility thresholds can be expressed on physical or perceptual decibel scales

Evidence

In 1951, results from the “Beasley survey” were adopted as the basis for calibration of audiometers in the United States and determined the relationship between dB SPL, which uses a physical reference, and dB HL, which uses the perceptual reference, in the ASA-1951 standard. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 103

Categorization

• DEFINE: Defines dB SPL (physical reference) versus dB HL (perceptual reference), a core distinction for hearing measurement.