Clinical threshold changes are measured relative to normative curves, not in absolute dB SPL

Evidence

It is clear from Figure 2–10 that auditory pathology can change the shape of the MAP and produce an upward shift in the MAP. However, because the MAP function is not constant across frequencies, we would need to subtract the shifted MAP values from the normative MAP values to determine the threshold change imposed by the auditory pathology. Although this calculation is fairly straightforward and is easily visible in Figure 2–10 when the normative MAP values are plotted along with the measured threshold values, measurements are not conducted in dB SPL in clinical applications, as we discuss in the next section. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 266