Loudness models estimate total neural activity since it cannot be measured directly

Evidence

These models are based on the premise that the loudness of a sound is based on the total neural activity evoked by the sound: Greater neural activity leads to a louder sound. Because we have no direct way to measure or quantify total neural activity in the human auditory system, a model of neural activity forms the basis of the loudness estimate. These models are psychophysical in nature and are based on psychophysical, not physiological, data. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 770