The cochlea’s sharp frequency selectivity needs an unexplained second filter.

Evidence

Georg von Békésy and followers first measured the vibratory patterns of the membrane using ears that were not in prime physiological condition. They found that the frequency selectivity of mechanical vibration was far poorer than the selectivity measured from auditory nerve responses. This was explained by a hypothetical ‘second filter’ somewhere between these points. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 755

Categorization

• QUESTION: The second filter is ‘hypothetical’ — worth chasing what it was eventually identified as (active cochlear mechanics/outer hair cells).
• corroborated: 2 independent passages support this claim — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 755