Cochlear pitch mapping exploits the mass of vibrating liquid varying with route length

Evidence

The vibration enters the liquid through the oval window and travels at a very high speed in the duct. The round window acts as a sink for the vibration, and one might think that the vibration finds a route there by penetrating through the basilar membrane. This vibrating liquid column can be seen as a mass–spring system, and, as shown by Equation (2.6), the lower the mass, the higher the resonance frequency. High frequencies thus favour a shorter route, since the mass of the vibrating liquid decreases if the route is shorter. In contrast, the lowest frequencies favour a longer route, since the mass of the system is higher there. Added to this, the vibration favours penetrating the basilar membrane at a position where its frequency matches the characteristic frequency of the membrane. Thus, many different characteristics are optimized to make the frequency-to-place conversion of acoustic vibrations. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 749

Categorization

• DEFINE: Explains the physical mechanism (liquid mass as mass-spring system) behind frequency-to-place coding.
• QUESTION: This explanation frames the traveling-wave route-selection physics — worth checking how this mass-to-resonance argument reconciles with the alternative phase-response account in para 753.