How the cochlea achieves its frequency selectivity
PAPER V — BIOLOGY · QUESTION 31 OF 33
In plain terms: The free review of the cells that amplify sound inside the ear — what makes hearing sharper than physics alone would allow.
The question as set
How does the cochlea achieve its frequency selectivity? Give the mechanics of the travelling wave, and say what the active process adds that passive filtering cannot.
The question in Paper V · Its section on the reference page
The question in context
This is question 31 of the 33 set in Paper V — Biology of The Truly Hardest Exam in the World, the examination of The College of All Minds. In the College’s subject index it is filed under Acoustics and Hearing and the ear. Like every question of the College, it carries three references of record, verified live at publication and described twice — technically, and in plain terms — so the ground can be judged before it is walked. The response required is an argument, not a survey, of at most three thousand words, as technical as the question demands and no more.
The references of record
The cochlear amplifier and outer hair cells
The active process characterised: prestin-driven electromotility supplying gain, sharpening tuning beyond the passive travelling wave.
In plain terms: The free review of the cells that amplify sound inside the ear — what makes hearing sharper than physics alone would allow.
Georg von Békésy — the travelling wave
The passive mechanics at source: the basilar membrane’s stiffness gradient and the place principle it establishes.
In plain terms: The free official record of the work that showed how the ear separates frequencies along a strip of membrane.
The travelling wave visualised, with the dispersion that produces a place code rather than a uniform response.
In plain terms: The free animations that make the cochlear mechanics intelligible before the mathematics.