Josh McDermott, teaching MIT's course 9.35 Perception, covers how the inner ear breaks sound into its component frequencies and why that process is nonlinear rather than a simple filter bank. The lecture works through the mechanics of the cochlea and the role of hair cells, explaining how the basilar membrane's tuning gives rise to frequency selectivity and how outer hair cells actively amplify and compress incoming signals. McDermott connects the physiology to psychophysical measurements, showing how properties like two-tone suppression and compressive growth of response with sound level emerge from the ear's biomechanics rather than downstream neural processing. The seventy-seven minute session builds toward the auditory system's tuning curves and their implications for how loudness and pitch are computed. It is the second lecture in the course, assuming only the anatomical basics of the ear as background.