Basilar-membrane frequency sorting is too crude to explain our pitch discrimination

Evidence

Even a pure sinusoid excites a fairly wide zone on the membrane, a region called the critical band. At 1000 Hz, the critical bandwidth is about 100 Hz (figure 21.9). Physically, this corresponds to a region about 4 mm long on the membrane, with a peak response in a 1-mm-wide zone, ending abruptly at the place where waves of that wavelength can no longer propagate. This is approximately true at any frequency. The membrane is about 35 mm long and is tuned along its length like the musical scale: frequency is scaled exponentially progressing up the membrane, starting at the apex—that is, the broad end of the coil (figure 21.9). Thus, a 1-mm-wide zone of excitation encompasses increasingly wide frequency zones, measured in Hz, as we progress up in frequency. However, the bandwidth does not vary too much when measured in cents (section 23.18; greater for low frequencies—but above 1000 Hz, the critical bandwidth is about 10 cents). In terms of Q, a frequency width of 100 Hz at 1000 Hz is a Q of 10. Our ability to distinguish one frequency from another is perhaps 20 to 30 times better than this, which requires some additional sharpening mechanism, since the tonotopic (correlation of frequency with place) localization on the basilar membrane is too crude. — Eric J. Heller, Why You Hear What You Hear- An…, 1953

Categorization

• DEFINE: Defines the critical band, Q, and tonotopy with concrete numbers.
• QUESTION: What is the additional sharpening mechanism that gives 20-30x better discrimination? Later sections presumably explain.