6Blurred frequency resolution

TThe ear as a spectral analyser

Normal hearing resolves frequency through a set of overlapping auditory filters, each sharply tuned by the active cochlear amplifier (Chapter 2). This sharp tuning is what lets the ear separate the formant peaksof a vowel — the spectral landmarks that carry much of speech's identity.

CFilters that broaden

With outer-hair-cell damage, the active tuning is lost and the auditory filters broaden — each place on the cochlea now responds to a wider band of frequencies. The exquisite frequency selectivity of the healthy ear gives way to a coarse, smeared analysis.[2003]

CSmearing speech

The consequence for speech is spectral smearing: formant peaks that should stand apart blur into one another, so vowels lose their distinctiveness and consonant cues are muddied. The listener hears sound, even loud sound, but the spectral detail that distinguishes similar words is gone. This is a failure of resolution, not of audibility — and noise makes it far worse, because the smeared target and the background overlap.[1991]

CWhy the aid can't fix it

A hearing aid changes the level of the signal; it cannot change the width of the ear's filters. Researchers have tried to compensate by pre-sharpening the spectrum — enhancing peaks and deepening valleys before amplification — but these schemes have not produced clinically useful gains, because the broadened cochlea re-smears whatever the aid sharpens. The distortion lives in the ear, beyond the aid's reach. An implant, by contrast, imposes its own place code on the nerve, sidestepping the broken filters (Chapter 8).