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Publication Detail
The effect of the helicotrema on low-frequency cohlear mechanics and hearing
  • Publication Type:
  • Authors:
    Marquardt T, Jurado C
  • Publisher:
    Amer Inst of Physics
  • Publication date:
  • Editors:
    Shera CA,Olson ES
  • ISBN-10:
  • ISBN-13:
  • Book title:
    What Fire Is in Mine Ears: Progress in Auditory Biomechanics:
For most of the hearing range, the cochlear traveling wave terminates well before reaching the tip of the cochlea. However, at very low frequencies (approx. < 80 Hz in humans, < 150 Hz in cat and guinea pig), its wavelength becomes large compared to the length of the basilar membrane and, consequently, perilymph is forced through the helicotrema. It is probably the related inertia that, in conjunction with the basilar membrane compliance, forms a resonator, whose effect is visible in form of an under-damped step in the forward middle-ear transfer functions (fMETF, which is otherwise monotonic below 500 Hz). At frequencies below this resonance, the shunting effect of the helicotrema causes in many species (including humans) an increased slope of the fMETF [1]. We wondered whether these details of the fMETF are reflected in behavioral measures of low-frequency sound perception, and obtained fMETF (using an OAE iso-suppression technique [2]) and equal-loudness contour (ELC at approximately 40 phon) of 14 individuals with high resolution (16 measurements between 20 Hz and 160 Hz). A non-monotonic resonance feature was visible in ten fMETFs and ten ELCs, whereby only eight individuals had the features in both. Seven of these individuals showed measurements with the expected match in resonance frequency and damping. Three other intervals had a consistent lack of the feature in both their measurements. In the four remaining subjects, however, there was a clear inconsistency between their two data sets. Nevertheless, we conclude that the resonance feature in individual fMETF affects the sound perception of many subjects. We will discuss possibilities to explain also the observed inconsistencies and show that the majority of the individual fMETFs can be fitted with a simple lumped-element model of the cochlear apex [3]. [1] Dallos (1970), J. Acoust. Soc. Am 48, 489-99. [2] Marquardt et al. (2007), J. Acoust. Soc. Am 121, 3628-38. [3] Marquardt and Hensel, Mechanics of Hearing Workshop, Keele 2008.
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