Transient otoacoustic emissions and audiogram fine structure in the extended high-frequency region.

Previous studies at conventional audiometric frequencies found associations between the ripple depth seen in audiogram fine structure (AFS) and amplitudes of both transient evoked otoacoustic emissions (TEOAEs) and overall hearing threshold levels (HTLs). These associations are explained by the cochlear mechanical theory of multiple coherent reflections of the travelling wave apically by reflections sites on the basilar membrane and basally by the stapes. The aim was to investigate whether a similar relationship is seen in the extended high-frequency (EHF) range from 8-16 kHz. Measurements from 8-16 kHz were obtained in normal-hearing subjects comprising EHF HTLs, EHF TEOAEs using a double evoked paradigm, and Bekesy audiometry to assess AFS ripple depth and spectral periodicity. Twenty eight normal-hearing subjects participated. Results showed no significant correlation between AFS ripple depth and either frequency-averaged EHF HTLs or EHF TEOAE amplitudes. The amplitude of AFS ripple depth was also lower than that seen in the conventional frequency region and spectral periodicity in the ripple more difficult to discern. The results suggest a weaker interference pattern between forward and reverse cochlear travelling waves in the most basal region compared to more apical regions, or a difference in cochlear mechanical properties.