The effects on transient evoked otoacoustic emissions following changes in external auditory canal acoustic impedance

The form of the transient evoked otoacoustic emission (TEOAE) generated by an acoustic impulse and recorded in the external auditory canal is dependent on many factors. Of these, the acoustic impedance of the external canal is possibly the most important. The effect of a change of acoustic impedance on the TEOAEs, induced by filling the external canal with helium or sulphur hexafluoride gas, is measured in a group of normal hearing subjects. The fundamental frequency of the TEOAEs is found to depend upon the density of the gas in the external canal. Compared to that of air (density 1.29 g/l) the frequency increased with helium (density 0.178 g/l) and decreased with sulphur hexafluoride (density 6.6 g/l). The amplitude of the TEOAEs was also found to depend upon the density of the gas in the canal, decreasing with helium and increasing with sulphur hexafluoride. These results suggest that the TEOAEs observed in the group of subjects tested arose within the external auditory canal. This does not imply that cochlear emissions are not generated in response to transient acoustic impulses, rather it implies that this study failed to show the presence of a cochlear-mediated emission.