Some studies of spontaneous and evoked acoustic emissions from the human ear

Abstract

Several studies of spontaneous and evoked oto-acoustic emissions (SOAEs and EOAEs) and their relationship to norsal and pathological hearing mechanism are described.
1. A computer model was developed to study the theory that the EOAE is produced by summation of responses from activity distributed along the cochlea. It was found that, whilst with a regular cochlea and mapping the summed response is small owing to phase cancellation, any irregularity of mapping or structure or sensitivity gives rise to a sharply-tuned response showing many of the characteristics of EOAEs.
2. A search for SOAEs In 17 normal subjects revealed a prevalence of 53% (42% of ears), which was significantly higher among the females.
3. A group of 49 clinical tinnitus sufferers was investigated to test the hypothesis that SOAEs sight be responsible in some cases. 3 such cases were found, all with tonal tinnitus and normal hearing at the tinnitus pitch (although 1 had a bilateral loss at other frequencies). The SOAEs in both studies were found between 680 and 9610 Hz at levels up to 24 dB SPL. They were not found at frequencies where hearing threshold was greater than 20 dB, and no consistent association with audiometric abnormalities was noted, implying that they should be seen as part of normal hearing variation, rather than the result of minor pathology.
4. Suppression effects were studied for (a) SOAEs in 2 ears and (b) both click-evoked and continuous-tone evoked OAEs in another ear. A number of features were observed, most notably multiple lobes in many of the suppression tuning curves. The implications of these results for theories of OAE generation are discussed.