'Open canal' measurements of click-evoked otoacoustic emissions

We report here for the first time measurements of click-evoked otoacoustic emissions (CEOAEs) in human ears recorded with a physically open ear canal. This work was inspired by the report of Withnell, Kirk and Yates (1998), who described such recordings from the guinea pig ear and further reported responses with energy at much higher frequencies than are traditionally observed in CEOAEs. Our aims were to determine whether it is possible to measure human CEOAEs using an open-canal technique; to compare any such recordings with those obtained in the same ears using the traditional, closed-canal technique; and to determine whether significantly higher-frequency CEOAEs can be measured in humans with this technique. CEOAEs were recorded at click rates up to 5000/s, using the maximum length sequence technique, in order to improve the signal to noise ratio of the recordings and to provide a ready means of assessing the physiological origin of the signals. Clear indication of CEOAEs in the open canal recordings were obtained in approximately half the 12 normal ears tested, with borderline/questionable CEOAEs in the others. The responses obtained from open canals had characteristics broadly similar to conventional closed-canal CEOAEs, albeit significantly lower amplitudes and higher noise levels. (This was the main reason why clear CEOAEs were not recorded in all normal ears.) However, the key difference between the recordings was the presence of significantly greater high-frequency energy in the open-canal recordings, as has been reported in the guinea pig. This new development in recording techniques for human CEOAEs bears significant promise as a means for assessing cochlear function at frequencies above the ~ 4 kHz limit that typically applies to conventional closed-canal techniques