Otoacoustic emissions (OAEs) are the sounds that the activity of the OHCs generate in the ear canal,¹⁷ which can be measured with a microphone and, with modern technology, separated from sounds entering the ear. Middle ear disorders affect OAEs, but are easily distinguishable from sensorineural hearing loss.

We used the Otodynamics ILO88 analyser (Otodynamics Ltd, Hertfordshire, England) in this study. A probe containing both a speaker (which delivers the stimulus) and a microphone (for recording the response) is sealed in the ear canal. We used the transient-evoked (or click-evoked) otoacoustic emission (TEOAE)^17,18 technique, which specifically targets the active OHC response and ignores the acoustic response of the external and middle ear. A standard 260 repetitions of an 80dB train of clicks lasting 1ms are delivered to the ear, and the otoacoustic emission response from the outer hair cells is detected by the microphone. The first 20ms of the response following the click is averaged to improve the quality of the signal. Stability of probe placement is routinely monitored and displayed as a percentage with the record. Data collection for approximately one minute gives a clinically useful waveform. Alternate responses are summed into two arrays and at the end of recording the correlation coefficient between the arrays is calculated. This coefficient (range, -1.0 to 1.0) is normally expressed by Otodynamics as a percentage, denoted by the term Waverepro% (designated by some authors as Wholerepro%), which is a measure of strength of the emission.

Complete sets of OHCs (such as in neonates) produce emissions near 100%. As the ear ages or is progressively damaged, the Waverepro% decreases. Waverepro% may be compared across the population¹⁹ provided a standard 80dB peak stimulus is used.

The ability of TEOAE to measure net activity of any ear is of particular relevance to this report, which relates otoacoustic emissions to age and two specific risk factors. The relationship between Waverepro% and pure tone threshold hearing levels is illustrated,^20,21 showing that the Waverepro% needs to decline to below about 35% before hearing loss is detectable by audiometry. Hence, TEOAE may be able to detect accumulated ear damage in the preclinical range. If so, the technique may prove useful in detecting latent damage caused by noise and other factors.