The Cochlear Microphonic (CM) has been found to be dominated by hair cells located in close proximity to the recording electrode, near the base of the cochlear, regardless of stimulus frequency. Whilst a tone will generate greatest excitation of the basilar membrane at the characteristic frequency of the stimulus, it has been found that phase-rotation for hair cells at apical regions of the cochlear causes the vector sum of their CM to be much less than if the hair cells were orientated in the same direction, as occurs at the base of the cochlea. Therefore the CM is dominated by contributions of outer (and inner) hair cells within the basal region of the cochlea regardless of stimulus frequency. For more detail regarding these concepts, please see Withnell (2001).
The Eclipse uses 30 kHz sampled sound files, so this implies a maximum of 15 kHz sound stimulation is possible without risk of aliasing.
References and caveats
Withnell, R.H. (2001) Brief report: The cochlear microphonic as an indication of outer hair cell function. Ear and Hearing, 22(1), 75-77.