Distortion product otoacoustic emissions in human newborns and adults. II. Level effects

This study varied the levels of the primaries and recorded distortion product otoacoustic emissions (DPOAEs) from human newborns and adults. Preterm as well as term newborns were tested. The 2 f1-f2 DPOAE was the most reliably recorded DPOAE, especially at low levels of the primaries. Amplitude and phase reproducibility deteriorated with decreasing level of the primaries. Newborn DPOAEs were slightly less reproducible than adult DPOAEs. The underlying DPOAE I/O functions were nonmonotonic for both newborns and adults. Unity gain characterized the initial increasing portion of those functions in most subjects. Although newborn and adult I/O functions were similar, they did differ. In particular, adult functions tended to be more linear with saturation at higher primary levels. Some of the newborn functions saturated at very low stimulus levels. Although differences in cochlear mechanics may explain developmental difference in DPOAE I/O functions, developmental differences in the resonance characteristics of the outer and middle ears may also be involved.