Distortion product otoacoustic emissions in human newborns and adults. I. Frequency effects

This study varied stimulus frequency and recorded distortion product otoacoustic emissions (DPOAEs) in human newborns and adults. Because of outer and middle ear acoustics, the same auditory input resulted in higher newborn stimulus sound pressure levels across a broad frequency range in the occluded outer ear canal. Noise levels in the canal were 5-15 dB lower for adults at frequencies less than about 3 kHz. The 2 f1-f2 DPOAE was the most reliably recorded DPOAE except at the lowest frequencies assessed. At the lowest frequencies the 2 f2-f1 DPOAE was more frequently recorded than any other DPOAE. There were no striking developmental differences in the kinds of DPOAEs that were recorded. The amplitudes of consecutively recorded 2 f1-f2 DPOAEs were generally within 1.5 dB of each other for all age groups (slightly better reproducibility for adults than newborns). The phases of consecutively recorded 2 f1-f2 DPOAEs were generally within 15 degrees of each other (often less than 10 and 5 degrees for newborns and adults respectively). At the highest frequencies assessed (f2 = 4.2-9.9 kHz) all subjects had similar amplitude 2 f1-f2 DPOAEs. At lower frequencies adult 2 f1-f2 amplitudes were significantly less than those of newborns. At the lowest frequencies reliably assessed (f2 = 1.5-2.1 kHz) term newborns had significantly larger 2 f1-f2 DPOAEs than preterm newborns. Newborn and adult 2 f1-f2 DPOAE amplitude X f2/f1, functions were quite similar although there were reliable differences. Age related differences in the outer and middle ears may explain some of the differences in DPOAEs that were observed.