Acoustic distortion products (ADP) from the ears of term infants and young adults using low stimulus levels

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Abstract
It has been shown previously that relatively low stimulus levels (L1 = 55, L2 = 40 dB SPL) elicit recordable levels of ADP from healthy adult ears (Gaskill and Brown, 1990). We have used the same stimulus levels in healthy term infants within 6 days of birth. The results from the 38 infants who provided data from both ears (from a total of 66 attempted) are reported here. The data were compared with those from 12 adults aged 20 to 30 years with normal audiometric thresholds. ADP was measured across frequency with f2 (the high frequency tone) between 1 and 8 kHz (infants) and 0.5 kHz and 8 kHz (adults) and f1 (the low frequency tone) determined by f2/1.225. ADP was also measured at a range of different stimulus frequency separations. The mean level of ADP was higher in infant than in adult ears but this difference was not significant. The f2/f1 ratio of approximately 1.2 which has been widely adopted for use in adults is also an appropriate frequency separation for term infants.