ABSTRACT
BACKGROUND AND OBJECTIVES: Microphones designed to measure otoacoustic emissions (OAEs) from the human ear canal typically sample the sound field in the canal some 15-20 mm away from the tympanic membrane. Discrepancies inevitably arise at high frequencies between the sound pressure at the tympanic membrane and at the entrance to the emission probe from sound reflected by the tympanic membrane due to incomplete absorption. A previous work on the ear canal acoustics suggests the emission probe underestimate the sound pressure level of the stimulus at the tympanic membrane by as much as 15-20 dB for the stimulus frequencies near 5-7 KHz. Materials and Method: This study checked the variation of transiently evoked otoacoustic emissions (TEOAEs) and distorsion product otoacoustic emissions (DPOAEs) for the situations of probe which were controlled by ear canal volume. RESULTS: All mean values on TEOAEs (stimulus level, echo response, reproducibility) significantly increased (p 0.05) as the ear canal volume decreased. The mean amplitude of DPOAEs significantly increased at 1.6, 2, 25, 3.2, 4 (F2 frequency) and the variation of the mean noise level was statistically significant at 1, 2, 4 KHz (F2 frequency). The mean signal to noise ratio on DPOAEs significantly increased at 1, 2, 3.2, 4, 6.3 KHz (F2 frequency). CONCLUSION: These results suggest that the reliability of measurements of otoacoustic emissions should be improved when the probe is located nearer to the tympanic membrane.