BACKGROUND: Up to now, laser interferometric vibration measurements of the human eardrum have not provided any information about cochlear function, because the measurement devices have not been sufficiently sensitive. METHODS: After designing a new type of laser Doppler vibrometer (LDV) that allows detection of displacement amplitudes down to about 1 pm, we used this device in 20 subjects to measure growth functions of the distortion products of otoacoustic emissions (DPOAE) as vibrations of the umbo. For comparison, DPOAE growth functions were also measured conventionally with an acoustic probe in the closed external auditory meatus. Hearing thresholds were estimated from both sets of measurements and compared with Békésy thresholds. RESULTS: The standard deviation of the threshold estimate obtained from the vibration DPOAEs was 8.6 dB, which is significantly smaller than that of the threshold estimate (16.7 dB) obtained from the acoustic DPOAEs. We attribute the smaller standard deviation for the LDV data to the fact that these measurements are made in an open sound field and are therefore less susceptible to pressure calibration errors. CONCLUSIONS: Being relatively free of sound-field measurement artefacts, the LDV method allows precise estimation of the hearing threshold. Vibration measurements of the umbo have, therefore, considerable potential for the differential diagnosis of mechanical dysfunction of the middle and inner ear.
BACKGROUND: Up to now, laser interferometric vibration measurements of the human eardrum have not provided any information about cochlear function, because the measurement devices have not been sufficiently sensitive. METHODS: After designing a new type of laser Doppler vibrometer (LDV) that allows detection of displacement amplitudes down to about 1 pm, we used this device in 20 subjects to measure growth functions of the distortion products of otoacoustic emissions (DPOAE) as vibrations of the umbo. For comparison, DPOAE growth functions were also measured conventionally with an acoustic probe in the closed external auditory meatus. Hearing thresholds were estimated from both sets of measurements and compared with Békésy thresholds. RESULTS: The standard deviation of the threshold estimate obtained from the vibration DPOAEs was 8.6 dB, which is significantly smaller than that of the threshold estimate (16.7 dB) obtained from the acoustic DPOAEs. We attribute the smaller standard deviation for the LDV data to the fact that these measurements are made in an open sound field and are therefore less susceptible to pressure calibration errors. CONCLUSIONS: Being relatively free of sound-field measurement artefacts, the LDV method allows precise estimation of the hearing threshold. Vibration measurements of the umbo have, therefore, considerable potential for the differential diagnosis of mechanical dysfunction of the middle and inner ear.