Volume 113, Issue 5, May 2003
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
Stimulus-frequency-emission group delay: A test of coherent reflection filtering and a window on cochlear tuning113(2003); http://dx.doi.org/10.1121/1.1557211View Description Hide Description
This paper tests and applies a key prediction of the theory of coherent reflection filtering for the generation of reflection-source otoacoustic emissions. The theory predicts that reflectionsource-emission group delay is determined by the group delay of the basilar-membrane (BM) transfer function at its peak. This prediction is tested over a seven-octave frequency range in cats and guinea pigs using measurements of stimulus-frequency-emission (SFOAE) group delay. A comparison with group delays calculated from published measurements of BM mechanical transfer functions supports the theory at the basal end of the cochlea. A comparison across the whole frequency range based on variations in the sharpness of neural tuning with characteristic frequency (CF) suggests that the predicted relation holds in the basal-most 60% of the cochlea. At the apical end of the cochlea, however, the measurements disagree with neural and mechanical group delays. This disagreement suggests that there are important differences in cochlear mechanics and/or mechanisms of emission generation between the base and apex of the cochlea. Measurements in humans over a four-octave range indicate that human SFOAE group delays are roughly a factor of 3 longer than their counterparts in cat and guinea pig but manifest similar trends across CF. The measurements thus reveal global deviations from scaling whose form appears quantitatively similar in all three species. Interpreted using the theory of coherent reflection filtering, the group delay measurements indicate that the wavelength at the peak of the traveling wave decreases with increasing CF at a rate of roughly 25% per octave in the base of the cochlea. The measurements and analysis reported here illustrate the rich potential inherent in OAEmeasurements for obtaining valuable information about basic cochlear properties such as tuning.
Measurements of human middle ear forward and reverse acoustics: Implications for otoacoustic emissions113(2003); http://dx.doi.org/10.1121/1.1564018View Description Hide Description
Middle and inner ears from human cadaver temporal bones were stimulated in the forward direction by an ear-canal sound source, and in the reverse direction by an inner-ear sound source. For each stimulus type, three variables were measured: (a) —ear-canal pressure with a probe-tube microphone within 3 mm of the eardrum, (b) —stapes velocity with a laser interferometer, and (c) —vestibule pressure with a hydrophone. From these variables, the forward middle-ear pressure gain (M1), the cochlear input impedance the reverse middle-ear pressure gain (M2), and the reverse middle-ear impedance (M3) are directly obtained for the first time from the same preparation. These measurements can be used to fully characterize the middle ear as a two-port system. Presently, the effect of the middle ear on otoacoustic emissions(OAEs) is quantified by calculating the roundtrip middle-ear pressure gain as the product of M1 and M2. In the 2–6.8 kHz region, decreases with a slope of −22 dB/oct, while OAEs (both click evoked and distortion products) tend to be independent of frequency; this suggests a steep slope in vestibule pressure from 2 kHz to at least 4 kHz for click evoked OAEs and to at least 6.8 kHz for distortion product OAEs. Contrary to common assumptions, measurements indicate that the emission generator mechanism is frequency dependent. Measurements are also used to estimate the reflectance of basally traveling waves at the stapes, and apically generated nonlinear reflections within the vestibule.