Volume 107, Issue 6, June 2000
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
Group delays of distortion product otoacoustic emissions: Relating delays measured with - and -sweep paradigms107(2000); http://dx.doi.org/10.1121/1.429402View Description Hide Description
A theoretical analysis is presented of group delays of distortion product otoacoustic emissions (DPOAEs) measured with the phase-gradient method. The aim of the analysis is to clarify the differences in group delays and obtained using the - and the -sweep paradigms, respectively, and the dependence of group delays on the order of the DPOAE. Two models are considered, the place-fixed and the wave-fixed models. While in the former model the generation place is assumed to be invariant with both - and -sweeps, in the latter model the shift of generation place is fully accounted for. By making a simple local approximation of the cochlear scale invariance, a mathematical conversion from phase-place to phase-frequency gradients is incorporated in the wave-fixed model. Under the assumption that the DPOAE (as recorded at the best ratio) is dominated by the contribution from the generation site and not by, e.g., reflection components, the analysis leads to simple expressions for the ratio and difference between and Validation of the models against experimental data indicates that lower sideband DPOAEs are most consistent with the wave-fixed model. Upper sideband components in contrast, are not properly described by either the place-fixed or the wave-fixed model, independent whether DPOAE generation is assumed to originate at the or at the more basally located characteristic place.
107(2000); http://dx.doi.org/10.1121/1.429403View Description Hide Description
Spontaneous otoacoustic emissions (SOAEs) were studied in humans during and after postural changes. The subjects were tilted from upright to a recumbent position (head down 30 degrees) and upright again in a 6-min period. The SOAEs were recorded continuously and analyzed off-line. The tilting caused a change in the SOAE spectrum for all subjects. Frequency shifts of 10 Hz, together with changes of amplitude (5 dB) and width (5 Hz), were typically observed. However, these changes were observed in both directions (including the appearance and disappearance of emission peaks). The most substantial changes occurred in the frequency region below 2 kHz. An increase of the intracranial pressure, and consequently of the intracochlear fluid pressure, is thought to result in an increased stiffness of the cochlear windows, which is probably mainly responsible for the SOAE changes observed after the downward turn. The time for the spectrum to regain stability after a postural change differed between the two maneuvers: 1 min for the downward and less than 10 s for the upward turn.
107(2000); http://dx.doi.org/10.1121/1.429404View Description Hide Description
Measurements from the 1–4-mm basal region of the chinchilla cochlea indicate the basilar membrane in the hook region (12–18 kHz) vibrates essentially as it does more apically, in the 5–9-kHz region. That is, a compressive nonlinearity in the region of the characteristic frequency, amplitude-dependent phase changes, and a gain relative to stapes motion that can attain nearly 10 000 at low levels. The displacement at threshold for auditory-nerve fibers in this region (20 dB SPL) was ∼2 nm. Measurements were made at several locations in individual animals in the longitudinal and radial directions. The results indicate that there is little variability in the phase of motion radially and no indication of higher-order modes of vibration. The data from the longitudinal studies indicate that there is a shift in the location of the maximum with increasing stimulus levels toward the base. The cochlear amplifier extends over a 2–3-mm region around the location of the characteristic frequency.