Volume 120, Issue 4, October 2006
Index of content:
- PHYSIOLOGICAL ACOUSTICS 
120(2006); http://dx.doi.org/10.1121/1.2258929View Description Hide Description
A standardized acoustical coupler should enable the calibration of audiometricearphones which ensures that the hearing thresholds determined in the audiometricmeasurement are independent of the earphone type. This requires that the coupler approximates the average human ear closely. Nevertheless, the differences among earphones as well as between human ears and the coupler affect the results of the audiometricmeasurements inducing uncertainty. As the mentioned differences are related to the coupling of different earphones to human ears and to a standardized coupler, the effects of this coupling are investigated by measuring the transfer functions from input voltage of the earphone terminals to the pressure at the ear canal entrance in two situations: open and blocked canals. Since the “ear canal entrance” is not well-defined for the coupler, transfer function measurements in the coupler were carried out in a similar way but at different depths. In order to describe and compare the earphone couplings, the pressure divisions at the entrance of the ear canal are calculated from the measured transfer functions. The results indicate that significant difference appears among sound pressures generated in different individuals’ ears. Also, the earphone couplings to human ears and to the coupler differ considerably.
120(2006); http://dx.doi.org/10.1121/1.2335421View Description Hide Description
The 2f1–f2 distortion product otoacoustic emission (DPOAE) is considered to consist of two components in normally hearingears, one having constant phase with changing DP frequency (wave fixed) and one having an increasing phase lag with increasing frequency (place fixed). The aim was to identify the wave-fixed and place-fixed components of both 2f1–f2 and 2f2–f1 DPs, and, in particular, to show whether a wave-fixed 2f2–f1 DP exists in normally hearing adults. DPOAE recordings were made in 20 ears of normally hearing young adults. Four frequency ratios were used and recording entailed fixed frequency-ratio sweeps. A separation into wave-fixed and place-fixed components was carried out using a time-window separation method. A method for estimating the noise floor after data processing was developed. Results confirmed the presence of wave-fixed and place-fixed components for 2f1–f2, consistent with previous studies. Both components were also present for 2f2–f1 in virtually all subjects. This latter finding conflicts with current models of DPOAE generation, and so a modified model is proposed. Unlike the 2f1–f2 emission, which has a wave-fixed component that is strongly dependent on the frequency ratio, neither component of the 2f2–f1 emission showed such a dependence. The proposed model explains these findings in terms of the overlap of the primary frequency traveling waves.
120(2006); http://dx.doi.org/10.1121/1.2338291View Description Hide Description
The distortion product otoacoustic emission (DPOAE) is thought to arise primarily from the complex interaction of components that come from two different cochlear locations. Such distortion has its origin in the nonlinear interaction on the basilar membrane of the excitation patterns resulting from the two stimulus tones, and . Here we examine the spatial extent of initial generation of the OAE by acoustically traumatizing the base of the cochlea and so eliminating the contribution of the basal region of the cochlea to the generation of . Explicitly, amplitude-modulated, or continuously varying in level, stimulus tones with and were used to generate the DPOAE in guinea pig before and after acoustically traumatizing the basal region of the cochlea (the origin of any basal-to- distortion product generators). It was found, based on correlation analysis, that there does not appear to be a basal-to- distortion product generation mechanism contributing significantly to the guinea pig OAE up to sound pressure level (SPL).