Investigation of the 2f1−f2 and 2f2−f1 distortion product otoacoustic emissions using a computational model of the gerbil ear

In this work, a three-dimensional computational model of the gerbil ear is used to investigate the generation of the 2f1−f2 and 2f2−f1 distortion product otoacoustic emissions (DPOAEs). In order to predict both the distortion and reflection sources, cochlear roughness is modeled by introducing random inhomogeneities in the outer hair cell properties. The model was used to simulate the generation of DPOAEs in response to a two-tone stimulus for various primary stimulus levels and frequency ratios. As in published experiments, the 2f1−f2 DPOAEs are mostly dominated by the distortion component while the 2f2−f1 DPOAEs are dominated by the reflection component; furthermore, the influence of the levels and frequency ratio of the primaries are consistent with measurements. Analysis of the intracochlear response shows that the distortion component has the highest magnitude at all longitudinal locations for the 2f1−f2 distortion product (DP) while the distortion component only dominates close to the DP best place in the case of the 2f2−f1 DP. Decomposition of the intracochlear DPs into forward and reverse waves demonstrates that the 2f1−f2 DP generates reverse waves for both the distortion and reflection components; however, a reverse wave is only generated for the reflection component in the case of the 2f2−f1 DP. As in experiments in the gerbil, the group delay of the reflection component of the DPOAE is between 1× and 2× the forward group delay, which is consistent with the propagation of DP towards the stapes as slow reverse waves.