Suppression of distortion product otoacoustic emissions (DPOAE) near 2f1−f2 removes DP-gram fine structure—Evidence for a secondary generator

Since the discovery of distortion product otoacoustic emissions (DPOAE) there has been a controversial discussion about their cochlear generation sites. Suppression experiments suggest that the place near $f_2$ is the main generation site. On the other hand, the fact that DPOAE can be perceived subjectively indicates that there is also a cochlear excitation at the place of ${\text{2f}}_1{\mathrm{-f}}_2$ resulting in a stimulus frequency otoacoustic emission (SFOAE). The contribution of this SFOAE to the overall emission is still unknown. Different studies showed contradictory results. We demonstrate a secondary generator by successive suppression of the SFOAE with a sine wave close to the frequency ${\text{2f}}_1{\mathrm{-f}}_2.$ Suppression growth functions (SGF) showed a three-step behavior. For low suppressor levels, the emission either decreased or increased when increasing the suppressor. For intermediate suppressor levels, DP amplitude was constant and independant of suppressor level. For high suppressor levels, the emission always decreased with further increase of the suppressor. The behavior of the SGF in the first section depends on the fine structure of the DP-gram, which shows minima and maxima. Emissions at a maximum decreased while emissions at a minimum increased in the first section of the SGF. We conclude that the fine structure of the DP-gram is produced by alternate constructive and destructive interference of the two generators. By adding a third tone near ${\text{2f}}_1{\mathrm{-f}}_2$ the SFOAE and thus the interference are suppressed. The fine structure of the DP-gram vanishes and the resulting DP-gram should be more closely related to the cochlear status near $f_2.$