"Single generator distortion products"(sgDPOAE). Precise measurements of distortion product otoacoustic emissions by three-tone stimulations

Distortion product otoacoustic emissions (DPOAE) are generated undoubtedly in the region of overlap between the two travelling waves of primaries f1 and f2 (the "primary" generator). Since DPOAE lead to a psychoacoustic perception, it is suspected that the cochlear amplifier is additionally activated at place 2f1-f2 (the "secondary" generator) leading to a stimulus frequency emission (SFOAE). In suppression experiments we demonstrated that a suppressor tone 25 Hz above the frequency of the postulated secondary generator alters DP-amplitude depending (1) on the DP-gram fine structure and (2) the sound pressure level of the suppressor (-20 to 80 dB SPL). At a maximum of the fine structure, low suppressor levels (< 50 dB SPL) reduced the DP-amplitude, whereas an increase occurred at a minimum. Intermediate suppressor levels (50-60 dB SPL) always clamped the DP-amplitude and extinguished the DP-gram fine structure. Our results support the hypothesis that conventional DPOAE, which are measured in the external ear canal, are the result of an interference of the primary and secondary generator. Depending on the relative phase between the two emissions, a constructive and a destructive interference of the two signals results in an emission pattern which closely resembles the DP-gram fine structure. When the secondary generator is suppressed by intermediate suppressor levels, DPOAE amplitude depends only on the status of the cochlear amplifier near f2. These emissions we call "single generator" DPOAE (sgDPOAE).