Effects of changes in stimulus level on phases of distortion product otoacoustic emissions

Effect of changes in stimulus levels of both lower (f₁) and higher (f₂) stimulus tones on phases of 2f₁-f₂ component of the distortion product otoacoustic emission (DPOAE) was examined in five normal hearing adults. The f₂ was fixed at 4004 Hz in all of the measurements, and the stimulus frequency ratio (f₂/f₁) was varied from 1.15 to 1.3. Change of the level of lower stimulus tone (L₁) and the level of higher stimulus tone (L₂) showed different effects on the DPOAE phases. The phase lags increased with increasing L₁, when f₂/f₁ was above 1.22, whereas the phase gains increased with increasing L₁, when f₂/f₁ was below 1.22. On the other hand, the difference in L₂ minimally affected DPOAE phase at most f₁s. The previous studies about basilar membrane vibration revealed that phase lags increase with increasing stimulus level, when the stimulus frequency is below the best frequency, while phase gains increase with increasing stimulus level, when the stimulus frequency is above the best frequency, and the effect of phase change in stimulus level diminished, when the stimulus frequency was far above the best frequency. Based on the comparison between the results of the present study and the previous findings of others concerning basilar membrane vibration, the DPOAE generation site is assumed to be located at apical of the peak of the f₂ traveling wave.