Distortion in the cochlea: acoustic f2-f1 at low stimulus levels

The stimulus level and frequency dependence of the quadratic difference tone (QDT) measured as an otoacoustic emission in the ear canal has been investigated in the guinea pig and compared with simultaneously measured cubic difference tone (CDT) and with the round window electrical response. Acoustic QDT level tended to be highly labile. Growth of the ear canal response with covaried stimuli was very gradual (slope < 0.5). Acoustic and CM responses showed similar behaviour when f2 alone was incremented. The QDT was strongly dependent on stimulus frequency separation for high frequency stimuli. It is suggested that, at low stimulus levels and high frequencies, the acoustic QDT may originate in the 'tonic' motile responses of outer hair cells as they follow the envelope of the two-tone stimulus.