Influence of primary frequencies ratio on distortion product otoacoustic emissions amplitude. I. Intersubject variability and consequences on the DPOAE-gram

Distortion product otoacoustic emissions (DPOAEs) are used widely in humans to assess cochlear function. The standard procedure consists of recording the 2f1-f2 DPOAE amplitude as a function of the f2 frequency, using a fixed f2/f1 ratio (DPOAE-gram), close to 1.20. DPOAE amplitude, as recorded in t he DPOAE-gram, shows a wide range of values in normal-hearing subjects, whi ch can impair the predictive value of the DPOAE-gram for hearing thresholds . This study is aimed at comparing intersubject variability in 2f1-f2 DPOAE amplitude according to three paradigms: a fixed f2/f1 ratio, such as the D POAE-gram, a variable ratio DPOAE-gram Cf2/f1 adapted to frequency) and an "optimum" DPOAE-gram, where the f2/f1 is adapted both to subject and freque ncy. The 2f1-S2 DPOAE amplitude has been investigated on 18 normally hearin g subjects at ten different f2 frequencies (from 0.75 to 6 kHz), using an f 2 fixed, fl sweep paradigm, and allowed to define, for each frequency, the f2/f1 ratio giving the greatest 2f1-f2 DPOAE amplitude (or optimum ratio). Results showed a large intersubject variability of the optimum ratio, espec ially at frequencies below 1.5 kHz, and a significant decrease of the optim um ratio with frequency. The optimum DPOAE-gram was underestimated by up to 5.8 dB on average (up to 14.9 dB for an individual subject) by the fixed r atio DPOAE-gram, and by up to 3 dB on average (up to 10.6 dB for an individ ual subject) by the variable ratio DPOAE-gram. Intersubject variability was slightly but significantly reduced in the optimum DPOAE-gram versus the fi xed-ratio DPOAE-gram. Lastly, correlations between tone-burst evoked otoaco ustic emission (TBOAE) amplitudes and maximum DPOAE amplitudes were signifi cantly greater than correlations between TBOAE amplitudes and fixed-ratio D POAE amplitudes. (C) 2000 Acoustical Society of America. [S9001-4966(00)031 02-7].