Sources of distortion product otoacoustic emissions revealed by suppression experiments and inverse fast Fourier transforms in normal ears

Primary and secondary sources combine to produce the 2f(1)-f(2) distortion product otoacoustic emission (DPOAE) measured in the ear canals of humans. DPOAEs were obtained in nine normal-hearing subjects using a fixed-f(2) par adigm in which f(1) was varied. The f(2) was 2 or 4 kHz, and absolute and r elative primary levels were varied. Data were obtained with and without a t hird tone (f(3)) placed 15.6 Hz below 2f(1)-f(2). The level of f(3) was var ied in order to suppress the stimulus frequency otoacoustic emission (SFOAE ) coming from the 2f(1)-f(2) place. These data were converted from the comp lex frequency domain into an equivalent time representation using an invers e fast Fourier transform (IFFT). IFFTs of unsuppressed DPOAE data were char acterized by two or more peaks. Relative amplitudes of these peaks depended on overall primary level and on primary-level differences. The suppressor eliminated later peaks, but early peaks remained relatively unaltered. Resu lts are interpreted to mean that the DPOAE measured in humans includes comp onents from the f(2) place (intermodulation distortion) and DP place tin th e form of a SFOAE). These findings build on previous work by providing evid ence that multiple peaks in the IFFT are due to a secondary source at the D P place. (C) 2001 Acoustical Society of America.