Suppression of human acoustic distortion product: dual origin of 2f1-f2.

In order to explore the origin of acoustic distortion product (DP) in the ear canal, the effect of the level and the frequency of the suppressor tone on the level of the DP was investigated. The results broadly support previous studies in showing that, with increasing suppressor level, the DP level shows an initial relatively stable plateau or gradual decline, followed by a rapid decline. The shape of the human iso-suppression contours derived from these "suppression-rate" curves was found to depend upon the size of the iso-suppression criterion. Low-level suppressor tones close in frequency to 2f1-f2 were effective at suppressing the DP by up to 3 dB in subjects where the distortion frequency fell at a region of strong stimulus frequency re-emission. It is probable that contributions from the DP frequency region, as well as components from the primary frequency region, make up the ear canal measured DP. With a 6-dB iso-suppression criterion, suppressor tones in the primary frequency region suppress DP most effectively.