SUPPRESSION OF DISTORTION-PRODUCT OTOACOUSTIC EMISSIONS (DPOAE) NEAR 2F(1)-F(2) REMOVES DP-GRAM FINE-STRUCTURE - EVIDENCE FOR A SECONDARY GENERATOR

Since the discovery of distortion product otoacoustic emissions (DPOAE ) there has been a controversial discussion about their cochlear gener ation sites. Suppression experiments suggest that the place near f(2) is the main generation site. On the other hand, the fact that DPOAE ca n be perceived subjectively indicates that there is also a cochlear ex citation at the place of 2f(1)-f(2) resulting in a stimulus frequency otoacoustic emission (SFOAE). The contribution of this SFOAE to the ov erall emission is still unknown. Different studies showed contradictor y results. We demonstrate a secondary generator by successive suppress ion of the SFOAE with a sine wave close to the frequency 2f(1)-f(2). S uppression growth functions (SGF) showed a three-step behavior. For lo w suppressor levels, the emission either decreased or increased when i ncreasing the suppressor. For intermediate suppressor levels, DP ampli tude was constant and independant of suppressor level. For high suppre ssor levels, the emission always decreased with further increase of th e suppressor. The behavior of the SGF in the first section depends on the fine structure of the DR-gram, which shows minima and maxima. Emis sions at a maximum decreased while emissions at a minimum increased in the first section of the SGF. We conclude that the fine structure of the DP-gram is produced by alternate constructive and destructive inte rference of the two generators. By adding a third tone near 2f(1)-f(2) the SFOAE and thus the interference are suppressed. The fine structur e of the DP-gram vanishes and the resulting DP-gram should be more clo sely related to the cochlear status near f(2). (C) 1998 Acoustical Soc iety of America.