Distortion product otoacoustic emission fine structure analysis of 50 normal-hearing humans.

When distortion product otoacoustic emissions (DPOAEs) are measured with a high-frequency resolution, the DPOAE shows quasi-periodic variations across frequency, called DPOAE fine structure. In this study the DPOAE fine structure is determined for 50 normal-hearing humans using fixed primary levels of L1/L2 = 65/45 dB. An algorithm is developed, which characterizes the fine structure ripples in terms of three parameters: ripple spacing, ripple height, and ripple prevalence. The characteristic patterns of fine structure can be found in the DPOAE of all subjects, though the DPOAE fine structure characteristics are individual and vary from subject to subject. On average the ripple spacing decreases with increasing frequency from 1/8 oct at 1 kHz to 3/32 oct at 5 kHz. The ripple prevalence is two to three ripples per 1/3 oct, and ripple heights of up to 32 dB could be detected. The 50 normal-hearing subjects were divided into two groups, the subjects of group A having slightly better hearing levels than subjects of group B. The subjects of group A have significantly higher DPOAE levels. The overall prevalence of fine structure ripples do not differ between the two groups, but are higher and narrower for subjects of group B than for group A.