Objective low-frequency audiometry by distortion-product acoustic emissions.

Area of Science:

• Audiology
• Otoacoustic Emissions
• Human Cochlear Physiology

Background:

• Distortion-product otoacoustic emissions (DPOAEs) are objective measures of cochlear function.
• Optimizing DPOAE measurement parameters is essential for reliable clinical application.
• Understanding the relationship between stimulus parameters and DPOAEs can reveal insights into cochlear mechanisms.

Purpose of the Study:

• To determine optimal stimulus parameters for measuring distortion-product otoacoustic emissions (DPOAEs) in a clinical setting.
• To investigate the behavior of DPOAEs across different frequency ratios and intensity levels.
• To explore the potential of DPOAEs for objective low-frequency audiometric testing.

Main Methods:

• Automatic realization of DPOE input-output functions to identify optimal f2/f1 frequency ratios.
• Measurement of DPOAEs across a range of primary tone intensities (below 60-dB SPL and above 66-dB SPL).
• Analysis of DPOE input-output functions for various DPOE frequencies (707.5 Hz down to 342 Hz).

Main Results:

• The optimal f2/f1 ratio for significant DPOE input-output functions was found to be near 1.22.
• DPOAEs exhibited a saturating portion below 60-dB SPL (threshold at 36-dB SPL) and a linear portion above 66-dB SPL for specific ratios.
• No saturating plateau was observed for DPOEs below 512.5 Hz, suggesting the absence of active mechanisms below 725 Hz in the human cochlea.

Conclusions:

• Precise stimulus parameters, specifically an f2/f1 ratio near 1.22 and primary intensities below 60-dB SPL, are necessary for accurate clinical DPOAE measurements.
• The saturating behavior of DPOAEs below 60-dB SPL likely originates from outer hair cell properties.
• These findings provide a foundation for developing an objective low-frequency audiometric test using DPOAEs.