Removing effects of ear-canal acoustics from measurements of otoacoustic emissions

Abstract

Otoacoustic emissions (OAEs) are the acoustic fingerprints of the inner ear—when carefully measured in healthy ears, their spectra, although highly individualized, remain stable over time. Thus, OAE changes usually indicate changes in cochlear function, e.g., due to efferent modulation, aging, noise trauma, and/or exposure to harmful agents. In humans, however, the reproducibility of OAE measurements is compromised by ear-canal standing waves at relevant frequencies. We show that even when stimulus levels are tightly controlled using methods designed to avoid standing-wave problems (forward-pressure-level calibration), distortion-product (DP)OAE levels vary by ~10–15 dB near half-wave resonant frequencies, depending on probe insertion depth (deep versus shallow). We propose a method, derived from a tube model of the ear canal, that separates the initial outgoing OAE pressure wave at the eardrum from reflected OAEs trapped in the residual ear-canal space. The emitted pressure level (EPL) represents the load-independent OAE level that would be recorded in an ideal, reflectionless canal. When DPOAE levels are converted to EPL, variability across insertion depths decreases by ~10 dB near half-wave resonant frequencies. EPL may provide a simple way to reduce confounding OAE variability across subjects and to improve the reliability of OAE measurements for detecting cochlear changes.