Measuring Distortion-Product Otoacoustic Emission With a Single Loudspeaker in the Ear: Stimulus Design and Signal Processing Techniques.

Yi-Wen Liu,Yung-Ching Chen,Wei-Chen Hsiao

doi:10.3389/fdgth.2021.724539

Abstract

The distortion-product otoacoustic emission (DPOAE) is a backward propagating wave generated inside the cochlea during the wave amplification process. The DPOAE signal can be detected rapidly under relatively noisy conditions. In recent years, the earphone industry demonstrated interest in adopting DPOAE as an add-on feature to make their product “intelligent” of inner-ear status. However, a technical challenge remains to be tackled—the loudspeaker in an earphone generates its own cubic distortion at the same frequency as DPOAE. Unfortunately, the intensity of loudspeaker distortion is typically comparable to that of the DPOAE, if not higher. In this research, we propose two strategies, namely compensation and cancellation, to enable DPOAE measurement with a single loudspeaker. The compensation strategy exploits the part of the growth function of the loudspeaker distortion which is almost linear, and thus suppresses the distortion it generates while retaining a larger portion of DPOAE in the residual signal. The cancellation strategy utilizes a one-dimensional Volterra filter to remove the cubic distortion from the loudspeaker. Testing on normal-hearing ears shows that the compensation strategy improved the DPOAE-to-interference ratio by approximately 7 dB, resulting in a cross-correlation of 0.62 between the residual DPOAE level and the true DPOAE level. Meanwhile, the cancellation strategy directly recovered both the magnitude and the phase of DPOAE, reducing the magnitude estimation error from 15.5 dB to 3.9 dB in the mean-square sense. These pilot results suggest that the cancellation strategy may be suitable for further testing with more subjects.

Highlights

Otoacoustic emissions (OAEs) are sounds generated in the cochlea that propagate backward to emit from the ear [1]
The two-speaker design seems necessary because, even with a high-quality headphone or earphone, the total harmonic distortion (THD) can reach 3% when driven to its full dynamic range [17]. This THD level is acceptable for listening to music; when delivering two pure tones simultaneously, we found that the distortion generated by such speakers would significantly interfere with the distortion-product otoacoustic emission (DPOAE) from the ear since the cubic distortion of the speaker occurs at fDP
We proposed two strategies to estimate the DPOAE level subject to interference from the loudspeaker IMD3

Summary

Introduction

Otoacoustic emissions (OAEs) are sounds generated in the cochlea that propagate backward to emit from the ear [1]. With appropriately chosen intensities and frequencies of the primary tones [e.g., f2/f1 = 1.22, [5]], the most prominent distortion product would occur at 2f1 − f2 and it can be recorded from a microphone in the ear canal. DPOAE serves as a robust and non-invasive tool for assessing cochlear functions in a frequencyspecific manner [4]. It has been applied clinically for hearing screening [7, 8], and diagnosis of acute hearing loss [9] and other kinds of hearing impairment [10, 11]

Methods

Results

Discussion

Conclusion