Characterization of non-linear distortion in hearing aids using coherence analysis. A pilot study

Area of Science:

• Auditory science
• Signal processing
• Hearing aid technology

Background:

• Coherence measures linear dependence between system input and output.
• It quantifies signal corruption like noise and non-linear distortion.
• A frequency-dependent signal-to-noise ratio (SNR) can be derived from coherence.

Purpose of the Study:

• To demonstrate the applicability of coherence measurements for assessing non-linear distortion in hearing aids.
• To investigate the influence of hearing aid gain and automatic gain control (AGC) on SNR.
• To analyze the impact of these factors on audibility and sound quality.

Main Methods:

• Utilized coherence function analysis in the frequency domain.
• Employed speech-shaped noise as the input signal for three different hearing aids.
• Varied hearing aid gain settings and evaluated the effect of automatic gain control (AGC).

Main Results:

• Hearing aid gain significantly affects the signal-to-noise ratio (SNR), particularly at maximum gain settings, due to non-linear distortion.
• Non-linear distortion introduced by hearing aids was effectively quantified using coherence.
• Automatic gain control (AGC) provided a partial reduction in non-linear distortion effects at high gain levels.

Conclusions:

• Coherence is a valuable tool for characterizing non-linear distortion in hearing aids.
• Gain settings critically influence hearing aid performance and SNR, especially near maximum amplification.
• Automatic gain control can help mitigate, but not eliminate, the negative effects of non-linear distortion at high gain.