Effects of noise and reverberation on speech recognition with variants of a multichannel adaptive dynamic range compression scheme

Abstract

Objective: Adaptive compression methods in hearing aids have been developed to maximise audibility while preserving temporal envelope modulations. Increasing the number of channels may improve listening comfort for loud sounds. However, the effects of this on speech recognition in different environmental conditions are unknown. This study evaluated the effects of different channel architectures and adaptive compression properties on speech recognition in noise and reverberation.

Design: Sentences were mixed with steady or modulated noise at three signal-to-noise ratios (SNRs). These were processed with and without reverberation and amplified with four proprietary adaptive compression methods or linear amplification.

Study sample: 36 listeners with mild to moderately-severe hearing loss.

Results: Adaptive compression improved speech recognition over linear amplification to a small extent, with no significant differences among methods using 4 or 24 channels or a combination thereof. These effects remained across the different background noise and reverberation conditions.

Conclusions: Increasing the number of channels does not negatively affect speech recognition in noise and reverberation when adaptive compression is used. If future research shows that increasing the number of channels improves listening comfort for loud sounds, these results indicate that adaptive compression methods with as many as 24 channels are viable options for hearing aids.

Keywords:

• Hearing aids
• speech perception
• adaptive compression
• multichannel

Acknowledgements

The authors wish to thank Kamilla Angelo, Anders Højsgaard Thomsen and Jan Petersen for sharing the code for the adaptive compression methods and for their fruitful discussions concerning the study. The authors also wish to thank Pavel Zahorik for sharing his expertise and MATLAB code that generated the different reverberation conditions.

Disclosure statement

The authors report that funding for the project as well as the adaptive compression algorithms were provided by Oticon A/S.

Notes

1 Attack time is the time it takes for gain to decrease after input level is abruptly increased. Typically, attack times are fast to minimise sound levels that are uncomfortably or dangerously high for the hearing aid user (Dillon 2012). Release time is the time it takes for gain to increase after input level decreases.

2 The amplification methods varied along two parameters i.e. channel number and time constants. Therefore, a fixed compression ratio was used to avoid additional confounds. A relatively higher value was chosen based on literature that has shown a greater effect of RT at higher compression ratios (Jenstad and Souza 2005). These approaches have been commonly adopted in previous studies (e.g., Reinhart and Souza 2016; Reinhart et al. 2016; Souza et al. 2015). If implemented clinically, the frequency-specific compression ratio can be set according to a validated prescriptive method for an individual listener.

3 The two participants also reported that they wore their hearing aids inconsistently. It is possible that these listeners had a wider frequency response with the headphones than with their hearing aids and might have been reacting to the amplified high-frequency components that they were not accustomed to hearing.

4 SNRs (0 and 5 dB) used in this study are representative of real-world scenarios encountered by hearing-impaired listeners (Smeds, Wolters, and Rung 2015; Wu et al. 2018). Similarly, the reverberation condition represents typical reverberation in an average room in a house. Based on previous research using fixed-fast and fixed-slow RTs (e.g. Reinhart et al. 2016; Reinhart et al. 2017), one can expect that there will be no significant differences among the adaptive methods at higher levels of reverberation.

Additional information

Funding

Funding for the project was provided by Oticon A/S.