Speech recognition in noise using bilateral open-fit hearing aids: The limited benefit of directional microphones and noise reduction

Abstract

Objective: To investigate speech recognition performance in noise with bilateral open-fit hearing aids and as reference also with closed earmolds, in omnidirectional mode, directional mode, and directional mode in conjunction with noise reduction. Design: A within-subject design with repeated measures across conditions was used. Speech recognition thresholds in noise were obtained for the different conditions. Study sample: Twenty adults without prior experience with hearing aids. All had symmetric sensorineural mild hearing loss in the lower frequencies and moderate to severe hearing loss in the higher frequencies. Results: Speech recognition performance in noise was not significantly better with an omnidirectional microphone compared to unaided, whereas performance was significantly better with a directional microphone (1.6 dB with open fitting and 4.4 dB with closed earmold) compared to unaided. With open fitting, no significant additional advantage was obtained by combining the directional microphone with a noise reduction algorithm, but with closed earmolds a significant additional advantage of 0.8 dB was obtained. Conclusions: The significant, though limited, advantage of directional microphones and the absence of additional significant improvement by a noise reduction algorithm should be considered when fitting open-fit hearing aids.

Key Words::

• Directional benefit
• directional microphone
• hearing aid
• IOI-HA
• Noise reduction
• omnidirectional microphone
• open fitting

Acknowledgements

The authors would like to thank Michael Valente, Kristi Oeding, and an anonymous reviewer for their important comments and helpful suggestions on an earlier version of the manuscript.

Declaration of interest: The authors report no conflict of interest. The authors alone are responsible for the content and the writing of the paper.

The study was supported by a grant from the Rune and Ulla Amlövs foundation for audiological research.

Appendix

Calculation of targets: Phonak Adaptive Digital

Rather than distinct, separate fitting formulas, the hearing instrument uses the Phonak Adaptive Digital fitting formula, which includes transitions for different hearing loss configurations. As such it recognizes configurations such as ‘standard’, ‘ski-slope’ ‘low-frequency’ and ‘profound’ hearing losses. The Phonak Adaptive Digital fitting formula also recognizes the importance of reaching gain targets at specific frequencies, depending on the hearing loss configuration. For example, the importance of reaching targets for speech-relevant inputs and frequencies (the so-called speech banana) is weighted higher than for frequencies outside this speech banana. The transitions from ‘standard’ to a different fitting pre-calculation are:

Transition to ski-slope criteria

• Hearing thresholds at or below 750 Hz to be less than 30 dB HL.

• Hearing thresholds at 4000 Hz or higher should be at least 50 dB more than hearing thresholds at 500 Hz or lower.

• The slope over two octaves between 500 Hz and 4000 Hz inclusive should be at least 50 dB.

Transition to low-frequency hearing loss criteria

• Hearing thresholds at or below 750 Hz should be at least 25 dB worse than at 1500 Hz or higher.

• Hearing thresholds between 2000 and 4000 Hz (inclusive) should be 20 dB HL or better.

• Weight shapes are low for regions with hearing loss of more than 45 dB HL over the minimum hearing loss between 500 and 4000 Hz.

Transition to profound hearing loss criteria

• Average hearing thresholds at 500, 1000, 2000, and 4000 Hz should be at least 75 dB HL.

• Hearing thresholds should be more than 55 dB HL.