An Improved Option for People with Severe to Profound Hearing Loss

Individuals with severe to profound hearing impairment have a reduced dynamic range that can make the selection and fitting of amplification challenging. When fitting this group of people with hearing aids, the goal is to provide as much auditory information as possible and thereby facilitate optimal speech recognition.¹

Frequency resolution is often reduced for these individuals due to the broadening of auditory filters, which increase the influence of masking noise. However, the ability to process sound in the temporal domain is less influenced, particularly in the lower frequencies. This ability to use information from the amplitude-time waveform helps in speech decoding; it aids the perception of suprasegmental speech cues (eg, stress and intonation).²

Using these listening cues is more challenging in the presence of background noise. Even when wearing hearing aids, detecting these speech decoding cues may be difficult and require more mental processing effort from the listener.

In a clinical study of Chili SP, the potential benefits of speech intelligibility, listening effort, and sound quality were explored by comparing Chili SP with the participants’ own hearing instruments.

The Effort of Listening

A common complaint by hearing aid users is that listening in noisy situations is an exhausting experience. Listening is a central part of conversation, and without a good representation of the spoken word, other parts of communication suffer greatly.

When having a conversation, the task of the listener is not merely listening to the speech signal; communication is a process that also taxes central auditory function such as focusing attention to specific sound sources, storing information in memory, using contextual information to improve understanding, resolving ambiguities, and preparing appropriate responses quickly.

For individuals with severe-to-profound hearing loss, additional tasks—such as making up for reduced audibility and compensating for impaired intelligibility by straining to catch features as stress, prosody, and intonation—are added to the listening/conversation process.

This requires significant additional effort for people with hearing impairment, especially in challenging sound environments. The cognitive system has limited resources available at any given time, and as one area is taxed more, the capabilities of other areas are negatively impacted (eg, a reduced capacity to store information).³ Greater effort of listening denies cognitive resources for other activities and may account for the self-reported increase in stress level and exhaustion reported by hearing-impaired listeners in noise.

When the information embedded in the speech signal is not effectively flowing through the periphery, cognitive processing becomes more reliant on the use of context and the listener’s own knowledge of the topic, and less on the information inherently available in the actual signal, had it not been degraded. Again, this makes listening more effortful.⁴ Thus, sufficient speech understanding may be obtainable even in situations with background noise; however, it is unlikely this comes without significant effort—effort that may not be directly observed in lab tests of speech intelligibility.

A hearing aid may help the user release working memory resources to allow for better storage and faster information processing in challenging situations.³ Additionally, it is possible that hearing aids contribute to an ease in perceived listening effort even if improvements of speech intelligibility may be less significant.

A Super Power Instrument With Specific Design Objectives

With a maximum output of 139 dB and a peak gain of 78 dB, the Oticon Chili SP hearing instrument is a non-linear super-power amplification system that features Speech Guard, split directionality, binaural processing, and connectivity (unpublished white paper by Donald Schum, PhD, available at ).

The Speech Guard processing strategy is designed to retain the speech envelope and thereby offer the hearing aid user the full dynamic properties of the speech signal.⁵ Chili SP features a dedicated prescriptive formula, the Dynamic Speech Enhancement Super Power (DSEsp). The device’s split directionality feature is designed to accommodate individuals who hear better in the low frequencies than in the high frequencies. Split directionality attenuates the high frequency input from the back and the sides, while still allowing the listener the full low frequency signal.¹

This following study sought to verify these benefits, as well as explore the effects of the hearing instrument on both objectively measured and subjectively perceived listening effort by individuals with severe to profound hearing loss.

Study Design

A total of 35 individuals with severe to profound hearing loss tested the Oticon Chili BTE SP9 against their own hearing aid at the Hörzentrum in Oldenburg, Germany (n=20) and at Oticon A/S, Denmark (n=15). Participants wore hearing devices of competing brands, as well as other Oticon hearing instruments.

As a starting point, all participants were satisfied with their own hearing aid performance in the sense that they had not contacted their dispensing professional for fine tuning or problem solving. The mean age of the participants at Hörzentrum was 64.8 years and the gender distribution was 12 males and 8 females; at Oticon A/S, the mean age was 57.1 years and the gender distribution was 9 males and 6 females.

Mean hearing thresholds levels (HTL) for the two test sites are shown in Figure 1. A significant difference is seen in HTL between the two populations. At Hörzentrum, the mean four-frequency hearing threshold level (4FAHL, the HTL average at 0.5, 1, 2, and 4 kHz) was 71.3 dBHL, and 91.2 dBHL at Oticon A/S. This difference has been taken into account in subsequent data analysis by using “site” as a predictor variable in the data analysis. The variation of hearing thresholds of the participants in the study reflects the full fitting range of Chili SP.

The participants were fitted with Chili SP and allowed fine tuning using the overall loudness trimmer as a primary tool for enabling user acceptance. The overall loudness trimmer operates on all underlying gain trimmers altering maximum power output (MPO) in 2 dB steps. For this test, only 4 of the possible 10 step-up/step-down fine tunings were used to promote consistency between participants. Fine tuning of noise reduction from moderate to maximum, change in directionality settings, and change of identity were also allowed due to the sometimes-detailed sound processing preferences of this group of hearing instrument users.

Before starting the test period with Chili SP, participants evaluated their own current hearing aid. The test period lasted 4 to 6 weeks. After the test period, participants completed laboratory tests and the comparative questionnaires. All participants were subjected to lab tests of speech intelligibility in noise, a lab test of listening effort, and questionnaires describing their experiences with and benefit from the hearing aids in real-life situations.

Lab tests of speech intelligibility in noise were performed with the Matrix sentences (Dantale II in Danish,⁶ and OlSa in German⁷) in modulated and unmodulated noise. The sentences were presented at a fixed level of 70 dBSPL, while the noise signal changed adaptively to obtain a SRT level of 50% correct. The tests were conducted with the participant’s own hearing aid in the omnidirectional setting and test hearing aid (Chili SP) in both omnidirectional and split directional setting. The speech was presented at 0° azimuth, and the background noise was presented at 110°, 180°, and 250° azimuth.

Listening effort tests were also performed with the Matrix sentences, requiring the participants to rate the listening effort in different situations on a visual-analog scale with anchors of “No effort” and “Max effort.” The measurement was conducted at both a reference SNR (the noise level of 80% correct found with the training part of the Matrix sentences at speech level of 70 dBSPL) and a SNR improved by 3 dB, obtained by turning down noise by 3 dB. The measurement was obtained in unmodulated noise with both the own aids and Chili SP hearing aids in omnidirectional setting.

After the test period, the participants completed an abbreviated version of the SSQ-C⁸ comparing the Chili SP to their own hearing aid on different parameters, and they stated their overall preference between the hearing aids. During the field test period, all features (directionality, noise management, etc) were enabled in both their own instruments and test instruments.

Study Results

Improved speech intelligibility in noise. Results from the speech-in-noise testing showed that mean scores were better with Chili SP than with their own device. Figure 2 shows the mean of the individual SRTs relative to presented noise level results from the speech intelligibility testing in modulated and unmodulated noise. Results were analyzed with repeated measures analysis of variance (ANOVA) and the main effects of these analyses were further explored using Scheffé post-hoc tests.

An effect of site was found in speech intelligibility results. This is possibly related to the difference in hearing thresholds and thus SRTs between the two sites. For all measures, the interaction between test site and hearing instrument was explored to make sure the site did not have an effect on the results. No interaction was found.

There was a significant overall effect of the hearing aid. The results for the modulated condition was significantly (p=0.040) improved, with a 0.9 dB better score for Chili SP, and the difference in unmodulated noise was non-significant. The participants scored significantly better in unmodulated noise than in modulated noise.

A highly significant effect (p<0.00001) of directionality was found in the comparison of Chili SP in omnidirectional mode versus Chili SP in split directional mode. Chili SP provided 3.5 dB benefit in the directional mode compared to the omnidirectional mode, or approximately 30% better speech understanding (assuming a 10% per dB sensitivity of the test).⁶

Subjective results (Figure 3) from the Speech section of the abbreviated SSQ-C show similar benefits of Chili SP. The participants rated Chili SP significantly better than their own hearing instrument on all speech items.

Less listening effort. The results of the listening effort test showed significantly less listening effort with Chili SP (p<0.001, Scheffé post-hoc test), both at the reference SNR and at the improved SNR setting as seen on Figure 4 (below). The data analysis (ANOVA) showed significant effects of hearing instrument and noise level, but no significant effect of site and no interactions.

Listening effort was also rated subjectively in the related items of the Qualities section of the SSQ-C: the need to concentrate when listening, the effort of conversation, and the ability to ignore competing sounds (bottom three items in Figure 5 above).

Sound quality and overall preference. On the remaining SSQ-C Qualities items (Figure 5), Chili SP was rated significantly better. Items concerning naturalness of the voices of others and clarity of sound were rated particularly well.

Overall, the results show a number of significant benefits with Chili SP compared to the participants’ own hearing instruments across a range of lab tests and self-assessment measures of real-world experiences. This was reflected in the participants’ choice of their overall preference.

The overall preference was chosen on a categorical scale from “own instrument much better” to “Chili SP instrument much better” (Figure 6). Of 35 participants, three quarters (74%, or 26 participants) found Chili SP to be “somewhat better” or “much better” than their own hearing instrument. Two participants did not prefer one to the other, and 7 found their own hearing aid to be “somewhat better” (none found their own device to be “much better” than Chili SP). When asked to provide a reason for the preference, the majority of the participants responded that Chili SP provided better speech understanding; others perceived improved sound quality, better audibility, and benefits from connectivity.

Summary

A clinical study comparing Chili SP to users’ own hearing aids was carried out. A total of 35 participants with severe to profound hearing loss participated in the two-site study. Results indicated that Chili SP provided significantly less listening effort both in lab tests and in the responses from the participants’ real-life experience with the aids.

Likewise, speech intelligibility results with the test aids were significantly better than the participants’ own hearing instruments, both in lab tests and when evaluated by the participants. Of the 35 users, 74% found Chili SP somewhat or much better than their own hearing aid in improved speech understanding and better sound quality.

The reduction in listening effort observed with Chili SP is a significant finding, as greater levels of listening effort deny cognitive resources for other activities and may account for the self-reported increase in stress level and exhaustion reported by hearing-impaired individuals when listening in noise.

The Effect of New Super Power Device

The significant benefits seen in speech intelligibility, listening effort measures, and questionnaire responses in this study can be attributed to the combination of the audiology and the hardware platform of Chili SP.

The perceptual effects of the more linear compression strategy, Speech Guard, were described by Sockalingam et al¹ in the January 2011 HR. These effects are likely also seen in the present study by the significant improvement in speech intelligibility observed with Chili SP on tests in modulated noise, where the mean improvement was 0.9 dB. In the unmodulated condition, the mean improvement was 0.4 dB. The great improvement in the difficult modulated condition will possibly provide more perceived benefit in similar noise conditions in real life (eg, situations with speech maskers in the background).

An improvement in perceived sound quality was found in the subjective ratings on the Qualities section of the SSQ-C questionnaire. The participants found Chili SP to provide more natural sound, more clarity, and better sound separation.

The split directionality also contributes to the positive results of Chili SP. Because the temporal frequency resolution in the low frequencies is similar for people with both severe-to-profound hearing losses and normal hearing,¹ high frequency attenuation of disturbance is desirable for the former group. High frequency directionality provides this attenuation, but leaves the low frequency input intact. The combined efforts of the split directionality and the noise management system are likely reflected in the subjectively perceived decrease in listening effort found in the SSQ-C items relative to concentration, effort of conversation, and ability to ignore competing sounds. The benefit of directionality for individuals with severe-to-profound hearing loss is probably pronounced because the split directionality facilitates selective attention (ie, the process of focusing on a speech signal and ignoring the noisy background).

Acknowledgements

The authors would like to thank Renskje Hietkamp and Niels Søgaard Jensen of Eriksholm Research Centre, Oticon A/S for their valuable contributions with the test planning and with data analysis and comments on earlier versions of this paper. Additionally, the authors would like to thank colleagues and collaborators at Oticon A/S and Hörzentrum for their hard work conducting this study.

References

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