Facilitating Spatial Hearing Through Advanced Hearing Aid Technology

This article was submitted to HR by Douglas L. Beck, AuD, director of professional relations at Oticon Inc, Somerset, NJ, and Ravi Sockalingam, PhD, AuD, senior audiologist at Oticon A/S, Smørum, Denmark. Correspondence can be addressed to Dr Beck at .

Spatial hearing ability is unique in every person and varies across every moment in time and every acoustic environment. It is much more than aided versus unaided, left versus right, or up versus down. Spatial hearing might be thought of as the “gestalt” auditory sensation; it identifies where things are in space and allows us to focus our attention on a primary sound source in difficult listening situations.

One of the primary challenges when sorting through speech sounds in difficult acoustic (ie, noisy) environments is to identify the location of the sound source. Once the brain has identified the location of the sound source of maximal interest, the brain focuses cognitive resources on that specific location to track (ie, follow) with precision that particular sound source over time. Additionally, after identifying the sound source of maximal interest, it is more likely one can ignore and dismiss less important auditory information.

Spatial Hearing and Amplification

Spatial hearing ability is a relatively new topic for many hearing care professionals. In years past, although a plethora of significant improvements have been made with regard to hearing aid technology, little was done to replicate and maintain the natural multi-dimensional acoustic cues and environments in which we listen. Fortunately, multiple technological advances now allow us to address these concerns. Specifically, binaural processing, extended bandwidths, open fittings, and receiver-in-the-ear (RITE) technologies each impact and facilitate spatial hearing ability.

Binaural processing allows the brain to compare and contrast sounds from each ear so as to detect and evaluate acoustic information. Typically, when considering binaural processing with regard to localization issues, one considers interaural loudness differences (ILDs) and interaural timing differences (ITDs). Indeed, ITDs are of greatest concern for sounds below 1500 Hz, and ILDs are of greatest concern for sounds above 1500 Hz. Interaural loudness differences are very significant with regard to speech sounds. Indeed, ILDs can be 20 to 25 dB from 4000 to 10,000 Hz (Figure 1). Preece¹ noted humans localize better along the horizontal plane than the vertical plane, and he noted for sounds above 1500 Hz, ILD differences are of primary importance and ITDs become increasingly ambiguous.

FIGURE 1. Significant interaural level differences (20 to 25 dB) often exist between the right and left ears for acoustic information up to 10,000 Hz. From Behrens.9

Extended bandwidths, as Beck and Olsen² noted, allow better sound quality and improved speech perception in quiet and noise. The use of extended bandwidth (to 10,000 Hz) in hearing aids is one way of delivering important ILD cues to the brain. Traditional bandwidths were often limited to approximately 4000 to 5000 Hz, and therefore could not transmit spectral or spatial information between 5000 and 10,000 Hz. Thus, naturally available high frequency information was imperceptible, and binaural processing of that information did not occur. Beck and Sockalingam³ noted hearing aid amplification should make all natural speech and spatial information audible, and it should maximize speech understanding through intelligent signal processing.

Open fittings provide significant and substantial solutions for the occlusion effect. However, open fittings also allow important spatial information to be maintained. Indeed, open fittings allow natural low frequency acoustic cues to enter the ear canal without amplification to provide a natural presence of sound in tandem with interaural timing and loudness differences.

Receiver-in-the-ear (RITE) fittings avoid resonant peaks, which may be present in tube or slim-tube fittings. Further, RITE fittings (as compared to tube fittings) permit a higher frequency response to be delivered to the ear.

Traditional WDRC and Spatial Hearing

People with hearing loss generally have decreased spatial hearing abilities.⁴ Wide dynamic range compression (WDRC) has been well accepted by professionals, hearing aid users, and the hearing aid industry, and has been a pivotal technology with respect to assuring audibility while protecting individuals from uncomfortably loud sounds.

However, WDRC can negatively impact the spatial hearing ability of hearing-impaired listeners despite well-accepted clinical protocols, such as setting independent kneepoints for the left and right hearing aids based on hearing thresholds, through unnatural (ie, artificial) compression ratios, and through release times, which may distort the relationships between and among sounds (see Behrens et al⁵ for a detailed discussion of this relative to ILDs).

The negative impact of traditional WDRC may be increased when binaural hearing aids are not synchronized, resulting in diminished spatial perception and localization ability,⁶ as well as artificial loudness percepts, which may negatively impact interaural loudness differences and binaural perceptual processes. Binaural synchronized digital signal processing features, such as compression, noise reduction, and directionality, can help minimize these same WDRC-based problems. Indeed, some hearing instruments offer a balanced sound picture via wireless binaural synchronization.^7-9

Complaints associated with WDRC include scenarios such as user dissatisfaction while wearing amplification in a crowded restaurant. Specifically, the wearer may hear the conversation from a few tables away louder than they hear their conversation partner across the table. The reason this may occur is WDRC applies the most gain to the quietest signals—including conversation occurring at other tables.

With regard to release times, slow-acting compression tends to preserve short-term level changes and may be better with regard to comfort and sound quality. Fast-acting compression may benefit speech intelligibility¹⁰ yet may disrupt or distort acoustic cues particularly when background noise is present.¹¹

New Developments in Compression to Facilitate Spatial Hearing

To improve speech intelligibility while providing maximal listening comfort and excellent sound quality, a new compression system, called Speech Guard, has been developed. Speech Guard is featured in Oticon’s Agil hearing instrument and is based on a “floating” linear gain adjustment system that combines the advantages of linear and WDRC systems. Speech Guard acts rapidly to protect against abrupt and significant changes in loudness, and it acts slowly when the input signal is relatively stable. The system has been described in detail by Simonsen and Behrens.¹¹

FIGURE 2. ILDs for Oticon Agil with Speech Guard and a conventional compressor. The bold line shows ILDs for experimental aids with Speech Guard and the lower line shows ILDs for traditional WDRC.

Figure 2 demonstrates ILDs observed at the output of a pair of Agil hearing aids using Speech Guard versus a conventional WDRC compressor, based on a continuous male talker stimulus interrupted by a loud train whistle. The input signal was sent through a speaker placed directly to the right of a HATS mannequin positioned in an anechoic room. For both compressors (Speech Guard and WDRC), bilateral fittings with two similar hearing aids were used. All automatic features were turned off and no synchronization took place between the two hearing aids. The Agil hearing aids using Speech Guard were measured to have average ILDs in the high frequencies (>1.5 kHz) of 3.4 dB more than the hearing aids using a conventional WDRC compressor.

Studies on the Effects of Spatial Sound and Compression Systems

As discussed above, Oticon’s proprietary Spatial Sound system uses binaural processing to help maintain the ILDs, which are naturally present at the ears. Other components of Spatial Sound include: extended bandwidth, RITE technology, and open fittings.

Hansen¹² evaluated 58 hearing aid wearers between the ages of 28 and 84 years (average age of 72). All participants were previously fitted binaurally with advanced hearing aids. Participants compared their own advanced hearing aids to Epoq with Spatial Sound. A total of 91% of all participants reported being accustomed to the sound of Epoq with Spatial Sound after only 1 week. Evaluation of spatial attributes using the Speech, Spatial and Qualities questionnaire (SSQ) revealed the group rated Epoq significantly higher than their own advanced hearing systems in complex listening environments.

ILDs	Speech	Train
Speech Gaurd	9.7 dB	10.8 dB
Conventional	5.9 dB	7.8 dB
TABLE 1. This table documents substantial benefit using Speech Guard in challenging acoustic situations in which spatial hearing is important. Thus, Agil preserves spatial cues and ILDs better than conventional WDRC compressors as measured in these experiments.

Sockalingam et al¹³ evaluated 30 subjects (average age of 66 years) with mild-to-moderate sensorineural hearing loss. Of the total 30 subjects, 14 were new users, and 16 were experienced hearing aid wearers. All were fitted with binaural Oticon Dual XW hearing aids. Localization ability and sound quality in noise were compared between two conditions: binaural wireless link “on” and binaural wireless link “off.” In the binaural wireless “on” condition, binaurally coordinated compression, noise management, and directionality were engaged. When the binaural wireless link was “on,” subjects made significantly fewer (p<0.05) localization errors in noise than when Spatial Sound was “off.” Moreover, subjects reported significantly greater “naturalness” (ie, sound quality) in challenging acoustic environments, such as a café setting, when the binaural wireless link was “on.”

More recently, in a two-site field study of 39 experienced hearing aid users, Oticon Agil with Speech Guard and Spatial Sound engaged showed significantly better performance in the spatial domain as compared to an advanced conventional hearing instrument. The SSQ-Comparison (SSQ-C) scale was used to compare the two instruments.¹⁴ For all investigated items of the spatial subscale, Oticon Agil was rated significantly better (p<0.01) than the conventional instrument.

Conclusion

The ability to identify the origin of sounds (with respect to their location in space) is a fundamental function and attribute of binaural hearing. Once the specific sound source has been identified, the listener can focus attention on the primary sound signal of interest, while ignoring or dismissing competing speech or background noise.

High frequencies (>1500 Hz) contain significant binaural natural loudness level differences, which should ideally be preserved through advanced hearing aid amplification systems. Advanced hearing instruments with functionality designed to improve speech and spatial information (such as Spatial Sound and Speech Guard in Oticon Agil) play an important role in facilitating spatial hearing and improving listening in challenging situations.

References

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11. Simonsen CS, Behrens T. A new compression strategy based on a guided level estimator. Hearing Review. 2009;16(13):26-31.
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13. Sockalingam R, Holmberg M, Eneroth K, Shulte M. Binaural hearing aid communication shown to improve sound quality and localization. Hear Jour. 2009;62(10):46-47.
14. Jensen NS, Akeroyd MA, Noble W, Naylor G. The Speech, Spatial and Qualities of Hearing scale (SSQ) as a benefit measure. Poster presented at: NCRAR conference on “The Ear-Brain System: Approaches to the Study and Treatment of Hearing Loss.” October 2009; Portland, Ore.

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This article was submitted to HR by Douglas L. Beck, AuD, director of professional relations at Oticon Inc, Somerset, NJ, and Ravi Sockalingam, PhD, AuD, senior audiologist at Oticon A/S, Smørum, Denmark. Correspondence can be addressed to Dr Beck at .