After this course learners will be able to:
- Name 3 ways in which digital technology has led to improvements in hearing aids.
- Explain how microphone location on hearing aids influence natural sound quality.
- Explain how directional microphone technology can both help and interfere with the listening goals of the hearing aid user.
Hearing aids provide the obvious functional benefit of better hearing. Not only that, wearing hearing aids is also strongly linked to improved hearing-related quality of life (Bisgaard & Ruf, 2017; Said, 2017; Brodie et al, 2018; Joanovic et al, 2019) and many who wear hearing aids report that they wished they had begun to do so earlier (Niebling, 2018). Better social connections and relationships, less strain and fatigue from listening, and usefulness when at work would seem to be compelling reasons to use hearing aids. Emerging evidence suggests that treating hearing loss with amplification in aging populations yields profound benefits for maintaining mental health and function (Glick & Sharma, 2020; Salant et al, 2020). Despite these positive effects, a broad expansion in the uptake of hearing aids has not yet occurred, even in countries where hearing aids can be attained at no or very low cost to the user (Valente & Amlani, 2017).
There are multiple barriers that likely limit greater adoption of hearing aids. In the US, lack of access to affordable interventions for hearing loss is one barrier. As a result the OTC Hearing Aid Act was passed as a way of improving access to affordable care. HCP-related behaviors and interactions are also likely to factor in; however, there is currently little research linking user outcomes with clinical practices (Ekberg & Barr, 2020; Ismail et al, 2019; Amlani, 2020). People with hearing loss may also consider or be told by trusted advisors (such as their general practitioners) that their hearing is not yet “bad enough” to warrant treatment (Jorgensen & Novak, 2020).
As hearing aid manufacturers, we observe but have limited influence over any of these barriers. The area where the hearing aid manufacturer does play the main role is in the design and production of the devices themselves. Therefore, it is of great interest to us to identify successes and remaining challenges related to the hearing aids and their usage. Potential users and people who have tried but rejected hearing aids may report that they simply don’t help enough relative to the burdens of acquiring and using them. What aspects of hearing aids might work for or against this outcome? In this article, we discuss the current status of hearing aids in three broad areas that relate to how readily acceptable they may be to users. First, the technology available in hearing aids today offers significant audiological benefits in terms of audio features and connectivity, while increasing the possibilities for them to meet hearing needs more effectively than could be imagined only 10 years ago. Second, there has been growing interest in the research community to explore hearing aid usage and benefit from an ecological perspective rather than a purely technical one. The impact of this research on hearing aid development can already be seen. Finally, hearing aids continue to bring basic challenges along with the benefits. Challenges such as amplification of unwanted sounds, lack of access to desired sounds, unnatural sound quality, appearance of the device, or physical annoyance might be enough to discourage individuals in using hearing aids.
Hearing Aids as Connected, High-Performance Audio Products
In the year 2000, the transition from analog and digital programmable hearing aids to fully digital hearing aids was well underway. Today it is complete. All hearing aids are digital, even those with the most basic technology. While the first digital hearing aids essentially replicated the functions of their analog predecessors, the initial promise of this technology has begun to be realized. Sound processing features that cancel feedback, analyze the acoustic environment, and manage unwanted noise are commonplace. The actual processing hardware – the digital signal processor (DSP) - has continually decreased in size while increasing in efficiency and capacity, which has allowed hearing aids to be made with wider input dynamic range, wider frequency bandwidth, and high resolution audio processing capabilities.
The advances in DSP that have led to audio quality improvements are also of significant audiological benefit for hearing aid users. A wider input dynamic range than was available in the first generations of digital hearing aids can provide a cleaner signal to the user. It can allow amplification of very low input levels with little added noise, as well as processing of very high levels without the distortion that results from overloading the digital conversion of the sound. Extended high frequency bandwidth is an example of an advancement that is far-reaching in terms of benefit. Boothroyd & Medwetsky (1992) suggested that the upper bandwidth of a high fidelity hearing aid should be at least in the region of 10 kHz but acknowledged that hearing aid technology and fitting practices presented limitations. The move to digital technology imposed further limitations, which eventually were overcome as efficiency of the digital processors increased. Today it is not unusual for hearing aids for mild-to-moderate hearing loss severity to have bandwidth extending to at least the frequency region recommended by Boothroyd & Medwetsky. The importance of audibility of these higher frequencies is subtle relative to the effects of improving audibility up to 3000 Hz, which was earlier thought to be a reasonable design criterion based on early research on understanding speech via the telephone. Although the effects are more nuanced, recent studies support that listeners are better able to pick out and understand speech from a multi-talker background with a broad bandwidth signal (Best et al, 2019; Zadeh et al, 2019). Further benefits of access to extended high frequency bandwidth have emerged as researchers continue to examine them in alternative ways. For example, the high frequency information can contribute to realizing when someone is talking specifically to you, which would allow you to turn your attention to that talker (Monson et al, 2019). It can even assist listeners in recognizing exactly who is talking to them as extended high frequency bandwidth has also been shown to improve talker identification (Schwartz et al 2018). Improved word recall and decreased listening effort (Cramer, 2018) as well as better sound quality (Moore et al, 2012; Ricketts et al 2008) are other potential benefits.
A recent feature that has been enabled by technology developments in the previous 10 years is digital wireless connectivity to smart devices and audio sources. The availability of digital wireless connectivity in hearing aids has obvious and easily demonstratable benefits for users. For example, studies have shown that wireless routing of phone signals to both ears provides benefit compared to acoustic telephone usage, especially if the signal goes to both ears (Picou & Ricketts, 2011; Picou & Ricketts, 2013; Jespersen & Kirkwood, 2016). The ability to add visual cues via video chatting enhances benefit even more (Jespersen & Kirkwood, 2016). Remote microphones that stream a talker’s voice (or other connected input) to users hearing aids are also well-established as beneficial (Rodemark et al, 2015; Ciorba et al, 2014). Hearing aid users have reported additional benefit via the use of wireless streaming accessories (Smith & Davis, 2014).
The emergence of lithium ion (Li-ion) batteries combined with the rise of digital hearing aid processing fulfills a longstanding user wish for rechargeable hearing aids. There have been numerous attempts at rechargeable hearing aids or hearing aid batteries in the past. However, rechargeable solutions for hearing aids have had an unfortunate history of being either clumsy, unreliable, providing too short use time or all of the above. Today’s Li-ion batteries have high capacity, support low voltages and have a very low self-discharge rate, which means they are suited to miniaturized devices like hearing aids. As part of their safety and reliability, this battery type has a self-management system that protects itself against issues with voltage, current load and excessive internal temperature. Furthermore, it can communicate this information to the hearing aid and to an external charger. Among other things, this allows the user to get information about battery and charging status from the hearing aid, charger or smartphone app that communicates with the hearing aid.
Researchers and development engineers have no shortage of ideas on ways to leverage digital technology. We are likely to see many incremental refinements of existing digital features in hearing aids as the processing becomes more powerful. For example, feedback cancellation processing may gain enough calculation power to be able to model effects at a distance away from the ear, allowing better performance in different types of acoustic environments. Sensors embedded in or connected to hearing aids may allow users to modify how sounds are enhanced or suppressed according to real-time measurement of their brain activity or behavior. In addition, connectivity with smart devices will increasingly provide an extra tool and an extra engine for improving on the ability of users to make more personalized adjustments to their hearing aids based on real-life data from their own environments and actions, as well as from the actions taken by others in similar environments.
Using Hearing Aids in Real Life
A primary complaint of people with hearing loss, regardless of whether they use hearing aids, is hearing in noisy situations. Logic would seem to dictate that if people can better understand speech in noisy backgrounds, then they will be happy with their hearing aids because they solved their main complaint. Unfortunately, this is not necessarily the case, evident by the history of directional microphone technology, which has consistently shown improved signal-to-noise ratio (SNR) and speech recognition in noise in laboratory studies (Nielsen, 1973; Valente et al, 1995; Wouters et al, 1999; Pumford et al, 2000; Walden et al, 2000). Despite the longstanding availability of this technology, people who wear hearing aids still name hearing in noise as their main challenge. While it should be recognized that hearing aid wearers are vastly more satisfied with listening in noise than non-wearers (Picou, 2020), there remains plenty of room for improvement in how hearing aids can help.
While auditory deficits certainly make listening more challenging, people who wear hearing aids are no different from anyone else when it comes to why and how they listen. There are many types of listening, very often with multiple other sounds in the environment. When listening, you may be interested in one detail, like when jotting down a phone number or catching someone’s name when being introduced. You may be interested in following a story in a larger context, such as when chatting with friends, listening to the news or watching a play. You may be trying to learn and attend to something in a more academic setting. You could be listening to music to enhance your mood. Or you could be relaxing outside, enjoying the sounds of nature and the neighborhood. Hearing aids should be able to support users in all the different ways they listen. While directional microphones have the potential to assist listening in noise, they rely on spatial separation of the desired sound and the competing sounds, and may render desired sounds less audible or make them difficult to follow if they are not in front, if they are moving, or if the desired sound changes from moment-to-moment. The rigid schemes by which hearing aid directionality typically is applied may not be the best way to support the ways in which users naturally listen.
As directional microphones began to become common in hearing aids, researchers studied acoustic, audiological, fitting and human factors that have impact on their measured and perceived benefit (Ricketts, 2000; Ricketts & Mueller, 2000; Ricketts and Hornsby, 2006; Wu & Bentler, 2010; Cord et al, 2004; Cord et al, 2002). Close study of these findings led ReSound to pursue a unique path in applying directionality in a way that focused on ecological utility rather than considering only laboratory performance. A strategy was developed that could leverage the sophisticated binaural listening ability of the brain rather than relying only on the ability of the technology to improve SNR (Cord et al, 2007; Groth, 2016). Natural binaural listening allows the healthy auditory system to identify features in a scene and focus attention on objects that are important while simultaneously monitoring objects that might become important.
The goal of the ReSound binaural strategy is straightforward: to seamlessly maximize the sound quality and benefit from the simplest to the most complex acoustic environments by imitating the native strategies of healthy auditory systems. This is accomplished in three ways:
- preserving and restoring important natural cues for properly analyzing the auditory scene,
- optimizing situational awareness and spatial unmasking by leveraging residual auditory function, imitating the natural head shadow effect, in which the brain weighs the information in the “better ear” over the noisy ear, and
- enhancing speech intelligibility for diffuse, challenging and complex listening environments.
Within this strategy, it is critical that intelligent switching among the three configurations is based on real-time awareness of the acoustic environment, and that it occurs in an unobtrusive way. Such a multi-faceted and organic approach is typically not present in traditional hearing aid solutions; however, it maximizes the opportunity for the auditory system and brain to enhance the acoustic scene, compensate for lost biological function, and immerse listeners in a rich, externalized sensory space.
Technically, the best enhancement of SNR using on-board hearing aid microphones can be achieved by combining the two-microphone directional signals from each ear to create a bilateral 4-microphone beamformer. This technology is often referred to as “bilateral” or “binaural beamforming” and has been commercially available in hearing aids for years. One major downside of binaural beamforming is that it creates one monaural signal that is presented to both ears, thereby robbing the brain of important binaural listening cues. In addition, its effectiveness in improving SNR for sound in front has the concurrent effect of further decreasing access to any sound not in front, which deceases usability of the technology in the real world. An obvious next step is to refine binaural beamforming technology in a way that may support native listening strategies in the real world as well as boosting SNR in a prototypical test environment. As such, further enhancement can be realized with the preservation of personal spatial listening cues that allow access to the acoustic scene in the vertical plane and beyond the look direction and horizontal plane.
Solving the “Side Effects” of Using Hearing Aids
Reducing burdens associated with hearing aids can be as big of a win as increasing performance. A striking example is the complete reversal in the industry from custom hearing aids to behind-the-ear styles fit with a cosmetic thin tube to the ear canal. ReSound introduced this concept in 1999 in an inexpensive, analog non-programmable, instant fit product intended for mild hearing loss, and then followed up by incorporating the small device, thin tube and instant fit concept in the hugely successful ReSoundAIR. Today, it is commonplace for hearing aids to place the receiver in the ear canal. In 2019, Receiver-in-the-Ear (RIE) made up 78.9% of the hearing aids dispensed in the US (Strom, 2020). There are good reasons for the popularity of the RIE style. Hearing Care Professionals (HCPs) can have them on hand to demo or even fit them instantly with a stock dome, the style is comfortable to wear and virtually invisible on most ears, the effects of BTE microphone tubing is bypassed, and the RIE often will have the best complement of noise management and wireless connectivity features compared to other hearing aid styles. The ability of the hearing aids to cancel feedback vastly expanded the pool of candidates suitable for open fittings, thereby eliminating issues associated with occluding the ear, enhancing sound quality via direct sound in the low frequency region, and contributing to physical comfort. All-in-all, this current most popular style has reduced the burdens of wearing hearing aids in many and important ways.
A fundamental disadvantage of the RIE design is that the microphones which pick up the sound are located above or even behind the outer ear. This is potentially detrimental to sound quality, as it is not where sound naturally enters the ear. Sound quality might seem like something worth sacrificing, as the term implies that it is an aesthetic quality rather than a critical one. But the loss of hearing compromises a person’s connection with the world in a way that extends beyond audibility of sounds. It impairs the ability to detect cues that comprise meaning in sensory experiences. The loss of these cues interferes with a person’s ability to analyze and orient to an external acoustic environment. Therefore, the perception of sound as natural means that the auditory system can organically select, separate, and integrate sonic features delivered via our ears to build mental models of the soundscapes in the environment. It encompasses the ability to orient to the surroundings, to identify, follow or even exclude auditory objects from our attention, and to appreciate depth and detail in the acoustic environment. The reason that microphone location matters is because the cues from the acoustic environment are highly personalized as they pass through the unique shape of each person’s body, head and ears, an effect that is further enhanced by how we move our bodies within acoustic scenes. In effect, everyone hears differently. Truly natural sound quality that results from this rich tapestry of cues gives us:
- immersion in an acoustic environment
- spatial orientation of the world around us
- an impression of the distance, texture, interactions and continuity of the physical items in our world
- an enhanced ability to tune out noise sources that crowd the sound we want to hear
Hearing aids today account for our loss of ability to hear soft sounds and provide features that can be helpful in hearing in noisy situations. But many of the cues that provide us with information about our external world are disrupted when a person wears what is today’s most popular hearing aid style. This can cause the acoustic world of the hearing aid user to collapse into a flat, internalized and spatially devoid experience. Many hearing aids with behind-the-ear microphone location offer processing that emulates average pinna cues. While this provides some benefit for front-back localization and sound quality, it is a non-personalized and incomplete solution that cannot provide a completely transparent listening experience for individuals, particularly in the vertical listening domain. As technologies have continued to advance and mature, the hearing aid industry appears ripe to introduce innovations that can restore or preserve some elements of natural sound quality without sacrificing the many benefits of what HCPs and users have come to appreciate in the RIE style.
Research such as MarkeTrak 10 and Eurotrak confirms the positive effects of wearing hearing aids in terms of both better hearing and communication as well as life quality. They also document trends supporting that users perceive and appreciate the advances made in hearing aids over the years. Although one of the first things that potential hearing aid users are counseled is that hearing aids won’t restore their hearing to normal, there are still a myriad of concepts put forth by hearing aid designers and researchers in amplification and hearing science that can further improve how well these devices help people with hearing loss. As technology continues to evolve, large and small improvements in hearing aids will also continue to make their debut.
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