OTC Hearing Aids a Year On and Their Effect on Hearing Aid Uptake

It has been almost a year since the Over-the-Counter (OTC) Hearing Aid Act came into effect. There was much excitement and expectation at the outset that OTCs would improve hearing aid uptake by bringing about greater affordability and accessibility. In the past year, several new entrants have made a foray into the hearing aid market and have introduced OTC devices, with many of the big players also adding OTC devices to their line-ups through a series of collaborations, acquisitions, and rebranding. Has OTC affected hearing aid uptake?

Survey Data

The most direct evidence for recent trends in hearing aid uptake that follows the OTC Hearing Aid Act comes from an ASHA online survey and a recent MarkeTrak survey. The ASHA survey, conducted on 2,228 American adults ages 40 and older over the summer this year, reported that although awareness of OTCs had increased from 2021, only 2% of those with hearing difficulty purchased OTCs (since they became available) and only 4% of those with hearing difficulty reported that they are likely to buy OTCs in the next year. These numbers seem low given that 56% of American adults above the age of 40 acknowledged having some hearing loss, and only 8% reported getting any treatment for it. However, they are in line with the 3.4% increase in adoption rate attributed to OTC or PSAPs reported by MarkeTrak.

MarkeTrak is a consumer survey that has investigated hearing aid users and non-users in the United States for over three decades. The 2022 survey (MT2022) reported an increase in hearing aid adoption from 22% in 2000 to 38% in 2022. MT2022 reports that hearing aid fitting by a hearing care practitioner (HCP) is still the most common (81%), followed by fitting remotely by HCP (12%) and self-fitting (7%). Those who preferred a self-fitting hearing aid also had lower incomes than those who preferred the traditional fitting. Most hearing aid owners who got their devices in the past five years were satisfied with the overall value (85%) and the out-of-pocket price (74%). However, non-owners indicated that hearing aids are too expensive (55%), not affordable (40%), or they lack coverage (31%).

A similar trend for adoption rate has been reported in the EuroTrak data from France (46%) and Germany (41%) despite sizable differences between the US and European markets concerning reimbursement for hearing aids. Whereas in the United States, MT2022 estimated that 54% of hearing aid owners had some level of third-party assistance covering the cost, EuroTrak reported the corresponding estimate to be 89% in France and 94% in Germany. This illustrates that although affordability is a barrier to hearing aid uptake, it is not the only barrier. In countries where healthcare models make hearing aids much more affordable, hearing aid adoption rates are only slightly higher than those in the United States, suggesting that factors other than cost may be at play. In France and Germany, the reasons cited for non-adoption have in common the following factors also noted in the United States:

• cost, which continues to be a barrier even with more affordability
• the opinion that the hearing aid does not sufficiently restore hearing
• that they (non-adopters) hear well enough in most situations

Additional factors include physician (ENT and GP) opinion, and the hearing aid being uncomfortable. Some researchers have suggested that (apart from perceived effectiveness/benefit), other factors such as the social value of hearing aids, and possibly an interaction between social stigma, personality traits, and locus of control¹ may contribute to hearing aid non-adoption (Sternasky and Dhar, 2021).

It is probably too early to see any improvements in the hearing aid adoption rate and it may take some time to see a change in consumer behavior. The increase in awareness of OTC hearing aids is encouraging. The data above suggests already that several factors contribute to the decision not to purchase hearing aids, so it is reasonable to think that it will take much more than a lower price tag to get people with hearing loss to buy into amplification. Some recent articles provide useful insight into the topic.

Evidence from Systematic Reviews and a Scoping Review

A systematic review published earlier this year considered audiological and non-audiological factors that affected hearing aid uptake in the past ten years (Knoetze et al., 2023). Forty-two articles met their inclusion criteria. The following factors were positively associated with hearing aid uptake:

• severity of hearing loss
• self-reported hearing disability
• access to financial support
• cognitive anxiety²
• urban residency
• perceived potential benefit of amplification

The same study also examined hearing help-seeking. The following factors were positively associated with help-seeking:

• perceived potential benefit of amplification
• cognitive anxiety
• social factors (e.g. social pressure)

Unsurprisingly, some factors associated with hearing aid uptake also were associated with hearing help-seeking. When we convince ourselves of the benefits of amplification and can relate the implications of hearing loss to situations that we find ourselves in, we are ready to seek help. Sometimes we are convinced by others.

A second unpublished systematic review investigating hearing aid uptake in the last decade (the same period as the first review) found 15 studies that met their inclusion criteria (Jiang, 2023). It reported two factors that had significant positive associations with hearing aid uptake:

• self-perceived hearing loss
• degree of hearing loss

Although systematic reviews are extremely useful in summarizing and critically appraising literature extant in the field, they do not serve to identify gaps in literature, clarify concepts, or investigate research conduct within a domain (Munn et al., 2018). A scoping review is more useful in that regard.

A scoping review sought to understand better the theories from behavioral sciences that informed hearing aid-related decision-making in audiology (Iankilevitch, Singh, and Russo, 2023). Over the past 20 years, they found 24 articles that met their inclusion criteria. These studies were mostly based on four theoretical models. Three of the four models were based on health psychology, leading to the insight that to understand decision-making regarding hearing aid adoption a variety of theoretical models (from diverse fields) are needed that look at prospective research, have an experimental design, and examine behavioral outcomes in addition to attitudes (e.g. purchasing or returning a hearing aid).

A Prospective Study

A prospective study that examined hearing aid uptake among a group of volunteers from the community did just that (Humes, 2021a). This convenience sample comprised 139 older adults who were first-time hearing aid users. As it is well known that the severity of hearing loss impacts hearing aid uptake, the study examined differences between adherents (those who were advised to pursue hearing aids and they did) and non-adherents (those who were advised to pursue hearing aids and they did not) after controlling for this factor. The distinguishing factors between adherents and non-adherents were:

• perceived difficulties and reactions to them,
• expectations for hearing aids, and
• high-frequency PTA

Non-adherents had better high-frequency hearing, perceived that they had better communication performance, and reported better adjustment to hearing problems. They also had lower expectations for hearing aids.

The same study investigated the differences between those study participants who chose to keep their hearing aids one month after the trial and those who didn’t. They reported minimal differences, except poorer aided outcomes at 1-month post-fit, specifically outcomes related to satisfaction, benefit, and usage. 

Apart from high-frequency PTA the other two factors that distinguish adherents and non-adherents are based on perception, and susceptible to change (Humes, 2021b). This study highlights the importance of the audiologist’s role in providing patients with appropriate counseling and education. The audiologist can counsel the patient on various strategies they might use to address their hearing loss and social and emotional consequences of hearing loss, set realistic expectations for hearing aid performance in a variety of hearing scenarios, and empower them and others around them to (take charge of and) facilitate situations that benefit them.

How to improve hearing aid uptake?

1. Change the perception of hearing loss: Note that both the prospective study mentioned above and the systematic reviews highlight the importance of the perception of hearing loss/difficulty as a factor that governs hearing aid uptake, which also forms the basis of device candidacy (perceived mild-to-moderate hearing difficulty) as defined in the Over-The-Counter Hearing Aid Act of 2017. Unless the person with hearing loss is convinced of their hearing loss and the need for its redressal they are unlikely to seek help. Apart from routine measures of hearing loss (audiometric thresholds) audiologists could administer self-report measures of hearing handicap in the clinic. For example, the Hearing Handicap Inventory for the Elderly (HHIE) has been widely used in literature. Some new measures based on HHIE and screener for the Communication Profile for Hearing Impaired (CPHI) are promising (Humes, 2021c). Additionally, realistic simulations of different hearing scenarios that users commonly encounter can be offered as a tool to help evaluate listening difficulty.
2. Set Realistic Expectations: Audiologists can help set realistic expectations for hearing aids. While it is important to highlight that the vast majority of hearing aid users are satisfied with their purchases, it is important also to convey that noisy situations such as restaurants can be challenging even with hearing aids and for normal hearing listeners. Yet noise reduction in hearing aids may improve ease of listening comfort and alleviate fatigue considerably. These expectations must be set prior to the trial of hearing devices. HearAdvisor offers recording of various listening scenarios, alongside the same scenarios in the aided condition both with the initial fitting and fine tuning. These recordings are available for a variety of hearing devices, including OTCs. Listening can help provide value for fine-tuning.
3. Provide support: Nothing can replace the experience of wearing hearing aids in your day-to-day environment. Most users find themselves in situations where their hearing aids do not perform at par with their expectations within the first few weeks. When this happens, the audiologist must be available to engage the user. This entails understanding the specific acoustic scenarios that were problematic and providing specific steps to improve the hearing experience (fine-tuning or other changes the user can make to their hearing devices), the acoustic scenario to make it more conducive for hearing, use of hearing accessories as appropriate, or resetting expectations.

References

1. Health, Center for Devices and Radiological (2023). OTC Hearing Aids: What You Should Know. FDA. https://www.fda.gov/medical-devices/hearing-aids/otc-hearing-aids-what-you-should-know
2. American Speech Language and Hearing Association (2023). ASHA OTC Hearing Aid Survey. Published 22 August, 2023 by YouGov.
3. Knoetze, M., Manchaiah, V., Mothemela, B., & Swanepoel, D. W. (2023). Factors Influencing Hearing Help-Seeking and Hearing Aid Uptake in Adults: A Systematic Review of the Past Decade. Trends in Hearing, 27, 23312165231157255. https://doi.org/10.1177/23312165231157255
4. Jiang, Z. (2023). An Updated Systematic Review of Factors Influencing Hearing Aid Uptake in Adults from 2011 to 2022. Master’s Thesis. University of Canterbury.
5. Humes, L. E. (2021a). Differences between older adults who do and do not try hearing aids and between those who keep and return the devices. Trends in Hearing, 25, 23312165211014329.
6. Munn, Z., Peters, M. D. J., Stern, C., Tufanaru, C., McArthur, A., & Aromataris, E. (2018). Systematic review or scoping review? Guidance for authors when choosing between a systematic or scoping review approach. BMC Medical Research Methodology, 18, 143. https://doi.org/10.1186/s12874-018-0611-x
7. Iankilevitch, M., Singh, G., & Russo, F. A. (2023). A Scoping Review and Field Guide of Theoretical Approaches and Recommendations to Studying the Decision to Adopt Hearing Aids. Ear and Hearing, 44(3), 460. https://doi.org/10.1097/AUD.0000000000001311
8. Powers, T. A., & Bisgaard, N. (2022). MarkeTrak and EuroTrak: What We Can Learn by Looking Beyond the U.S. Market. Seminars in Hearing, 43(4), 348–356. https://doi.org/10.1055/s-0042-1758361
9. Sternasty, K., & Dhar, S. (2021). Barriers to Hearing Aid Adoption Run Deeper Than the Price Tag. JAMA Otolaryngology–Head & Neck Surgery, 147(6), 498. https://doi.org/10.1001/jamaoto.2021.0172
10. Humes, L. E. (2021b). Insight into the Reluctance of Hearing Aid Use in Older Adults. The Hearing Journal, 74(11), 6–8. https://doi.org/10.1097/01.HJ.0000800716.75423.49
11. Humes, L. E. (2021c). An Approach to Self-Assessed Auditory Wellness in Older Adults. Ear & Hearing, Publish Ahead of Print. https://doi.org/10.1097/AUD.0000000000001001

1. Locus of control is the degree to which you feel that events are within your control as opposed to outside your control. ↩︎
2. Cognitive anxiety is defined as a transient state where a person struggles to interpret situations meaningfully and judge their implications (Viney & Westbrook, 1976 in Konetze et al., 2023) ↩︎

Noise Reduction in Hearing Aids

Current Challenges

Hearing in noise continues to be one of the biggest challenges for hearing aid users. Hearing aids predominantly use single-microphone noise reduction, which estimates the presence or absence of noise from a single input signal and reduces hearing aid gain accordingly, without affecting speech if present (Brons, 2013). It uses as basis the environmental classification of input sound into speech, noise, or speech in noise to estimate the actual signal-to-noise ratio (SNR) in each channel. Then it adjusts the gain based on estimated SNR, and this is manifest as a trade-off between noise reduction and speech quality to the listener.

The goal of noise reduction is to reduce unwanted noise and retain speech that is of interest to the listener. Determination of what counts as unwanted noise and speech of interest to the listener at any given moment of time is challenging. This is because human attention is limited in capacity. Furthermore, listening environments that are noisy often involve many people talking (multi-talker speech), either taking turns or simultaneously, where the talker the listener is attending to (i.e., of interest to the listener) changes with time. Speaking in a noisy environment makes people speak louder and in a higher pitch, in an attempt to make themselves heard. The instantaneous SNR also changes with time, and often sounds appear, fade out, or disappear from the scene or soundscape. This makes it difficult to simulate a realistic environment when testing devices. To simplify the setup that is used for testing several assumptions are routinely made. This includes the assumption that the source (talker of interest, labeled the ‘target’) is in front of the listener (or in the frontal hemifield), that the target talker does not change, the SNR does not change, the source is static, and that the noise environment remains stable. To make it easier to separate the effect of the algorithm of interest measurements are typically made with other algorithms turned off.

These test scenarios and device settings are far from ideal and not how hearing aid users actually experience hearing in noise with their devices. For instance, it has been shown that compression may interact with noise reduction, so should be considered in the evaluation of noise reduction algorithms (Brons et al., 2015). It has been shown that user settings for listening in noise might be different when considering sound quality versus speech intelligibility, with there being a trade-off between noise reduction and speech distortion. In other words, settings that might be preferable in terms of sound quality may be sub-optimal in terms of speech understanding. Furthermore, hearing impaired users may have a different understanding of the term “speech distortion” than those with normal hearing (Huber et al., 2018). In fact it is known that single-microphone noise reduction does not improve intelligibility for speech in noise, but may provide benefit for listening effort and comfort (Brons , 2013). Thus, users prefer noise reduction measures in hearing aids to be on even though it has not been shown to increase speech intelligibility (Luts et al., 2010, Magnusson et al., 2013, Desjardins and Doherty, 2014, Wu et al., 2019).

A machine learning approach

Recently, researchers have been exploring the use of machine learning to improve noise reduction performance in hearing aids. Deep neural networks are employed in a commercially available hearing aid, Oticon More (Andersen et al., 2021). However not much is known about the actual architecture of the network employed, or its implementation for proprietary reasons. Recently, Healy (2023) reported improvement in speech intelligibility for both normal hearing and hearing impaired listeners for a novel deep-learning based noise reduction algorithm. They used reported intelligibility improvements of 46 to 58% for hearing impaired listeners, where intelligibility generalized to novel talkers.

A three depth level neural network, Loxaxs, CC0, via Wikimedia Commons

A comparison between current hearing aids and DNN-based enhancement approaches have shown an advantage for the latter. Specifically, objective metrics showed that hearing aids worsened performance in a “bypass” condition (with all hearing aid algorithms deactivated except feedback cancellation and linear amplification of 20 dB) due to the difficulty estimating the direction of arrival of target speech. However, DNNs (processed offline) were unaffected by the nonstationarity of noise and competing talkers and showed an increase in objective metrics of intelligibility and speech separation quality (MSTOI and SISDR), outperforming current hearing aids (Gusó et al., 2023).

What does the future hold?

The roadblock to implementing DNN networks on hearing devices is computational complexity. The size of networks required to achieve considerable noise reduction is large, and is out of reach for current hearing aids due to their small size. Consequently much of the processing is moved to high-powered devices such as the phone to improve efficiency. However, in the future, with work on DNN model compression and more efficient algorithms we might see DNN processing being used more on hearing devices (Diehl et al., 2023; Healy et al., 2023). DNN performance may be further improved by combining it with the traditional noise reduction approach of beamforming and using the user’s preference to guide a more personalized approach to noise reduction (Diehl et al., 2023). Work toward this end is ongoing (cf. Clarity Challenge).

Work is also ongoing to use ecologically valid environments for device as well as listener testing with the use of augmented and virtual reality (Keidser et al., 2020, Korzepa et al., 2018, Mehra et al., 2020). This includes research on trying to decrypt attention (attention decoding), the use of wearable sensor technology to obtain indices of listener attention, effort, fatigue, stress, and indeed, cognizance (cf. Fuglsang et al., 2020, Geirnaert et al., 2021), and open hearing aid platforms (e.g., openMHA) that enable devices to be developed and evaluated in real time with low processing delays (Herzke et al., 2017, Kayser et al., 2022).

References

1. Brons, I. (2013). Perceptual evaluation of noise reduction in hearing aids. Universiteit van Amsterdam [Host].
2. Brons, I., Houben, R. and Dreschler, W.A. (2015). Acoustical and Perceptual Comparison of Noise Reduction and Compression in Hearing Aids. Journal of Speech, Language, and Hearing Research 58, no. 4: 1363–76. https://doi.org/10.1044/2015_JSLHR-H-14-0347.
3. Huber, R., Bisitz, T., Gerkmann, T., Kiessling, J., Meister, H., & Kollmeier, B. (2018). Comparison of single-microphone noise reduction schemes: Can hearing impaired listeners tell the difference? International Journal of Audiology, 57(sup3), S55–S61. https://doi.org/10.1080/14992027.2017.1279758
4. Luts, H., Eneman, K., Wouters, J., Schulte, M., Vormann, M., Buechler, M., Dillier, N., Houben, R., Dreschler, W. A., Froehlich, M., Puder, H., Grimm, G., Hohmann, V., Leijon, A., Lombard, A., Mauler, D., & Spriet, A. (2010). Multicenter evaluation of signal enhancement algorithms for hearing aids. The Journal of the Acoustical Society of America, 127(3), 1491–1505. https://doi.org/10.1121/1.3299168
5. Magnusson, L., Claesson, A., Persson, M., & Tengstrand, T. (2013). Speech recognition in noise using bilateral open-fit hearing aids: The limited benefit of directional microphones and noise reduction. International Journal of Audiology, 52, 29–36.
6. Desjardins, J., L., & Doherty, K., A. (2014). The Effect of Hearing Aid Noise Reduction on Listening Effort in Hearing-Impaired Adults. Ear and Hearing, 35, 600–610.
7. Wu, Y.-H., Stangl, E., Chipara, O., Hasan, S. S., DeVries, S., & Oleson, J. (2019). Efficacy and Effectiveness of Advanced Hearing Aid Directional and Noise Reduction Technologies for Older Adults With Mild to Moderate Hearing Loss. Ear and Hearing, 40(4), 805–822. https://doi.org/10.1097/AUD.0000000000000672
8. Andersen, A. H., Santurette, S., Pedersen, M. S., Alickovic, E., Fiedler, L., Jensen, J., & Behrens, T. (2021). Creating Clarity in Noisy Environments by Using Deep Learning in Hearing Aids. Seminars in Hearing, 42(03), 260–281. https://doi.org/10.1055/s-0041-1735134
9. Healy, E. W., Johnson, E. M., Pandey, A., & Wang, D. (2023). Progress made in the efficacy and viability of deep-learning-based noise reduction. The Journal of the Acoustical Society of America, 153(5), 2751-2751.
10. Gusó, E., Luberadzka, J., Baig, M., Saraç, U. S., & Serra, X. (2023). An objective evaluation of Hearing Aids and DNN-based speech enhancement in complex acoustic scenes. arXiv preprint arXiv:2307.12888.
11. Diehl, P. U., Singer, Y., Zilly, H., Schönfeld, U., Meyer-Rachner, P., Berry, M., … & Hofmann, V. M. (2023). Restoring speech intelligibility for hearing aid users with deep learning. Scientific Reports, 13(1), 2719.
12. Keidser, G., Naylor, G., Brungart, D. S., Caduff, A., Campos, J., Carlile, S., Carpenter, M. G., Grimm, G., Hohmann, V., Holube, I., Launer, S., Lunner, T., Mehra, R., Rapport, F., Slaney, M., & Smeds, K. (2020). The Quest for Ecological Validity in Hearing Science: What It Is, Why It Matters, and How to Advance It. Ear and Hearing, 41, 5S. https://doi.org/10.1097/AUD.0000000000000944
13. Korzepa, M. J., Johansen, B., Petersen, M. K., Larsen, J., Larsen, J. E., & Pontoppidan, N. H. (n.d.). Learning preferences and soundscapes for augmented hearing. 7.
14. Mehra, R., Brimijoin, O., Robinson, P., & Lunner, T. (2020). Potential of Augmented Reality Platforms to Improve Individual Hearing Aids and to Support More Ecologically Valid Research. Ear and Hearing, 41, 140S. https://doi.org/10.1097/AUD.0000000000000961
15. Fuglsang, S. A., Märcher-Rørsted, J., Dau, T., & Hjortkjær, J. (2020). Effects of Sensorineural Hearing Loss on Cortical Synchronization to Competing Speech during Selective Attention. The Journal of Neuroscience, 40(12), 2562–2572. https://doi.org/10.1523/JNEUROSCI.1936-19.2020
16. Geirnaert, S., Vandecappelle, S., Alickovic, E., de Cheveigné, A., Lalor, E., Meyer, B. T., Miran, S., Francart, T., & Bertrand, A. (2021). EEG-based Auditory Attention Decoding: Towards Neuro-Steered Hearing Devices. arXiv:2008.04569 [Eess]. http://arxiv.org/abs/2008.04569
17. Herzke, T., Kayser, H., Loshaj, F., Grimm, G., & Hohmann, V. (2017, July). Open signal processing software platform for hearing aid research (openMHA). In Proceedings of the Linux Audio Conference (pp. 35-42).
18. Kayser, H., Herzke, T., Maanen, P., Zimmermann, M., Grimm, G., & Hohmann, V. (2022). Open community platform for hearing aid algorithm research: open Master Hearing Aid (openMHA). SoftwareX, 17, 100953.

Copyright © 2023 Vidya Krull. All Rights Reserved.

Fatigue and Hearing Loss

What is fatigue?

Fatigue is a term used to describe tiredness, typically over a long period of time. It can be used to refer to physical or mental fatigue, a combination of both, or a symptom. Broadly, it can be used to refer to feelings, or behaviorally, as various measures of physical or mental performance [1].

How is hearing loss related to fatigue?

There is evidence to suggest that people with hearing loss are at a higher risk of experiencing fatigue than those who have normal hearing, due to increased listening effort [1, 2, 3]. A recent cross-sectional study examining the association between hearing loss and self-reported fatigue in 3,031 participants in the US (as part of the National Health and Nutrition Examination Survey) showed that those with hearing loss were more likely to report fatigue for more than half the days and nearly every day than not having fatigue even when other factors such as age, sex, race, ethnicity, education, smoking, drinking, noise exposure, and body mass index was accounted for [4]. The study reports the relative risk ratio (RRR), which is the probability of the event occurring in the group with exposure (in this case hearing loss) versus those without. A relative risk ratio of greater than 1 means that the event is more likely to occur if there was exposure. Every 10-dB HL–worse of audiometric hearing was associated with a higher likelihood of reporting fatigue for nearly every day (RRR = 1.24; 95% CI, 1.04-1.47) but not for more than half the days [4].

What implications does fatigue have for those with hearing loss?

Fatigue, particularly chronic fatigue can result in:

1. reduced quality of life
2. deficits in cognitive processing (maintaining attention, thinking quickly, clearly, or efficiently)
3. reduced workplace productivity and safety [1]

Do hearing aids help?

There is not definitive evidence yet. The quality of evidence available from a systematic review to answer the questions of whether hearing loss has an effect on fatigue and whether hearing device fitting has an effect on fatigue is “very low” [5]. Having said that the review did highlight support for these questions.

There is some support that hearing loss increases fatigue; the “very low” risk categorization is due to lack of homogeneity among studies, and that there haven’t been any randomized clinical trials conducted thus far. Evidence from self-report measures did not support the hypothesis that hearing aids reduced fatigue in full, although the evidence was more promising for cochlear implants than hearing aids. There was a positive result from one study that used behavioral measures, suggesting that more studies with validated and consistent fatigue measures are needed to examine this hypothesis cogently.

A longitudinal study by the same group looked at the effect of hearing aids before fitting, at 2 weeks, 3 months, and 6 months post-fitting and found that hearing aid fitting significantly reduced listening (-related) fatigue but not general fatigue. Social activity and participation levels also were shown to be increased in the hearing aid group relative to the control group [6].

Given the role of motivation in the framework of listening effort, it is possible that motivation may directly or indirectly contribute to listening fatigue [3]. The interplay between fatigue and motivation to wear hearing aids has yet to be examined.

What to do about fatigue?

The following practices help with general fatigue, and likely listening fatigue:

• healthy diet and exercise
• good and consistent sleep schedule
• lower stress
• socializing that gives enjoyment

If you have hearing loss and own hearing aids, wear that hearing aid you purchased…and do things you love.

References

1. Hornsby, Benjamin W. Y., Graham Naylor, and Fred H. Bess. “A Taxonomy of Fatigue Concepts and Their Relation to Hearing Loss.” Ear & Hearing 37, no. 1 (July 2016): 136S-144S. https://doi.org/10.1097/AUD.0000000000000289.
2. Hornsby, B. W. (2013). The effects of hearing aid use on listening effort and mental fatigue associated with sustained speech processing demands. Ear Hear, 34, 523–534.
3. Pichora-Fuller, Kathleen, M., Sophia E. Kramer, Mark A. Eckert, Brent Edwards, Benjamin W. Y. Hornsby, Larry E. Humes, and et al. (2016). “Hearing Impairment and Cognitive Energy: The Framework for Understanding Effortful Listening (FUEL).” Ear and Hearing 37, no. 1: 5S-27S.
4. Jiang, K., Spira, A.P., Lin, F.R., Deal, J. and Reed, N. S. (2023). Hearing Loss and Fatigue in Middle-Aged and Older Adults. JAMA, 149, 8, 758-760
5. Holman, J.A., Drummond, A., and Naylor, G. (2021). The Effect of Hearing Loss and Hearing Device Fitting on Fatigue in Adults: A Systematic Review. Ear andHearing, 42 (1), 1-11.
6. Holman, J. A., Drummond, A., and Naylor, G. (2021). Hearing aids reduce daily-life fatigue and increase social activity: a longitudinal study. Trends in Hearing, 25, 23312165211052786.

Copyright © 2023 Vidya Krull. All Rights Reserved.

Of flies and crickets: Directional microphones for hearing aids

English: A gravid female Ormia ochracea resting on a fingernail. (Photo credit: Wikipedia)

The female parasitic fly, Ormia ochracea, can hear the high-pitched chirp of a male cricket (between 4-5 kHz) as far away as the length of a football field. The fly is sensitive to changes in the direction of sound of up to 1-2 degrees, precision similar to sensitivity in humans (Mason et al., 2001). It uses its detection and sound localization skills effectively to find a male cricket and deposits its larvae on the cricket’s back. The larvae penetrate the body of the host and once inside, feed and grow, ultimately leaving the shell of the dead cricket behind when they emerge as pupae in 10 days (Robert et al., 1992). The ability of the female Ormia to hone in with great accuracy on its host, the male cricket, has to do its extraordinary ears (Miles et al., 2009).

http://www.sciencenewsforkids.org/2006/09/how-to-silence-a-cricket-3/

Unlike human ears, that are separated  by our head, the ears of the Ormia are located on its chest. Its eardrums (tympanum) are adjacent to each other and separated by an ‘intertympanal bridge’ (Robert et al., 1992), about which it executes two different types of motion: a rocking motion, where the two eardrums move in different directions about the pivot (i.e., pitching up and down like a seesaw), and a translational motion where the two eardrums move in the same direction (e.g., flapping like a bird) (Miles et al., 2009).

Humans use interaural time differences (ITDs) and interaural level differences (ILDs) to localize sound sources. That is, they use timing differences based on when the sounds arrive at each ear as well as level differences between the two ears to calculate the direction of the sound. However, ITDs are dependent on the distance between the two ears, i.e., the size of the human head. It is remarkable that the fly, which has its eardrums separated by only 0.5 mm (Mason et al., 2001), is equally sensitive to localization cues as humans. It achieves this feat by effectively amplifying the time difference between its ears by several factors using a combination of mechanical and physiological adaptations.  Mechanically, the eardrum closest to the sound vibrates by 10 dB greater amplitude than the eardrum that is farther for sounds that come from an angle of 45-90 degrees. Furthermore, physiologically, these pressure differences are further processed in the fly’s auditory system as timing (latency) differences in the response of groups of auditory neurons (Mason et al., 2001). Together, these mechanical and physiologic mechanisms make the fly extremely sensitive to tiny pressure (i.e., sound level) differences between its two ears to which it would otherwise be insensitive.

Hearing aids use directional microphones to enhance the intelligibility of speech and reduce unwanted background noise, effectively increasing the signal-to-noise ratio (Ricketts and Hornsby, 2006). Conventional microphones use a design where sound differential on two sides of a diaphragm is used to calculate sound pressure level. Directional microphones are sensitive to differences in sound pressure at sound ports (usually two ports). Directional sensitivity is a function of the distance between the two ports where sounds enter the microphone. One of the drivers of sales in the hearing aid industry is the cosmetic appeal of the aid, which typically means a smaller size. It is extremely difficult to design small directional microphones because the reduction in size comes at the cost of sensitivity to sound pressure changes. Microphone noise becomes an issue, making it difficult to extract the signal from the noise (Miles et al., 2009).

Using biological-mimicry based on the hearing mechanism of the parasitic fly Ormia ochracea, scientists are designing micro-scale directional microphones that could be used in a hearing aid.  Using microfabrication technology, researchers have designed a differential microphone diaphragm that is light and pivots about a central hinge, much like the pivoting motion of the eardrums about the intertympanal bridge of the Ormia. These fly-inspired directional microphones are much smaller than current microphones used in hearing aids, but still have a lower noise differential as a result of low microphone noise (Miles et al., 2009). These directional microphones also employ an optical sensor that reduces electronic noise (Miles et al., 2009). The technology is still under development, and much more work needs to be completed before a commercial product is available. Ultimately, the crucial test will be the demonstration of better hearing aid outcomes when listening in fluctuating background noise, one of the big challenges faced by hearing-aid users.

REFERENCES:

1. Mason, A.C., Oshinsky, M.L., and Hoy, R.R. (2001). Hyperacute directional hearing in a microscale auditory system. Nature, 410, 686-690.
2. Miles, R.N., Su, Q., Cui, W., et al. (2009). A low-noise differential microphone inspired by the ears of the parasitoid fly Ormia ochracea. Journal of the Acoustical Society of America, 125, 4, 2013-2026.
3. Ricketts, T. and Hornsby, B.W. (2006). Directional hearing aid benefit in listeners with severe hearing loss. International Journal of Audiology, 45, 190-197.
4. Robert, D., Amoroso, J., and Hoy, R.R. (1992). The evolutionary convergence of hearing in a parasitoid fly and its cricket host. Science, 258, 1135-1137.

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