The World Health Organization estimated that approximately 20% of the world's population, or around 1.6 billion people, have some degree of hearing loss, with roughly 90% of them experiencing mild-to-moderate hearing loss (WHO, 2021). The number of people with hearing loss is expected to reach ~2.5 billion by 2050. As a result, hearing loss stands as the third-largest cause of years lived with disability, and untreated hearing loss imposes an annual global economic burden of US$1 trillion. This amount includes personal costs and additional costs related to health care, education support, lost wages and reduced quality of life (McDaid et al., 2021; Wilson and Tucci, 2021).

Hearing aids have been in existence for over a century, dating back to the invention of telephones. Modern digital prescription hearing aids, recognized as safe and effective medical devices, play a crucial role in mitigating the personal and societal burdens associated with hearing loss (e.g., Ferguson et al., 2017; Holman et al., 2021; Nixon et al., 2021). However, the adoption rate of these prescription hearing aids has remained relatively low, varying from 1 to 2% in low-income countries (Chadha et al., 2021) to 38% in the USA (Carr and Kihm, 2022). Several factors contribute to this low adoption rate of hearing aids, including limited awareness of the importance of hearing, restricted access to hearing care, and the high cost of hearing aids (Valente and Amlani, 2017; Jorgensen and Novak, 2020; Zheng et al., 2023).

To enhance public access to hearing aids and improve hearing, the FDA introduced a new category of over-the-counter (OTC) hearing aids on October 17, 2022, specifically designed for adults aged 18 or older with perceived mild-to-moderate hearing loss (https://www.fda.gov/medical-devices/hearing-aids/otc-hearing-aids-what-you-should-know). The OTC hearing aid is a medical device that utilizes air-conduction to amplify soft sounds, may feature a user-adjustable volume control, and, if inserted in the ear canal, must remain at least 10 mm away from the tympanic membrane. For the OTC hearing aid, FDA registration is required with clear package labeling specifying user age (18 or older). In addition, FDA listed the symptoms of perceived mild-to-moderate hearing loss as: “You have trouble hearing speech in noisy places, you have trouble hearing on the phone, you find it is hard to follow speech in groups, listening makes you tired, you need to turn up the volume on the TV or radio, and other people complain it's too loud”. Importantly, FDA listed warning signs that indicate the need to seek professional health care services (e.g., inability to hear speech even in a quiet room, observation of blood, pus, or fluid coming out of the ear). Finally, note that, unlike the OTC hearing aid, all other types of hearing aids necessitate a prescription from a licensed hearing health care professional.

Undoubtedly, the OTC hearing aid stands as a landmark achievement in hearing healthcare policy, ushering in an alternative option in hearing aid delivery for a large percentage of individuals with hearing loss and potentially transforming the entire hearing care and service market (Tucci and Califf, 2023). In this article, we will first examine the one-year-old market trends of OTC hearing aids. Subsequently, we will analyze the technical implementations of OTC hearing aids, focusing on amplification and fitting. Finally, we will explore the opportunities for OTC hearing aids, considering not only technical perspectives such as alternative smartphone implementations and the application of artificial intelligence but also their potential impact on global hearing healthcare and dementia.

It has now been a year since the FDA OTC hearing aid regulation went into effect. To assess its impact, let's first examine the supply side. A search on the FDA Establishment Registration and Device Listing database revealed a total of 113 entries related to OTC hearing aids (see Table 1). Among these entries, 74 manufacturers were identified, with 21 based in the USA, two in Europe, 49 in Asia, one in Mexico, and one in South Africa.

Significantly, four of the “Big Five” traditional hearing aid manufacturers have ventured into the OTC hearing aid market. At present, the “Big Five” (Sonova, Demant, Sivantos/Widex, GN ReSound, and Starkey) controls the majority of the global market share for prescription hearing aids. GN ReSound and Sonova are selling OTC hearing aids through the acquisition of former audio brand names, Jabra and Sennheiser, respectively. Starkey, on the other hand, introduced a new line named “Start Hearing One.” Additionally, Widex, through a merger with Signia and agreements with SONY, is marketing several OTC hearing aids (https://electronics.sony.com/otc-hearing-aids), although none could be found on the FDA database. Demant is the only one among the “Big Five” yet to enter the OTC market. While several mid-sized companies like Eargo are also making a presence, most registrants are startups, particularly from Asia, striving to establish themselves in this emerging market.

Curiously absent from the OTC hearing aid database are USA consumer electronics manufacturers. Bose, the first company selling the OTC hearing aid, sold its self-fitting hearing aid technology to Lexie (part of HearX) and totally exited the hearing aid market. Apple's absence in the OTC hearing aid market is particularly surprising, because not only do they own more hearing aid-related patents than any hearing aid manufacturer (Zeng, 2015), but also are more than capable of producing a premium-quality hearing aid (Lin et al., 2022).

Second, the OTC hearing aids have brought about some small but desired changes on the demand side. When compared to new prescription hearing aid users, new OTC hearing aid users, on average, were 2 years younger (64 vs. 62) and have milder hearing loss, but more meaningfully, they had a significantly shorter duration of hearing loss [20.9 vs. 14.4 years reported by Knoetze et al. (2023)]. Note that the duration of 14.4 years was obtained from a population in South Africa, which was much longer than the 4- to 8.9-year delay reported in the USA (Simpson et al., 2019; Powers and Carr, 2022). Nevertheless, the overall uptake of OTC hearing aids is still low. Only 2% of American adults aged 40 and older with hearing difficulties reported purchasing an OTC hearing aid in the last 6 months, and just 4% may buy one in the next year (ASHA, 2023). An independent survey also indicated a negligible impact of OTC hearing aids on existing audiological practices (LaMartina, 2023).

Third, a reputable pair of OTC hearing aids costs between $800 and $3,000, but most consumers are only willing to pay around $200 for them (ASHA, 2023). Apple seems to have discovered this desirable price point for the AirPods, pricing them between $129 and $249. This price contributed to Apple's sales of over 80 million AirPods, or $15 billion in revenue in 2022, which was more than the combined revenue of $12 billion from all hearing aid and cochlear implant companies worldwide. Unless the OTC hearing aid label can increase the unit price, there is no incentive for companies like Apple and other consumer electronics giants to enter the OTC hearing aid market. The potential return does not outweigh the risks associated with the liabilities of a medical device. Both technical and business innovations are required for the OTC hearing aids to make the intended impact on the market.

One key technical ruling for the OTC hearing aid was to specify the maximum output level without limiting the gain (FDA, 2022). To ensure safety, the FDA set the maximum output level at 111 dB SPL at any frequency for OTC devices with linear amplification or 117 dB SPL for that with compression and a user-adjusted volume control when measured in a 2-cc coupler. To improve efficacy and broaden the range of intended users, the FDA declined to set the gain limit for it “reduces the ability to adequately amplify soft sound inputs in some cases” and may “constrain device design and innovation”. Here we demonstrate from two different perspectives that defining the maximum output level in a 2-cc coupler without limiting the gain allows amplification far more than what is needed for people with mild-to-moderate hearing loss.

We first examined the maximum output level at the ear drum that can be produced by an OTC hearing aid. Since the FDA ruling defined the maximum output level that is measured in a 2-cc coupler, the actual output level at the ear drum can exceed this maximum level because the effective ear canal volume is likely reduced by the presence of a hearing aid in the ear canal.

Table 2 shows our analysis results. For the linear OTC hearing aid, the FDA ruling set the maximum output level to be 111 dB SPL (Row 1) at any frequency using pure tones. The American National Standards Institute characterized the average real-ear-to-coupler difference or RECD (Row 2) between a 2-cc coupler (e.g., HA1) and the ear canal at octave frequencies from 250 to 8,000 Hz (ANSI, 2013). The addition of 111 dB SPL and the RECD gives the actual maximum output in dB SPL at the ear drum (Row 3). Assuming that the loudspeakers of OTC hearing aids have an insertion depth similar to that of an ear pod, the 0-dB HL can be estimated in terms of dB SPL (i.e., the reference equivalent threshold sound pressure level, or RETSPL, Row 4, Ho et al., 2017). The difference between Row 3 and Row 4 gives the maximum hearing loss in dB HL that a linear OTC hearing aid can potentially compensate in terms of sound awareness at thresholds (Row 5). The averaged degree of hearing loss is 114 dB HL, ranging from 102 dB HL at 250 Hz to 121 dB HL at 2,000 Hz. Because the FDA ruling allows compression OTC hearing aids to have a maximum output of 117 dB SPL (Row 6), they can produce 6-dB higher maximum output level at the ear drum (Row 7) and compensate for 6-dB more hearing loss than the linear counterpart, accordingly (Row 8). In either case, the >102 dB HLs are much more than the amplification needed to compensate for mild-to-moderate hearing loss (20–49 dB HL by WHO or 26–55 dB HL by ASHA).

The function of a hearing aid is not to provide sound awareness at the threshold, but rather to improve speech intelligibility at suprathreshold levels. Therefore, we derived a typical input-output function and gain associated with a compression hearing aid that produced a maximum output level of 117 dB SPL in a 2-cc coupler. Figure 1 shows the input-output function (blue line), which has an expansion region (1:2.5) between 0 and 20 dB SPL to reduce environmental and circuit noise or pumping noise associated with fast release times, a linear (1:1) and low-compression (3:1) region at moderate input levels for conversational speech, and a high-compression region (10:1) at high levels to limit output level (e.g., Villchur, 1973; Keidser et al., 2012). The gain function (orange line) is the difference between input and output levels. The maximum gains for soft (50 dB SPL), conversational (65 dB SPL), and loud (80 dB SPL) speech are 56, 42, and 31 dB, respectively. The maximum gain in the linear amplification region is 56 dB for input levels between 20 and 45 dB SPL.

We then used NAL-NL2 to derive the amount of gain required for a hypothetic person with a severe 80-dB HL flat hearing loss (see Supplementary Table S1 for details). Figure 1 shows the gain required for such a person at six octave frequencies from 250 to 8,000 Hz at soft (circles), conversational (triangles) and loud (squares) speech. Regardless of speech levels and frequency channels, the prescribed gain for a person with 80-dB HL monaural hearing loss is at or below that is available for a compression OTC hearing aid with the 117-dB SPL maximum level even without the adjustment for the RECD. Also, when a binaural hearing aid fitting is assumed, the prescribed gains would be 2–7 dB lower than the prescribed gain for monaural hearing aid fitting, suggesting that the fitting range of OTC hearing aids can be even higher than an 80-dB HL flat hearing loss.

The above analyses showed that while OTC hearing aids are intended for people with mild-to-moderate hearing loss, the current FDA guideline in fact allows sufficient amplification for people with severe or profound hearing loss. On the one hand, the OTC hearing aids may present an opportunity for providing amplification for people with a wide range of hearing loss. On the other hand, the OTC hearing aid manufacturers and users need to be aware of the possibility and the danger of overamplification.

Another key technical ruling was loosely defined specifications to differentiate between customizing and fitting the OTC hearing aids (FDA, 2022). The differentiator between customizing and fitting is “the number of presets” in the OTC hearing aids. The “presets” can be frequency-dependent amplification, noise reduction, scene selection or other parameters in typical prescription hearing aids. If the number of presets for a parameter is limited, “for example, three”, then this process would be considered as customization not self-fitting, thus belonging to the OTC hearing aid category. If the number of presents is large, or controlled by software, apps, or similar tools, then the OTC hearing aids would become “self-fitting hearing aids” that must “comply with applicable requirements, including FDA 510(k) requirements and compliance with special controls”.

In practice, all “self-fitting” hearing aids would require FDA clearance even if they would be marketed as OTC hearing aids. Interestingly, FDA explicitly stated that for “devices intended fitting based off of a user-supplied audiogram, a requirement for the involvement of a licensed person to produce the audiogram may cause the device not to be an OTC hearing aid” (FDA, 2022). We feel that this explicit exclusion of a licensed person's produced audiogram seemed to be more about politics than technicality because the device itself is the same. Nevertheless, these rules pose serious technical challenges in designing the OTC hearing aids to meet an individual user's needs. For example, can we fit a hearing aid without an audiogram? Can a limited number of hearing aid presets meet the needs of most people with mild-to-moderate hearing loss?

Twenty years ago, we proposed that cochlear implant manufacturers should implement advanced hearing-aid front-end processing from directional microphones to multi-channel noise reduction algorithms to improve cochlear-implant performance (Chung et al., 2004, 2006). Today, we argue that hearing aid manufacturers should borrow the “mapping” algorithm in cochlear implants to fit the OTC hearing aids. In cochlear implants, each electrode may require different minimal and maximal electric current levels to reach hearing threshold (T level) and most comfortable loudness (M or C level). The cochlear implant speech processor has to compress a 50–60 dB sound dynamic range into a 10–20 dB electric dynamic range between the T and M levels (Zeng et al., 2002). Because only two of the three “mapping” parameters, namely, compression, T and M levels, are independent, cochlear implant manufacturers do not use the T level in their mapping, rather simply setting the T level to be either 0 or 10% of the M level (Vaerenberg et al., 2014). This mapping strategy is equivalent to fitting a hearing aid without an audiogram.

In fact, the FDA rulings have made the OTC hearing aid fitting easier and simpler than the cochlear implant mapping. Figure 2 illustrates how this borrowed idea can fit a compression OTC hearing aid for its targeted users. According to the FDA, the maximal level is either 111 dB SPL for a linear device or 117 dB SPL for a compressive device, which is considered here. The minimal level is either mild or moderate hearing loss (=blue or red triangles on the y-axis, corresponding to 35 or 50 dB SPL threshold, respectively). The overall input-output function includes four regions: (1) an expansion region at low input levels (e.g., 0 to 30 dB SPL), (2) a compression-amplification region at low-to-moderate levels, (3) a no-amplification region at high levels, and (4) a hard limit at 117 dB SPL output or above.

First, the expansion region minimizes the effects of background noise or pumping sounds associated with amplification. Second, the compression region provides audibility to soft sounds, especially consonants that are critical to speech intelligibility. For example, without amplification, a 30-dB SPL normally soft sound (vertical black line) would be barely or not audible to a person with mild-to-moderate hearing loss. With proper amplification, the 30-dB SPL sound is not only audible but also at approximately “soft” loudness level for a person with mild (arrowed horizontal blue line or 10 dB above the 35 dB SPL threshold) or moderate hearing loss (arrowed horizontal red line or 10 dB above the 50 dB SPL threshold). In other words, as Gain = Output - Input, the hearing aid provided 45–30 = 15 dB of gain to the user with mild hearing loss and 60–30 = 30 dB of gain to the user with moderate hearing loss to make low-level sounds audible for them.

For high-level input sounds, because of loudness recruitment, there is no need to provide any amplification for individuals with mild-to-moderate hearing loss (Figure 2). Additionally, the output limiting region not only conforms to the FDA's OTC hearing aid guideline, but also helps reduce exposure to loud sounds. We note that this four-stage fitting is similar to an early implementation (Killion, 1993), which provides more amplification for low-level sounds and no gain or linear amplification for loud sounds. Our proposed fitting method (Figure 2) improves Killion's implementation and uses expansion to reduce the gain applied to very soft sounds that may not carry much linguistic information.

Most importantly, our proposed method allows a simple and quick OTC hearing aid fitting process. Specifically, the user only needs to adjust the gain at 30 dB SPL input level to make sure they can hear soft sounds and then the gains for conversational level sounds and loud sounds are automatically set. This adjustment can be performed at different frequency channels to suit the individuals' configuration of hearing loss and listening needs. Should a volume control be installed, our proposed fitting method can be used for individuals with more than mild-to-moderate hearing loss.

The next question we address is: can one design an OTC hearing aid with a limited set of frequency responses without requiring an audiogram? In response, we propose a straightforward solution involving high-frequency compression amplification to cater to the needs of most, if not all, OTC hearing aid users. Figure 3 illustrates two levels of one-channel amplification for sounds above 1,500 Hz, providing 20 dB or less amplification for soft sounds in mild loss cases (top panel) and 40 dB or less amplification for soft sounds in moderate loss cases (second panel). This high-frequency amplification is designed to be effective not only for individuals with mild-to-moderate high-frequency loss but also for those with the same degree of flat loss. Previous studies have demonstrated that the exact frequency response is not a critical parameter as long as speech sounds fall within a listener's dynamic range, without too much low or high frequency gains (van Buuren et al., 1995, 1996).

While most potential OTC hearing aid users have high-frequency or flat hearing loss, some may have relatively steep sloping or notched hearing loss (Cruickshanks et al., 2020; Parthasarathy et al., 2020; Wang et al., 2021). To further address the need for those with sloping or notched hearing loss, additional two-channel amplification can be provided at high frequencies (bottom two panels in Figure 3). Together, these four preset amplification profiles can not only satisfy the FDA OTC hearing aid requirements, but more importantly likely improve both speech intelligibility and sound quality for people with different configurations of mild-to-moderate hearing loss. Future investigation is needed to compare performance between hearing aids using these four presets and that fitted with an audiogram in the targeted population.

The FDA's establishment of the OTC hearing aid category opens up a myriad of opportunities for applying innovative technologies to enhance ear and hearing healthcare, not only in the USA but also globally, particularly in low-middle-income countries (LMICs). In this discussion, we explore opportunities related to smartphones and True-Wireless-Stereo (TWS) devices, delve into the role of artificial intelligence, and assess the potential impact of OTC hearing aids on hearing care in LMICs and their role in delaying or reducing cognitive decline.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding author.

KC: Writing – original draft, Writing – review & editing. F-GZ: Writing – original draft, Writing – review & editing.