Hearing aids have undergone remarkable transformation over the past 25 years. Today’s devices incorporate cutting edge technologies that deliver individualized amplification, advanced noise reduction, and a range of intelligent features.
However, no matter how sophisticated the internal electronics are, the sound ultimately reaches the ear through the earpiece. This earpiece plays a crucial role in shaping the effectiveness of the entire hearing solution through what we call acoustic coupling.
In this article, part of the Shaping Sound series, we focus on low-frequency amplification and the critical role of acoustic coupling. As open and instant-fit solutions have become increasingly common, an important question remains: are we sacrificing too much low-frequency amplification in pursuit of comfort and openness? Historically, less low-frequency gain was often used to reduce unwanted noise. But with today's advanced noise reduction systems, should we still limit the low-frequency speech cues that contribute to voice quality, speaker identity, and the natural character of sound?
The hearing aid industry has seen a strong shift toward instant-fit solutions and increasingly open acoustic couplings. These approaches have delivered important benefits in comfort, fitting efficiency, and occlusion management. However, they also raise an important clinical question: are we unintentionally sacrificing low-frequency audibility in the pursuit of openness?
Historically, reduced low-frequency gain was often accepted as a practical way to limit unwanted noise. Today's hearing aids, however, combine sophisticated noise reduction with highly accurate gain prescriptions. This raises a new clinical challenge: how can clinicians ensure that low-frequency speech information reaches the eardrum while still maintaining comfort and wearer acceptance?
Increasingly, the answer depends not only on the hearing aid itself, but on the acoustic coupling that connects the device to the patient.
Low-frequency amplification has often been approached with caution. In the era before non-linear amplification and effective noise reduction, increasing low-frequency gain often amplified unwanted noise alongside speech, leading to concerns about boominess and masking. As a result, modern fittings frequently favour more open couplings and reduced gain below 500 Hz.
While this approach may improve comfort for some users, it can also reduce the effectiveness of hearing loss compensation, particularly for patients whose hearing loss requires substantial low-frequency support. Speech may remain audible, yet voices can sound thinner, less natural, and less distinctive, reducing some of the richness and character of conversation.
This is because important components of speech are carried by low-frequency information, including speaker recognition, intonation, prosody, and the natural character of speech (Mesiano et al., 2023). When these cues are under-amplified, listeners may retain audibility while losing some of the qualities that make speech sound natural, recognizable, and engaging.
Research consistently highlights an important distinction: hearing-aid settings do not determine amplification on their own. Acoustic coupling plays an equally important role. Two fittings configured with the same gain prescription can deliver very different low-frequency outputs at the eardrum if their acoustic coupling differs.
A vent is a small opening in the earpiece designed to reduce the occlusion effect: the sensation that one's own voice sounds hollow, boomy, or unnatural when the ear canal is blocked. However, the same opening also provides an escape path for low-frequency sound. Kuk et al. (2009) demonstrated that venting does more than simply reduce occlusion. As vent diameter increases, low-frequency output reaching the eardrum decreases systematically, reducing the amount of amplification available where many users still need support.
However, vent diameter is only part of the story. Importantly, the same nominal vent size can produce very different acoustic outcomes depending on the coupling design. Hollow earmoulds produce substantially greater low-frequency attenuation than traditional solid earmoulds because their shorter vent length creates a lower acoustic mass.
The key acoustic challenge is that a more open coupling allows amplified low-frequency sound to escape before it reaches the eardrum. As a result, the hearing aid may be programmed to provide the prescribed gain, but the user may receive significantly less low-frequency amplification than intended. Kuk and colleagues found that hollow earmoulds with 2-3 mm vents reduced low-frequency output by as much as 25 dB at 200 Hz, producing an effect similar to an open fitting.
Perhaps more importantly, these findings highlight a broader clinical challenge: predictability. When acoustic coupling varies, the relationship between prescribed gain and delivered amplification becomes less certain. Achieving consistent hearing outcomes therefore requires more than selecting the appropriate hearing-aid settings. It requires understanding, measuring, and increasingly controlling the acoustic pathway between the device and the ear.
Historically, reduced low-frequency amplification was often accepted as a trade-off for improved comfort and reduced occlusion. The widespread adoption of instant-fit solutions has made this trade-off more complex, as acoustic outcomes now depend heavily on the interaction between the dome and the individual ear canal. Unlike custom earmoulds, whose acoustic properties can be more precisely controlled, instant-fit couplings introduce greater variability between users.
Open domes gradually evolved into increasingly occluding designs, including so-called "power" domes intended to behave more like closed earmoulds. Yet a more occluding dome does not necessarily behave like a closed earmould, and substantial low-frequency leakage can remain.
Cubick et al. (2022) demonstrated that this behaviour varies considerably between individuals. For some users, a power dome may function almost like a closed fitting, while for others it remains acoustically much more open. As a result, two patients fitted with the same hearing aid and the same dome style may experience very different low-frequency amplification at the eardrum.
Two patients fitted with the same hearing aid and the same gain prescription can experience very different acoustic outcomes. The challenge is not simply providing amplification but ensuring that the intended amplification reaches the eardrum.
The implications extend beyond low-frequency amplification. A more open acoustic pathway also allows unprocessed environmental sound to reach the eardrum directly (Blau, et al. 2008), creating the open-fit paradox and reducing the benefit of features such as directional microphones and noise reduction.
As discussed in the previous Shaping Sound article, tighter acoustic coupling helps preserve both low-frequency audibility and signal-processing effectiveness by keeping the processed signal dominant at the eardrum.
Modern hearing aids can deliver sophisticated noise reduction, highly accurate gain prescriptions, and advanced signal processing. Yet these capabilities only provide value if the intended signal reaches the eardrum as designed.
The challenge is therefore no longer simply providing amplification but ensuring that amplification reaches the eardrum as intended. This requires clinicians to consider acoustic coupling as an integral part of the hearing solution.
Low-frequency performance is not determined by vent size alone. Acoustic outcomes depend on the interaction between vent diameter, vent length, acoustic mass, retention, and overall coupling design. Understanding and controlling these relationships is essential for achieving the desired balance between comfort, occlusion management, and hearing performance.
Historically, low-frequency amplification was often reduced to avoid amplifying environmental noise. Modern hearing aids can now apply sophisticated noise reduction while preserving important speech information. The goal should therefore be to maintain access to low-frequency speech cues while minimizing the impact of low-frequency noise through the combined use of signal processing and appropriate acoustic coupling.
The objective is not to close every fitting, but to provide sufficient acoustic sealing to deliver the intended amplification while maintaining comfort and wearer acceptance. Venting is only one factor influencing own-voice perception; insertion depth, shell geometry, and coupling design can also help manage occlusion while preserving low-frequency audibility.
Research on instant-fit solutions has demonstrated substantial variability in low-frequency behaviour between users, even when the same dome type is used. Rather than assuming that a selected coupling will achieve the intended outcome, clinicians should verify how much amplification actually reaches the eardrum through real-ear measurements and confirm that fitting targets have been achieved.
As coupling strategies become more sophisticated, digital workflows become increasingly important. Accurate ear scanning, advanced modelling tools, and highly reproducible manufacturing allow clinicians to move beyond generic dome selections and create individualized acoustic couplings tailored to each patient's anatomy and hearing needs. Vent geometry, retention, and sealing characteristics can be controlled with greater precision, improving acoustic consistency.
Ultimately, solving the low-frequency dilemma is not about choosing between comfort and amplification. It is about designing an acoustic coupling that delivers the intended low-frequency speech information while supporting comfort, reducing occlusion, and enabling modern hearing-aid processing to perform at its full potential.
As hearing-aid technology continues to advance, performance is no longer defined by signal processing alone. The acoustic pathway between the device and the ear determines whether prescribed amplification is ultimately delivered as intended.
The challenge is no longer how to prescribe amplification; it is how to ensure that the prescribed amplification reaches the ear as intended.
By carefully managing acoustic coupling, clinicians can preserve important speech information, improve fitting consistency, and enable modern hearing-aid technology to perform closer to its full potential.
Ultimately, improving outcomes requires treating acoustic coupling not as a compromise between comfort and amplification, but as a critical part of achieving both.
1. Low-frequency amplification depends on acoustic coupling, not just gain settings. Understanding acoustic coupling is essential to ensure prescribed amplification reaches the eardrum.
2. Modern noise reduction changes the equation. The goal is to preserve low-frequency speech cues while minimizing low-frequency noise.
3. There is a fundamental trade-off between openness and audibility. The goal is optimization through an as-closed-as-needed approach, not eliminating venting.
4. Acoustic performance is determined by the whole coupling design. Vent diameter, vent length, acoustic mass, retention, and insertion depth all matter.
5. Real-ear verification and digital workflows enable predictable outcomes. Accurate scanning, custom design, and reproducible manufacturing turn acoustic coupling from an approximation into a controlled variable.
Blau, M., Sankowsky, T., Stirnemann, A., Oberdanner, H., & Schmitt, N. (2008). Acoustics of open fittings. The Journal of the Acoustical Society of America, 123(5_Supplement), 3011–3011. https://doi.org/10.1121/1.2932603
Cubick, J., Caporali, S., Lelic, D., Catic, J., Vikær Damsgaard, A., Rose, S., Ives, T., & Schmidt, E. (2022). The Acoustics of Instant Ear Tips and Their Implications for Hearing-Aid Fitting. Ear & Hearing, 43(6), 1771–1782. https://doi.org/10.1097/aud.0000000000001239
Kuk, F., Keenan, D., & Lau, C. (2009). Comparison of Vent Effects between a Solid Earmold and a Hollow Earmold. Journal of the American Academy of Audiology, 20(8), 480–491. https://doi.org/10.3766/jaaa.20.8.3
Mesiano, P. A., Zaar, J., Bramsløw, L., Relaño-Iborra, H., & Dau, T. (2023). The role of average fundamental frequency difference on the intelligibility of real-life competing sentences. Journal of Speech, Language, and Hearing Research, 66(7), 2521-2534. https://doi.org/10.1044/2023_jslhr-22-00219