Clinical practices have increasingly shifted toward instant fit and open-fit solutions, driven by comfort, speed, and ease of fitting. However, this trend introduces a fundamental acoustic challenge. The effectiveness of modern noise reduction depends not only on the signal processing algorithms, but also on how sound physically enters the ear canal and reaches the eardrum, which is the role of the acoustic coupling.

This raises an important question for clinicians, researchers, and manufacturers: are we unintentionally limiting the clinical value of noise reduction by overlooking acoustic coupling?
As signal processing becomes more advanced, an overlooked challenge emerges. When the ear canal is too open, environmental sound can bypass the hearing aid entirely, mixing with the processed signal and reducing its effectiveness.

Understanding and controlling this acoustic interface is therefore essential, not just for comfort, but for real-world speech understanding in noise.

The open-fit paradox

Here lies the “wicked problem”: while signal processing has improved significantly, the widespread adoption of open-fit and instant-fit designs introduces a parallel pathway for sound to enter the ear. 
In open fittings, external sound travels directly into the ear canal without being processed. This creates a mixing effect between:

• Processed sound from the hearing aid
• Unprocessed environmental noise entering through venting or open domes

This creates the open-fit paradox in today’s fittings: while open designs improve comfort and own-voice perception, they can undermine the noise reduction features they are meant to support.

Noise reduction is an acoustic feature, not just signal processing

Research has consistently shown that increasing openness reduces the measurable benefit of directional microphones and noise reduction systems, particularly in complex listening environments (Winkler et al., 2016; Kuk et al., 2020). More recent findings further confirm that tighter acoustic coupling is directly associated with improved speech-in-noise outcomes.

The core acoustic challenge in open fittings is that direct, vent-transmitted sound can dominate the low frequencies, effectively bypassing the hearing aid's processing (Keidser et al., 2007; Winkler et al., 2016). When this unprocessed environmental noise enters the canal and mixes with the device's output, it dilutes the acoustic "buffer" the hearing aid is trying to create (Winkler et al., 2016). Research based on technical evaluations of hearing aids indicates that the efficacy of directional algorithms (which reduced the noise coming from specific directions) drops significantly as vent size increases (Ricketts, 2000; Keidser et al., 2007). Without a tight seal, the hearing aid simply cannot prevent external noise from dominating the sound path reaching the eardrum.

While open designs are excellent for user comfort and reducing the occlusion effect, they can negatively impact the noise reduction features meant to support speech understanding in noise (Winkler et al., 2016; Jürgens et al., 2025). Clinical studies have shown that using a closed-fit instead of an open-fit can improve the benefit derived from combined directional and noise reduction features, ensuring that the processed, cleaned signal remains dominant at the eardrum (Magnusson et al., 2013). When there is little to no acoustic “buffer” between these pathways, the effectiveness of noise reduction drops significantly.

More recent research further reinforces that the “closedness” of the acoustic coupling is directly associated with improved speech-in-noise outcomes, emphasizing that physical fit plays a decisive role in auditory performance (Jürgens et al., 2025). Besides, the physical seal of the earpiece within the ear canal, was the strongest predictor of the individual speech-in-noise benefit a patient receives from directional microphones and noise reduction (Jürgens et al., 2025). This means that, by measuring this "closedness" through real-ear occluded insertion gain (REOIG), researchers were able to predict hearing-aid benefit better than traditional metrics like age or audibility.

Ultimately, improving outcomes requires us to treat acoustic design not as a constraint, but as a central part of the solution. We must find the optimal balance for each user, optimizing coupling to ensure that the hearing aid remains the dominant source of sound at the eardrum.

The core trade-off: openness vs control

At the heart of noise reduction lies a fundamental trade-off:

As with occlusion, the goal is not to fully close or fully open the ear, but to find the right balance for each user.

Solving the open-fit paradox

Noise reduction is no longer just a comfort feature, it has become a critical component of speech understanding in challenging listening environments. For many users, especially those with hearing impairment, improvement in signal-to-noise ratio (SNR) is essential for communication, cognitive effort reduction, and overall quality of life.

This creates the core challenge for modern audiology. The goal is not to eliminate openness entirely, but to find the optimal balance for each user, optimizing the acoustic coupling to ensure that the hearing aid remains the dominant source of sound at the eardrum especially for people struggling in hearing in noise. Importantly, SNR improvement is increasingly being integrated into hearing aid prescriptions and fitting rationales. Emerging research and industry practices, recognize that optimizing speech perception in noise is a core audiological goal, not an optional enhancement. We need to provide the right amount of SNR to the people who need it and that requires not only an algorithm but also the appropriate acoustic coupling.

Custom-fit solutions offer a powerful approach: They allow clinicians to precisely control how much external sound leaks into the ear canal, provide a stable acoustic “buffer” that ensures the processed signal remains dominant, and enable tailored venting strategies that balance occlusion, comfort, and overall acoustic performance.

1. Shift from "elimination" to "optimization"

The "solution" is not to fully close every ear, but to find the optimal balance for everyone. While open designs are often the default for comfort, the sources emphasize that "closing the canal is not a mistake" - it is often a requirement for effective processing, especially for patients who struggle with speech in noise.

2. Implement the "as-closed-as-acceptable" principle

Recent landmark research suggests a clear clinical path to solving the performance gap: fitting patients with an "as-closed-as-acceptable" acoustic coupling. By measuring the physical "closedness" (via REOIG) and ensuring it is tight enough to let the processed signal dominate the eardrum, clinicians can maximize the benefit of modern noise reduction and directional microphones.

3. Leverage digital design and customization

The paradox is solved through better acoustic design, not just better algorithms. Modern digital workflows allow for:

• Precise venting: Designing vents based on acoustic mass (length and diameter) rather than just diameter alone to reduce occlusion while maintaining a stable acoustic buffer.
• Custom-fit stability: Using custom-fit earpieces instead of instant-fit domes to provide repeatable positioning and consistent leakage control, ensuring the hearing aid remains the dominant source of sound at the eardrum.

4. Control the interface, not Just the signal

The ultimate solution lies in recognizing that the challenge is no longer how to process sound, it is how to control what reaches the ear. By treating the acoustic interface as a central part of the solution rather than a comfort constraint, you "solve" the paradox by ensuring the device's sophisticated processing actually reaches the patient's eardrum without being diluted by environmental noise.

Conclusion

As hearing aid technology continues to advance, performance is no longer defined by processing alone. The acoustic interface between the device and the ear determines whether these advanced features translate into real-world benefit.

Noise reduction systems can only improve what they control. When sound bypasses the hearing aid, their effectiveness is fundamentally limited. The challenge is no longer how to process sound; it is how to control what reaches the ear.

By carefully managing openness, leakage, and acoustic coupling, clinicians can ensure that processed signals remain dominant, allowing modern algorithms to deliver their full potential.

Ultimately, improving outcomes requires treating acoustic design not as a constraint, but as a central part of the solution.

Take Home Messages

1. Noise reduction performance depends on acoustic coupling - not just algorithms. Open fittings can significantly reduce real-world SNR benefit.
2. Processed signals must dominate at the eardrum to be effective, unprocessed sound leakage creates a critical limitation compromising directivity and noise reduction.
3. SNR improvement is increasingly part of hearing aid prescriptions, making acoustic control a clinical, not just comfort, priority.
4. There is a fundamental trade-off between openness and performance. The goal is optimization by precisely controlling openness, balancing comfort with a stable acoustic buffer not elimination.
5. Custom-fit and digital workflows enable precise, repeatable control of acoustic coupling, unlocking the full value of modern hearing aids.

References

Jürgens, T., Ihly, P., Tchorz, J., Nishiyama, T., Tanaka, C., Suzuki, D., Shinden, S., Kitama, T., Ogawa, K., Zaar, J., Laugesen, S., Jones, G., Vatti, M., & Santurette, S. (2025). Closedness of acoustic coupling and audiological measures are associated with individual speech-in-noise benefit from noise reduction in hearing aids. Trends in Hearing, 29, 1-15. https://doi.org/10.1177/23312165251325983

Keidser, G., Carter, L., Chalupper, J., & Dillon, H. (2007). Effect of low-frequency gain and venting effects on the benefit derived from directionality and noise reduction in hearing aids. International Journal of Audiology, 46(10), 554–568. https://doi.org/10.1080/14992020701481698,

Kuk, F., Ruperto, N., Slugocki, C., & Korhonen, P. (2020). Efficacy of directional microphones in open fittings under realistic signal-to-noise ratios using Widex MOMENT hearing aids. Hearing Review. https://hearingreview.com/hearing-loss/patient-care/hearing-fittings/efficacy-of-directional-microphones-in-open-fittings-under-realistic-signal-to-noise-ratios-using-widex-moment-hearing-aids

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(1), 29–36. https://doi.org/10.3109/14992027.2012.707335,

Ricketts, T. (2000). Directivity quantification in hearing aids: Fitting and measurement effects. Ear and Hearing, 21(1), 45–58. https://doi.org/10.1097/00003446-200002000-00008,

Winkler, A., Latzel, M., & Holube, I. (2016). Open versus closed hearing-aid fittings: A literature review of both fitting approaches. Trends in Hearing, 20, 1–13. https://doi.org/10.1177/2331216516631741,