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Occlusion effect
Occlusion effect
Occlusion effect
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The occlusion effect occurs when an object fills the outer portion of a person's ear canal, causing that person to perceive echo-like "hollow" or "booming" sounds generated from their voice, chewing, footsteps, or any other sounds originating in their own body.

The bone-conducted sound travels to the cochlea through different pathways. The outer ear pathway corresponds to the sound pressure generated in the ear canal cavity due to the vibration of the ear canal wall, which constitutes the source of the occlusion effect. At low frequencies, the outer ear pathway is negligible when the ear canal is open but dominates when the ear canal is occluded. The occlusion effect is thus objectively characterized by an acoustic pressure increase in the occluded ear canal at low frequencies and which can be measured with a probe-tube microphone.[1]

Considering that the vibrating ear canal wall acts as an ideal source of volume velocity (also known as volumetric flow rate), the occlusion device increases the “opposition” of the ear canal cavity to the volume velocity imposed by its wall and thus increases the amplitude of the acoustic pressure that is generated in reaction, leading to the occlusion effect.[2]

The acoustic impedance of the ear canal cavity represents its “opposition” to the volume velocity transfer and governs its reaction in terms of acoustic pressure. In other words, the occlusion effect is mainly caused by the increase of the acoustic impedance of the ear canal cavity when it is occluded.[2][3][4][5]

A person with normal hearing can experience the occlusion effect by sticking their fingers into their ears and talking. Otherwise, this effect is often experienced by hearing aid users who only have mild to moderate high-frequency hearing loss but use hearing aids which block the entire ear canal. The occlusion effect is also deemed to be a notable source of discomfort to workers wearing shallowly inserted passive occlusion devices such as earplugs.[6][7]

Active occlusion algorithms are needed to adequately help people with severe hearing loss. If a person suffers from "near-normal low-frequency hearing and mild to moderate hearing loss of up to 70 dB at mid and high frequencies," hearing aids with increased vent size or hollow ear-molds/domes are more suitable for them in lessening the extent of the occlusion effect.[8] In the latter case, the open-fitting decreases the ear canal acoustic impedance and thus lessens the occlusion effect.

For earplug users, an incomplete seal has a similar effect at frequencies lower than the Helmholtz resonance formed by the system (with the neck of the resonator corresponding to the incomplete seal at the interface of the earplug and ear canal wall, and the resonator cavity being the partially occluded ear canal). In general, deep-fitting reduces the occlusion effect because the volume velocity imposed by the ear canal wall to the occluded ear canal cavity decreases since the surface as well as the vibration amplitude of the remaining ear canal wall diminish with the insertion depth.[citation needed]

See also

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References

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Occlusion effect

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The occlusion effect is a perceptual auditory characterized by an increase in the of low-frequency sounds, particularly self-generated ones like one's own voice or chewing, when the external is blocked or occluded. This results in a distinctive hollow, booming, or barrel-like quality to the sounds, often experienced by users of hearing aids, earplugs, or in-ear devices. The effect primarily stems from bone-conducted vibrations—transmitted through the skull and soft tissues—that generate air pressure waves trapped within the occluded ear canal, which then drive the tympanic membrane and inner ear more efficiently. In a normally open ear canal, low frequencies (below approximately 1 kHz) are attenuated by a natural high-pass filtering mechanism due to acoustic radiation impedance; occlusion eliminates this filter, boosting sound pressure levels at the eardrum by 10–25 dB in the low-frequency range. The magnitude is greatest with shallow occlusions in the cartilaginous portion of the canal and diminishes with deeper placements reaching the bony section or in cases of middle ear disorders, where sound transmission to the cochlea is impaired. In and otolaryngology, the occlusion effect holds clinical significance for accurate threshold testing, fittings, and user comfort, as it can lead to complaints of overly amplified own-voice and reduced satisfaction with amplification devices. strategies include incorporating vents (at least 2 mm in diameter) in ear molds to allow low-frequency escape, using canal extension tips for bony contact, or advanced receiver-in-canal designs that minimize canal resonance, often reducing the effect to near-zero levels. Research continues to refine models of this effect, emphasizing its role in both diagnostic assessments and the design of modern hearing technologies.

Definition and Characteristics

Definition

The occlusion effect is an auditory characterized by the perceived enhancement of low-frequency sounds, particularly those generated by one's own body such as voice or chewing, when the is partially or fully obstructed by devices like earplugs, hearing aids, or helmets. This blockage creates a sensation of increased intensity, often described as a hollow, boomy, or echoing quality in the affected sounds. The effect primarily impacts frequencies below 2 kHz, where bone-conducted sounds—transmitted through the skull rather than air—are amplified due to the sealed environment of the occluded canal. It is most prominent with self-generated noises like speech, swallowing, or mastication, as these involve significant bone conduction components that resonate within the trapped space. Initial observations of the occlusion effect date back to the early in and acoustics studies, first reported by the English physicist in 1827, who observed enhanced bone-conducted sounds with occluded ears. The term gained prominence in hearing protection and audiological research by the early , as investigations into earplugs and hearing devices highlighted its implications for user comfort.

Perceptual Effects

The occlusion effect manifests perceptually as an altered sensation of one's own voice, often described by users as sounding muffled, bass-heavy, boomy, hollow, or akin to speaking in a barrel. This arises from the amplification of low-frequency components in self-generated sounds, leading to reports of increased and unnatural that can evoke discomfort and during verbal interactions. Additionally, it may reduce the perceived clarity of , prompting individuals to adjust their speaking volume or style unconsciously. In daily activities, the occlusion effect disrupts natural communication, making conversations feel strained or artificial, especially in noisy environments or prolonged discussions where voice monitoring is essential. This issue is particularly evident in bilateral fittings, where is subjectively rated as more intense compared to unilateral configurations, exacerbating the sense of auditory isolation. Perceptual variations occur across individuals, with being more pronounced in those with normal hearing or mild low-frequency , as their intact sensitivity amplifies the trapped bone-conducted sounds compared to those with significant hearing impairment who may perceive less deviation.

Mechanisms

Physiological Mechanisms

The occlusion effect arises primarily through the pathway, where mechanical vibrations generated by sources such as the jaw, , or one's own voice are transmitted directly through the bones of the to the , bypassing the outer and to stimulate the 's sensorineural structures. In normal conditions with an open , a portion of these vibrations radiates as into the external auditory (EAC) and vents outward, reducing the overall intensity perceived at the . However, when the is occluded—such as by an , , or earmold—this venting is blocked, trapping the radiated within the and amplifying its transmission to the via air conduction pathways, resulting in heightened perception of low-frequency bone-conducted sounds, typically by 10-20 dB below 1000 Hz. This enhancement occurs because the trapped energy reinforces the bone-conducted signal through additional compressional mechanisms involving the EAC walls and vibrations. The prevailing understanding attributes this to waves generated by vibrations of the walls and adjacent structures, such as the , which are trapped in the occluded volume and drive the tympanic , reinforcing the cochlear input. Anatomically, the ear canal functions as a resonator for low-frequency sounds, with its cartilaginous outer third particularly responsive to vibrations from adjacent structures like the mandible, which exhibits resonance around 200 Hz during speech or chewing. Occlusion alters pressure dynamics in the canal, leading to increased sound pressure that drives the tympanic membrane and ossicular chain (malleus, incus, and stapes) more vigorously, thereby enhancing the inertial and compressional forces on the cochlear fluids and basilar membrane. The ossicles play a key role in this process by converting the elevated canal pressure into mechanical motion that couples with bone-conducted vibrations, contributing to the overall threshold shift observed in occluded conditions. These anatomical interactions explain the effect's prominence at low frequencies, where long wavelengths lead to uniform pressure buildup in the closed canal volume from trapped vibrations. Two historical theories have sought to explain the physiological basis of the occlusion effect, though both have faced challenges. The outflow theory, proposed by in the mid-19th century, posits that internal physiological sounds generated near the propagate outward through the open , and occlusion prevents this outflow, causing a buildup of that intensifies at the . This theory was later invalidated by experiments showing the effect diminishes with deeper occlusions, suggesting it does not fully account for the trapped radiation mechanism. The masking theory, advanced by in the 1930s, argues that ambient environmental noise normally masks the faint bone-conducted vibrations radiating into the from its vibrating walls; occlusion reduces external noise ingress, thereby unmasking these internal sounds and making them more audible. However, this was refuted in 1960 by Egbert Huizing's tests, which demonstrated the occlusion effect persists even in complete silence, highlighting the dominance of canal-trapped energy over masking alone.

Acoustic Principles

The occlusion effect arises from the acoustic behavior of sound waves within an enclosed space, such as the when blocked by an object like an or component. When occluded, the functions as a closed acoustic cavity, altering the and reflection of sound waves. This closure prevents from radiating outward, leading to increased levels inside the cavity, particularly for low frequencies where the exceeds the canal dimensions, resulting in uniform pressure buildup from bone-conducted vibrations. A key acoustic principle underlying this phenomenon is the amplification in the closed cavity, where the occluded traps air waves generated by vibrations of the canal walls due to . The —typically approximately 2.5 cm in length—behaves as a closed volume that enhances from incoming low-frequency vibrations, such as those in the 200–1000 Hz range, by 10–20 dB, as the trapped air reinforces transmission to the . The 's resonances occur at higher frequencies (around 3–7 kHz, depending on the exact boundary conditions), but the occlusion effect is most prominent below 1 kHz due to efficient trapping without radiation loss. The effect is frequency-specific, with amplification diminishing above 2 kHz as shorter wavelengths no longer result in uniform pressure and experience more dissipation or leakage. This transition highlights how the occluded cavity's shifts, favoring low-frequency retention while higher frequencies experience less enhancement. Bone-conducted vibrations contribute to this pressure rise but are amplified primarily through these acoustic trapping mechanisms.

Measurement and Assessment

Objective Measurement

Objective measurement of the occlusion effect involves instrumental techniques to quantify the increase in levels within the due to occlusion, providing precise data on its acoustic impact. One primary method is real-ear measurement, which utilizes probe inserted into the to record levels (SPL) under occluded and unoccluded conditions. This approach captures the direct acoustic changes in the , typically employing a small-diameter tube connected to a sensitive positioned near the tympanic membrane to minimize . The procedure follows standardized protocols outlined in ANSI/ASA S3.46, which specifies methods for measuring real-ear characteristics of hearing aids, including occlusion-related gains. Measurements are conducted by presenting bone-conducted or low-frequency stimuli while alternating between open and occluded states, often using earmolds or inserts to simulate occlusion. This technique is particularly effective for assessing the effect across frequencies, revealing pronounced increases at low frequencies due to trapped vibrations. Another established objective method employs bone oscillator tests to evaluate the occlusion effect through pathways. In this procedure, a bone vibrator is applied to the mastoid process behind the ear, delivering pure-tone stimuli to measure hearing thresholds with and without ear canal occlusion. The occlusion is achieved using inserts or earplugs, and thresholds are compared to determine the bone-conduction gain induced by the blockage. This quantifies the enhancement in perceived bone-conducted sound, as the occluded canal prevents energy dissipation, amplifying internal vibrations. Studies indicate that the occlusion effect typically results in a 5-15 dB increase in bone-conduction sensitivity at 500 Hz when using mastoid placement, with variations depending on occlusion type and individual anatomy. The test is standardized in clinical and helps isolate the contribution of bone-conducted signals to the overall effect. Key metrics derived from these measurements include the occlusion effect magnitude, calculated as the difference in SPL between occluded and open conditions: OE = SPL_occluded - SPL_open. This value is plotted as curves, typically spanning 125 Hz to 4 kHz, to illustrate the effect's spectral profile, which peaks in the low-frequency range before diminishing at higher frequencies. These curves provide quantitative insights into the effect's extent, aiding in the validation of acoustic models and device performance. For instance, real-ear measurements often show OE values exceeding 20 dB below 250 Hz, establishing critical context for low-frequency amplification needs. Such metrics ensure reproducible, instrument-based assessments independent of subjective variability. Recent advancements as of 2025 include the development of anatomically realistic acoustical test fixtures (ATF) for more precise quantification of the objective occlusion effect in simulated environments.

Subjective Evaluation

Subjective evaluation of the occlusion effect relies on self-reporting techniques to capture individuals' perceptions of altered own-voice , such as increased boominess or reduced naturalness, during audiological assessments. Common tools include structured questionnaires administered post-fitting, like the User's Questionnaire (HAUQ), which probes satisfaction with own-voice sound and occlusion-related discomfort on a Likert-style scale. Additionally, dedicated scales such as the Occlusion Effect Scale rate the intensity of occlusion sensations during vocalization, using categories from 0 (no occlusion) to 4 (complete occlusion), often applied after sustained tasks like producing the /i/. Visual analog scales (VAS) provide a continuous measure for rating specific attributes, with users marking positions on a 0-10 line to indicate perceived boominess, hollowness, or naturalness of their voice while wearing hearing aids. These scales are particularly useful in clinical settings for quantifying subtle perceptual differences, as they allow nuanced self-assessment beyond categorical responses. For instance, patients may rate voice quality after comparing occluded and non-occluded conditions, highlighting how occlusion amplifies low-frequency self-generated sounds. In clinical protocols, subjective evaluation often incorporates speech-based tests where patients read aloud passages and subsequently rate their voice quality on scales assessing clarity, comfort, and realism. Another approach involves comparing preferred gain settings with and without occlusion; users adjust amplification levels to optimize comfort, revealing higher low-frequency gain reductions needed under occluded conditions to mitigate unnatural voice . These tests emphasize real-time feedback, enabling audiologists to tailor fittings based on individual reports of perceptual discomfort. Factors influencing subjective reports include adaptation over time, where initial occlusion sensations often diminish after 1-2 weeks of consistent hearing aid use as the auditory system acclimates to altered bone-conducted sound transmission. Bilateral occlusion typically elicits stronger subjective effects than unilateral, with users reporting greater voice unnaturalness due to symmetric low-frequency amplification in both ears. These variations underscore the importance of repeated evaluations to account for experiential changes in self-reporting accuracy.

Applications and Contexts

In Hearing Aids

The occlusion effect is a common challenge in hearing aid use, particularly affecting users of in-the-canal (ITC) and completely-in-canal (CIC) styles, which seal the more completely than receiver-in-canal (RIC) or behind-the-ear (BTE) designs with open fittings. Studies indicate that moderate to severe own-voice problems, a primary manifestation of the occlusion effect, are reported by approximately 18% of both first-time and experienced users. The severity is closely linked to vent size, with smaller vents (under 3 mm) exacerbating the effect by trapping low-frequency bone-conducted sounds, while larger vents reduce it by allowing acoustic venting. Similarly, shallower insertion depths in the cartilaginous portion of the canal increase the effect compared to deeper placements that better seal against vibrations. This phenomenon significantly impacts user satisfaction and retention, as the amplified, hollow quality of one's own voice can feel unnatural and lead to discomfort during speaking, chewing, or swallowing. Surveys show that dissatisfaction with own-voice quality affects around 30% of hearing aid users, contributing to broader issues like reduced perceived naturalness and pleasantness of speech. It is especially bothersome for individuals with mild hearing loss, who retain normal low-frequency sensitivity and thus notice the distortion more acutely than those with greater losses. Consequently, closed-canal fittings, which heighten the occlusion effect, are associated with higher return rates—11.3% compared to 1.8% for open fittings—often due to these perceptual issues. The occlusion effect has been recognized in hearing aid design since the 1970s, when open earmold fittings emerged as a response to complaints of unnatural own-voice in traditional occluded styles, with early studies highlighting higher dropout rates among users of fully occluded devices. This issue stems from low-frequency in the occluded , amplifying bone-conducted sounds by 20-30 dB below 1000 Hz.

In Other Audio Devices

The occlusion effect manifests prominently in noise-isolating in-ear monitors (IEMs) and earbuds, where the tight seal in the traps low-frequency bone-conducted sounds, such as one's own voice or chewing noises, creating a hollow, "head in a bucket" sensation. This perceptual distortion is particularly disruptive during phone calls, music listening, or live performances, impacting musicians who rely on IEMs for stage monitoring and gamers seeking immersive audio without external distractions. Research shows that the effect is more pronounced with shallower insertions, amplifying bone-conducted speech by 10-20 dB below 1 kHz, though it diminishes with deeper fits. In protective like earplugs and helmets, the occlusion effect arises from similar canal blockage, enhancing internal sounds in noisy environments such as industrial sites or motorsports. For instance, occlusion with in-ear devices can produce a low-frequency boost of up to 20 dB, amplifying bone-conducted vibrations and leading to discomfort and altered for users, where earplugs are worn under helmets to attenuate external hazards while inadvertently intensifying self-generated acoustics. In (VR) headsets, the occlusion effect compounds immersion challenges by unnaturally boosting low-frequency cues from head movements or voice, disrupting spatial audio realism. Emerging bone-conduction technologies in VR aim to bypass canal occlusion, leveraging direct vibration for more natural sound externalization without the typical low-end amplification.

Mitigation and Solutions

Design Strategies

Venting techniques represent a primary strategy to mitigate the occlusion effect by incorporating acoustic vents or into earpieces, which permit low-frequency generated by the user's own voice to escape the rather than being amplified internally. These vents create a controlled acoustic pathway that balances sound leakage with necessary isolation for amplified audio delivery. Studies have shown that vent diameters in the range of 2 to 4 mm can reduce the occlusion effect by approximately 10 dB at low frequencies (around 250-500 Hz), particularly for in-the-ear and in-the-canal shells, without substantially compromising noise isolation or feedback control. For instance, increasing vent diameter by 1 mm typically yields a 4 dB reduction in objective occlusion measures, allowing designers to optimize based on requirements. Open-fit designs further address occlusion by employing receiver-in-canal (RIC) or behind-the-ear (BTE) configurations with non-occluding tips, such as vented domes or skeleton-style earmolds, which maintain partial openness of the to prevent full sealing. In RIC styles, the receiver is positioned deeper in the canal while the housing sits externally, connected via thin tubing that minimizes blockage and incorporates phase-cancellation ports to counteract internal buildup by aligning destructive interference for low-frequency bone-conducted vibrations. BTE open-fit variants use similar tubing to route without enclosing the canal, reducing occlusion by up to 15-20 dB compared to fully occluded styles, while preserving natural and own-voice quality. These designs are particularly effective for users with mild to moderate high-frequency , as they prioritize low-frequency venting over complete canal occlusion. Material and shape innovations enhance these strategies through the use of soft tips that enable partial occlusion, conforming to the 's contours while allowing subtle air escape to dampen amplification. Ultra-soft domes, often with integrated venting, achieve occlusion levels comparable to an open , minimizing the boomy sensation by distributing pressure evenly across the canal walls. Complementary advances involve computational modeling of geometries using finite element analysis to simulate acoustic propagation and customize tip shapes, predicting occlusion reductions tailored to individual canal variations for up to 5-10 dB improvement in low-frequency response. Such modeling integrates 3D scans to optimize vent placement and material compliance, ensuring devices balance of external with occlusion mitigation.

Clinical Approaches

Clinical approaches to managing the occlusion effect emphasize patient-centered strategies employed by audiologists during fitting and follow-up care. Fitting protocols typically begin with initial real-ear verification using microphone measurements to objectively assess the occlusion effect by comparing the real-ear unaided response (REUR) to the real-ear occluded response (REOR), ensuring the does not excessively amplify low-frequency bone-conducted . This is followed by subjective scaling, where patients rate their own-voice perception using validated tools like the Client Oriented Scale of Improvement () to identify discomfort levels and guide adjustments. periods, often lasting 1-2 weeks, incorporate adjustable vents—such as flexible or collection vents—to iteratively reduce occlusion while monitoring patient feedback for optimal balance between own-voice naturalness and external sound amplification. Adaptation training plays a crucial role in helping patients acclimate to the altered voice perception caused by occlusion. Audiologists provide counseling to explain the temporary nature of the effect, typically resolving within 1-2 weeks as the brain adapts to the new auditory input, thereby reducing reported subjective discomfort. Additionally, (DSP) features are utilized to equalize voice frequencies by applying targeted low-frequency gain reductions (e.g., 3-5 dB), verified through follow-up real-ear measures, to restore a more natural own-voice quality without compromising overall audibility. For cases where standard adjustments prove insufficient, advanced interventions include fabricating custom molds that extend into the bony portion of the ear canal to minimize low-frequency trapping, often combined with integrated feedback cancellation to prevent whistling that can exacerbate perceived occlusion. If anatomical issues, such as narrow ear canals or excessive cerumen impaction, are identified during otoscopy and contribute to persistent occlusion, audiologists refer patients to an otolaryngologist (ENT) for further evaluation and potential medical intervention. Post-fitting follow-ups, scheduled at 2-4 weeks, involve reassessing occlusion via subjective questionnaires and objective measures, with counseling to reinforce adaptation and address any residual concerns.

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