SISB Lec 6: Problem Solving & HA Fine Tuning (Part II)

Occlusion, Feedback, and Fitting: Key Concepts and Practical Approaches

• Learning outcomes for week 3, part 2:

  • Diagnose and resolve common hearing aid problems (feedback, occlusion, and amp-occlusion).

  • Develop a systematic approach to problem-solving using hearing aid software to fine-tune gain, compression, and frequency responses.

  • Adjust settings based on user feedback and specific complaints; extend clinical skills with plan B strategies.

  • Understand how to balance occlusion and feedback risks when fitting devices for severe losses.

• The big three problems addressed today: occlusion, feedback, and amp-occlusion (amp-occlusion is the perceived voice amplification that accompanies occlusion).

• A US-based survey (Hearing Industries Association) tracked barriers to obtaining hearing aids and rehabilitation; focus on user dissatisfaction metrics from 2010:

  • Sound of voice: about $73\%$ satisfied; $9\%$ dissatisfied.

  • Feedback: about $18\%$ dissatisfied.

  • Chewing sounds (in-ear resonance): about $14\%$ dissatisfied.

  • These data underscore the clinical relevance of occlusion and feedback issues in real-world use.

• Occlusion effect: what it is and how to evaluate it

  • Definition: Occlusion effect is the reflection of vibration from soft tissue in the ear canal back to the tympanic membrane, increasing perceived low-frequency energy when the ear canal is blocked.

  • Practical check: place a finger on the ear canal to feel the occlusion effect; if you can’t, the canal is not effectively occluded.

  • Common patient complaints:

  • Perceived unnatural self-voice (they hear themselves as if trapped in a bucket or barrel).

  • Chewing sounds appear louder.

  • Acoustic profile of occlusion:

  • The occlusion boost is greatest in the low-frequency region (roughly $250-500\ \text{Hz}$), with a center around $f_c \approx 300\ \text{Hz}$.

  • Formula representation (conceptual): the occlusion effect creates an additional low-frequency SPL boost $\Delta\text{SPL}<em>{LF}(f)$ centered near $f</em>c \approx 300\ \text{Hz}$, diminishing as venting increases.

  • Formant reference for testing:

  • First formants near $300\ \text{Hz}$ are informative for occlusion testing (e.g., vowels like i/u).

  • The first formant around $500\ \text{Hz}$ (as in "ah") is less useful for diagnosing occlusion.

  • Typical speech acoustics reminder:

  • Vowels with F1 near 300 Hz (e.g., i/ɪ, u) are more diagnostic for detecting occlusion than those with F1 near 500 Hz.

• Ear canal anatomy and its significance for fitting

  • Temporal bone components relevant to the ear canal:

  • External canal, with a cartilaginous outer third and a bony inner two-thirds.

  • The tympanic plate forms the floor and the anterior wall of the ear canal.

  • The squamous part forms the roof and the posterior wall.

  • The petrous part houses the inner ear.

  • Boundaries of the residual ear canal when an ear mold is in place:

  • Medial boundary: eardrum (tympanic membrane).

  • Lateral boundary: shell/ear mold.

  • Lateral canal walls: cartilaginous (outer) and bony (inner) portions.

  • Why boundaries matter:

  • If the mold’s boundary or vent resonates with the tympanic membrane, reflections are introduced between the mold and tympanic membrane, contributing to occlusion or feedback.

  • Jaw dynamics:

  • The jaw is mobile and less massive than the skull (~1/5 of skull mass); jaw movement during speech/chewing perturbs soft tissues in the ear canal, affecting occlusion/feedback.

• Feedback: mechanism, causes, and management

  • Mechanism: feedback is an acoustical loop where sound emitted by the hearing aid leaks out, re-enters the microphone, is re-amplified, and re-emitted, creating a self-sustaining whistling effect.

  • Leakage routes:

  • Vent leakage and gaps between the ear canal wall and the shell.

  • Common causes (to rule out in clinic):

  • Wax

  • Poor ear mold insertion

  • Cracked tubing

  • Use of adaptation/acclimation managers (gain increases over time)

  • Patient turning up volume (user-driven gain increase)

  • Basic preventive steps:

  • Measure feedback limits at initial fitting.

  • Run feedback tests with the aid in the ear; warn the patient about a brief loud noise during testing.

  • Feedback management technologies:

  • Static vs dynamic limits: static limits are always the same; dynamic limits adjust in real time based on environment.

  • Phase cancellation (classic) and frequency shifting (modern) as primary strategies.

  • Active feedback cancellers may introduce entrainment when long musical tones or tones are misidentified as feedback.

  • Diagnostics and headroom concepts:

  • Dashed purple line in target plots often shows the maximum gain without feedback (headroom limit).

  • If the actual gain (bold line) hits the headroom boundary, feedback is likely.

  • Practical considerations when headroom is insufficient:

  • Reduce gain at problem frequencies (often high frequencies for speech intelligibility or low frequencies for occlusion).

  • Do not rely solely on automatic fine-tuning; manual fine-tuning allows better control of individual bands and compression ratios.

  • Reducing vent size can help with feedback but worsens occlusion; balance is key.

  • Testing and validation:

  • Use real-ear verification and feedback tests with the client in the ear to ensure headroom is adequate in real life.

  • Artifacts and caveats:

  • Feedback cancellation may degrade speech signals if misapplied (entrainment).

  • Some manufacturers’ claims lack independent evidence; rely on evidence-based practice and objective/subjective testing.

• Occlusion vs amp-occlusion: diagnosing and distinguishing

  • Occlusion (physical) vs amp-occlusion (amplification-induced self-voice change):

  • If the hearing aid is off and the user’s voice is normal, occlusion is likely the issue if the voice sounds boomy when the aid is on.

  • If the user reports their own voice sounds boomy only when the device is on, amp-occlusion is likely contributing.

  • Diagnostic approach:

  • If there is a loud, boomy self-voice with the aid on, but not with it off, consider amp-occlusion in addition to occlusion.

  • If the jaw movement remains noisy and chewing sounds are loud despite occlusion compensation, both issues may co-exist.

• Practical fitting strategies to manage occlusion

  • Avoiding or reducing occlusion at the time of fitting:

  • Choose venting strategically; larger vents increase leakage and risk of feedback but reduce occlusion.

  • Consider a MAWL-based approach (open venting) to balance comfort and occlusion.

  • If using a receiver-in-canal (RIC) or BTE with vent, consider changing the dome to an open dome to encourage venting.

  • Acoustic mass concept:

  • Acoustic mass is influenced by vent diameter and length; increasing vent size or length lowers acoustic mass, enabling low-frequency sound to escape.

  • A longer canal stock reduces occlusion because the vent is effectively longer and venting occurs closer to the bony canal where reflections are minimized.

  • Canal stock length and occlusion: longer canal stock tends to reduce occlusion; shorter always increases occlusion risk.

  • Earpiece modifications and lab communication:

  • When occlusion persists, discuss possible REM-driven adjustments and communication with the lab for longer canal stock, deeper impressions, or open jaw impressions.

  • Techniques include tapering the earmold tip, using acrylic builds, PhotoPlast, or other accommodations to alter the shell for better venting and seal.

  • Alternatives and compromises:

  • Different hearing aid styles or brands with advanced occlusion compensation features.

  • If needed, revert to a stronger occlusion compensation in software and fine-tune gain manually across channels to preserve intelligibility while reducing occlusion.

  • acclimatization and counseling:

  • Educate patients about acclimatization periods for new devices; some self-voice changes may improve with time.

  • If self-voice sounds different but comfortable, monitor and re-evaluate in 1–3 weeks.

• Fine-tuning workflow for occlusion and feedback (practical steps)

  • Software approaches:

  • Phonak Target: occlusion compensation can be turned on to reduce low-frequency gain for occlusion; see target vs actual curve overlays (faint lines vs bold lines).

  • Manual fine-tuning: adjust G80 (loud input), G65 (mid input), G50 (soft input) gains, and observe how frequency-specific gain moves the curves and affects compression ratio (CR).

  • Be mindful: CR > 3:1 can introduce distortion; check how changes in one band affect other bands due to channel count and interaction between bands.

  • Fine-tuning examples discussed:

  • If the client reports a too-teeny or very boomy voice, reduce low-frequency gain for the high-intensity input (G80) while watching CR, which may increase distortion if CR becomes too high.

  • If automatic fine-tuning is used (auto fine tuning), be cautious as it may modify gains without clearly showing per-band changes; manual fine-tuning can be more precise.

  • Other software tools mentioned:

  • Oticon Genie 2: own voice manager that can adjust loudness preferences and lower low-frequency amplification to reduce own-voice issues.

  • Signia: Own Voice Processing (OVP) with training; claims of improved adaptation lack robust evidence.

  • Take-home for clinicians:

  • Always verify with the patient using the aid in the ear.

  • Document changes and follow up to ensure comfort and intelligibility.

  • Evaluate the impact on compression ratios and speech clarity when making frequency-specific adjustments.

• Case highlights and scenarios

  • Case 1: young adult prefers small completely-in-canal (CIC) device; occlusion risk high, no directional microphones; counseling about downsides (occlusion, Rieschlems/REMs) and scheduling follow-ups for adjustment.

  • Case 2: patient reports mixed occlusion and amp-occlusion after initial fitting; use occlusion compensation to reduce low-frequency gain; test with occlusion on/off to separate occlusion vs amp-occlusion contributions; adjust venting and canal stock accordingly.

  • Case 3: severe high-frequency loss with feedback risk; software shows headroom limits; consider vent adjustments, canal stock changes, or alternative shell for improved fit and reduced feedback. Emphasize patient comfort and safe headroom for gain.

• Practical implications and evidence considerations

  • Manufacturer claims vs evidence: many features (OVP, advanced venting, etc.) are marketed with limited independent evidence; rely on objective testing (REM, feedback tests) and patient-reported comfort.

  • Evidence-based practice: use REMs and real-ear measures to validate fitting and ensure amplified targets meet prescribed standards (NL2/DSL, adaptive DSP behaviors) within safe headroom.

  • Counseling priorities: audibility, speech intelligibility, and listening comfort (including own-voice perception) are primary goals; the patient’s subjective experience takes precedence over purely numerical targets.

• Quick reference equations and relationships (conceptual)

  • Occlusion frequency band of concern: $f \,\in\, [250\ \text{Hz}, 500\ \text{Hz}]\quad (\text{center } f_c \approx 300\ \text{Hz})$

  • Compression distortion risk threshold: $CR > 3:1\quad \text{(high risk of distortion)}$

  • Representing gains at different input levels (illustrative example):

  • For a given frequency, soft input gain $G<em>{soft}$ (50 dB SPL) may be amplified to a target; mid input gain $G</em>{mid}$ (65 dB SPL); loud input gain $G<em>{loud}$ (80 dB SPL). The actual outputs follow the processor’s compression curve and may be read as: $Output</em>{soft} = 50\ + G<em>{soft},\ Output</em>{mid} = 65\ + G<em>{mid},\ Output</em>{loud} = 80\ + G_{loud}.$

  • Open vs closed venting and acoustic mass intuition:

  • Larger vent diameter and longer vent length reduce acoustic mass, facilitating low-frequency venting and reducing occlusion, but potentially increasing leakage and feedback risk.

• Final takeaways for practice

  • Start with a systematic assessment: determine whether occlusion, amp-occlusion, or both are present; test with the aid off and on; test with jaw movement; use vowel-based occlusion tests and REMs.

  • Balance is key: occlusion-reducing vent changes can increase feedback risk; use targeted fine-tuning to avoid over-distortion (watch CR).

  • Use venting and shell modifications strategically (MAWL, open domes, deeper canal stocks) to reduce occlusion while preserving fit and comfort.

  • Leverage software tools with caution: occlusion compensation and own-voice features can help but require careful verification and patient feedback.

  • Maintain documentation and follow-up: issues are patient-specific and dynamic; keep detailed notes and adjust plans as needed.

• Ethical and practical implications

  • Emphasize evidence-based practice; be cautious about marketing claims without robust evidence.

  • Prioritize patient comfort, acceptance, and safety; obtain informed consent about potential changes in voice perception and venting trade-offs.

  • Ensure realistic expectations: acclimatization periods are normal; set expectations during the initial fitting to minimize dissatisfaction.

• Quick glossary of terms to remember

  • Occlusion: physical blockage of the ear canal causing low-frequency boost; often seen as “boomy” voice.

  • Amp-occlusion: self-voice perception change due to amplification itself when the device is on.

  • Real Ear Measurements (REMs): objective verification of a hearing aid’s output in the ear canal.

  • Vowels for testing occlusion: prefer vowels with F1 near 300 Hz (e.g., i, u) for detecting occlusion.

  • MAWL: open venting strategy parameter in fitting (larger venting for occlusion relief).

  • Venting: the opening through which sound can escape; relates to acoustic mass and occlusion.

  • Acoustic mass: a concept describing the resistance to low-frequency sound flow through the vent; reduced by larger/longer vents.

  • Feedback manager: system that limits gain to prevent feedback; can be static or dynamic; includes phase cancellation and frequency shifting.

  • Own Voice Processing (OVP): manufacturer feature to improve the user’s own voice quality; evidence for adaptation benefits is limited.

• End of section: questions and further discussion

  • If you have questions about a specific case, post them on Ed Discussion or bring them to the next session; practical cases will help solidify these concepts.

Lecture Focus

• Diagnose and resolve common hearing-aid problems (feedback, occlusion, inclusion).

• Apply systematic approaches using fitting software for gain, compression, and frequency response adjustments.

• Adjust settings based on user feedback and specific complaints.

• Develop practical plan-B strategies and clinical reasoning for fine-tuning.

1. Overview: Client Dissatisfaction Data

• U.S. MarkTrak survey (Hearing Industries Association) identifies hearing-aid user issues:

  • Voice sound: 9 % dissatisfied.

  • Feedback: 18 % dissatisfied.

  • Chewing/swallowing sounds: 14 % dissatisfied.

• Highlights: common barriers to full hearing-aid benefit include occlusion effect and feedback.

2. The Occlusion Effect

Definition

• Reflection of vibration from soft tissue in the ear canal back to the tympanic membrane when the canal is blocked.

• Maximal around 300Hz

• More likely if low freq. hearing thresholds are 40dB or better

Key Signs

• User hears own voice as “boomy,” “hollow,” “in a barrel/bucket.”

• Chewing or jaw movement sounds uncomfortably loud.

Why It Happens

• When ear canal is closed, bone- and tissue-conducted vibrations from speech/chewing are trapped.

• Without hearing aid: vibrations leak out.

• With hearing aid/plug: vibrations reflect between mould tip and tympanic membrane.

Relevant Anatomy

• Outer 1/3 = cartilaginous (vibrates).

• Inner 2/3 = bony (does not vibrate).

• Occlusion arises from vibration of the cartilaginous portion when sealed.

Acoustic Pattern

• Increased low-frequency SPL (250–500 Hz) when canal occluded.

• Smaller vent → greater occlusion; larger vent → less occlusion.

• Use vowel sounds /i/ or /u/ (F1 ≈ 300 Hz) to test; /a/ less useful (F1 ≈ 500 Hz).

3. Feedback (Basic Concept)

• Whistling sound due to acoustic leakage:

  • Amplified sound leaks from ear canal → mic → re-amplified → infinite loop.

• Leak points: vent, poor seal, loose mould, or tubing crack.

Clinical Dilemma

• Large vent: ↓ occlusion → ↑ feedback risk.

• Small vent: ↓ feedback → ↑ occlusion.

• Goal = balance comfort & stability.

• Getting the balance (between occlusion effect and feedback risk) right → tricky for individuals with steeply sloping losses

• Anticipating the problem

  • Choosing the right features when deciding on hearing aids (i.e., quality of feedback managers)

  • Making appropriate decisions regarding venting options

  • Setting realistic expectations

  • Avoiding the problem

4. Predicting Risk

Occlusion

Own voice sounds:

• like it’s coming through a microphone/amplifier

• too loud

• too lispy, sharp, metallic, brassy, tinny, pitchy.

• too echoey, boomy, hollow, like head is in a bucket

5. Ampclusion (“Amplified Occlusion”)

Definition

• Amplification of the user's voice

• Occlusion + amplification → own-voice distortion caused by excessive low- / high-frequency gain.

• Appears only when HA is ON (not physical sealing alone).

Differentiate

6. Managing Ampclusion (Fine-Tuning)

Initial Checks

• Identify whether issue is high-frequency (tinny/metallic) or low-frequency (boomy/hollow).

• Review REM to ensure no over-amplification.

Software-Based Steps (Phonak Target example)

1. Occlusion Managers:

   • Automatically reduces low-frequency gain across input levels.

   • Use cautiously—manual fine-tuning preferred.

2. Manual Fine-Tuning:

   • Reduce 2–3 dB LF gain for loud inputs (own voice > 60dBSPL)

   • For high-pitched complaints → reduce loud HF gain.

   • Always monitor compression ratios—avoid > 3:1 (distortion → loss of clarity).

3. Alternate Approaches:

   • Adjust moderate input (G65) slightly if issue persists.

   • Individualise based on real-ear verification and patient feedback.

Manufacturer-Specific Tools

7. Occlusion-Specific Adjustments

If purely physical (HA off still boomy):

1. Counsel & acclimatisation:

   • Clients often adapt mild occlusion over time

   • arrive at a sound level that is different but comfortable

   • Ampclusion can be acclimatised, occlusion less so.

2. Modify ear mould acoustics:

   • Increase vent diameter → ↓ occlusion (↓ acoustic mass).

   • Open dome for RIC aids if clinically suitable.

   • Diff. size of dome

3. Modify canal stock length:

   • Short stock = ↑ occlusion (~ +10 dB @ 300 Hz)

     • Deep fit (> second bend) = ↓ occlusion (~ 0 dB boost)

     • Longer canal stalk (longer cut) → less volume (space) in ear canal → less occlusion

   • Because deeper fit lies in bony canal → less vibration.

4. Take open-jaw impression to capture deeper canal for next mould.

5. Taper mould tip to create “virtual vent” if both occlusion + ampclusion coexist.

6. Clearly state issues you have experienced to laboratory

8. Feedback Management

Common Causes

• Wax blockage

• Poor earmould insertion / loose mould

• Tubing crack

• Gradual gain increase via adaptation manager or user volume control

Modern Feedback Control Types

Verification & Adjustment

1. Run Feedback Test in Software

   • Measures maximum usable gain per frequency before feedback.

   • Warn client—brief loud tone.

2. Interpret Curves

   • Dashed purple line = feedback threshold.

   • If target curve exceeds it → risk of feedback.

   • Feedback manager may limit HF gain, reducing audibility of soft/moderate sounds.

3. Clinician Actions

   • Ensure proper fit & seal.

   • Consider premium HA with stronger cancellation if limits reached.

   • Run real feedback test in-situ (software estimate ≠ reality).

4. Risks of Over-cancellation

   • Reduced headroom → less HF amplification.

   • Entrainment: canceller misidentifies tonal sound (e.g., music) as feedback → introduces distortion.

Physical & Fitting Solutions

• Remake / re-coat ear mould for tighter fit.

• Posterior-bite impression to capture deeper jaw position.

• Acrylic build-up or fotoplast repair gel for small leaks.

• Smaller vent / more occluding dome (but trade-off: occlusion↑).

• Softer materials improve seal comfort.

• Try a different style of hearing aid

9. Practical Balancing Strategies

10. Clinical Practice Principles

• Prioritise comfort and clarity.

• First fit ≠ perfect fit—expect multiple follow-ups.

• Document adjustments and rationales.

• Provide counselling and coaching:

  • Acclimatisation takes time.

  • Reinforce management skills (insertion, cleaning, volume use).

• Use evidence-based, not manufacturer-marketing, claims.