Amplification/Sensory Systems – Audiology Chapter 14

Learning Objectives

• 14.1 Differentiate analog vs. digital hearing-aids (HA)
• 14.2 Define electro-acoustic properties & measurement methods
• 14.3 Identify HA styles (BTE, RIC, etc.)
• 14.4 Describe implantable devices & candidacy
• 14.5 Outline selection & verification factors for adults/children
• 14.6 Recognize hearing-assistance technologies (HATs) that augment/replace traditional amplification

Hearing-Aid Development & Historical Context

• Early sound collectors: horns & seashells → purely acoustic funnels
• First electronic HAs: table-top units; vacuum-tube era → smaller, more powerful bandwidth
• 1950s transistor revolution → modern BTE (portable, battery-powered)
• 1980s in-the-ear (ITE) overtakes BTE in U.S. popularity
• Integrated circuits + computer tech → rapid miniaturization, programmable gain, multi-memory devices
• Clinical challenge: match an amplification/sensory system to patient’s communication needs, age, cognition & motivation (Martin & Clark, p. 389)

Core Components of Any Hearing-Aid

• Microphone – converts acoustical input → electrical signal
• Amplifier/Signal Processor – increases/reshapes signal (analog or digital)
• Receiver (loud-speaker) – reconverts electrical output → acoustic energy

Signal-Processing Technology

Analog

• Output voltage is analogous to incoming waveform
• Frequency-dependent gain fixed at manufacture; adjustments via on-board dials

Digital

• A/D converter transforms waveform → binary bits (0/1) representing f, intensity & time
• DSP chip executes high-speed algorithms (noise reduction, feedback cancellation, adaptive directionality)
• Multiple user programs stored (quiet, noise, music, telecoil, etc.)
• Fine-tuning via PC software; firmware updates possible

Today’s DSP Advantages

• Enhanced signal-to-noise ratio (SNR) & clarity
• Selective filtering of unwanted signals
• On-board data-logging & connectivity (Bluetooth®, telehealth)

Key Electro-Acoustic Terms

• Acoustic Gain (G) – added intensity G = P{out} - P{in} (dB)
• High-Frequency Average Gain (HFA) – \displaystyle HFA = \frac{G{1000}+G{1600}+G_{2500}}{3}
• Frequency Response – usable pass-band between intersection points 20\,\text{dB} below HFA line
• Output Sound-Pressure Level (OSPL90 / SSPL / MPO) – max output with 90\,\text{dB SPL} input & VC full-on
• Feedback – acoustic loop → whistling if gain too high or fit too loose
• Directional Microphones – suppress off-axis noise, improve SNR
• Compression (WDRC, AGCi/o) – non-linear circuit keeps loud sounds within patient’s dynamic range, compensating for loudness recruitment
• Distortion –
– Frequency: altered spectrum
– Amplitude: non-linear gain
– Harmonic: % additional unwanted harmonics
• Telecoil (t-coil) – electromagnetic pickup bypassing mic for phones & loops
• Bluetooth / 2.4-GHz / NFMI – wireless streaming, remote programming

Measuring Electro-Acoustic Performance

• Test box (ANSI coupler) 125\text{–}10,000\,\text{Hz}; compare to norms
• Primary measures: {\text{OSPL},\,G,\,\text{Frequency Response},\,\text{Distortion}}
• Real-ear (probe mic): actual SPL in ear canal accounting for individual resonance

Styles of Air-Conduction Hearing-Aids

Earmolds & Acoustic Coupling

• Pure coupler (no electronics); tubing length/diameter, venting & material alter acoustics (low-frequency venting, HF damping)
• Critical for pediatric growth; easy replacement

Bilateral vs Unilateral Amplification

• Goal: recreate binaural hearing
• Benefits: binaural summation (louder/clearer), improved localization, better speech-in-noise, reduced auditory deprivation
• \approx84\% of patients fitted binaurally
• CROS – routes signal from unaidable ear → normal ear (unoccluded canal for natural hearing)
• BiCROS – same routing but better ear also amplified (aidable HL)
• Limitations: all sound to one side → poor localization, S-in-N challenges

Bone-Conduction (BC) Hearing-Aids

• Indications: conductive HL, chronic otitis, atresia, drainage
• Vibratory oscillator on mastoid stimulates both cochleae; true binaural cues reduced (bilateral BC aids partly compensate)

Hearing-Aid Selection & Fitting Workflow

Adult Considerations

• Case history (perceived handicap, motivation)
• Diagnostic results: pure-tones, speech discrimination → prognosis
• Choose circuit, style, accessories (remote, streamer) per dexterity, aesthetics, budget
• Counseling critical for expectation management

Pediatric Challenges

• Limited behavioral thresholds, language delay, co-morbidities
• Need family support & early fitting (critical language period)

Verification & Validation

• Electro-acoustic analysis – meet target recipe (e.g., DSL, NAL)
• Real-ear measures – probe mic at eardrum
• Functional tests (sound-field aided thresholds, SRS)
• Questionnaires: APHAB, COSI (adult); PEACH, CHILD (peds)

Pediatric Assessment Tools

• Meaningful Auditory Integration Scale (MAIS/IT-MAIS) – parent questionnaire on auditory behavior & device use
• Early Speech Perception (ESP) test – closed/open-set for children ≥ 3\,\text{yr} with severe-profound HL
• Lexical Neighborhood Test / Multisyllabic LNT – open-set word recognition; used for CI candidacy & benefit tracking

Carhart’s Four Goals for HA Fitting

• Restore audibility of soft speech/environmental cues
• Secure clarity in quiet
• Maintain clarity in noise
• Avoid intolerable amplification of loud sounds

Common Post-Fit Complaints & Adjustments

• “Occlusion / barrel” – vent size, low-freq gain ↓, open-fit option
• “Too sharp/tinny” – HF gain ↓, adjust smoothing, increase low-freq
• “Ear plugged” – add venting, switch to RIC
• “Poor in background noise” – enable adaptive directionality, noise reduction, add remote mic
• “Fan too loud” – tweak expansion/noise floor, compression kneepoint

Over-The-Counter (OTC) Hearing-Aids

• 2017 U.S. law: adults with mild → moderate HL may purchase OTC devices; FDA sets safety & labeling standards
• Public-health rationale: reduce untreated HL linked to depression, dementia, isolation

Surgical / Implantable Hearing Devices

Types & Indications

• Cochlear Implant (CI) – bypass hair cells, stimulate auditory nerve directly
• Bone-Conduction Implant (Osseointegrated) – significant ABG, good BC thresholds; \ge5 yrs old FDA
• Middle-Ear Implant (MEI) – ages \ge18; eliminates occlusion/feedback; higher cost/surgical risk
• Auditory Brain-Stem Implant (ABI) – no cochlear nerve (e.g., NF2); stimulates cochlear nuclei

Cochlear Implants – Components & Process

• Internal receiver beneath skin; electrode array in scala tympani
• External BTE mic + speech processor → RF link → electrodes
• Mapping – audiologist programs max/threshold current levels

Candidacy (Cochlear Corp., 2020)

• Age 12\text{–}24 mo: bilateral profound SN HL + limited aided benefit (per MAIS/ESP)
• \ge2 yr: severe→profound bilateral HL; aided word score \le30\% on MLNT/LNT
• Contraindications: absent cochlea, auditory nerve lesion, active ME infection, perforation w/ disease

Prognosis Factors

• Shorter duration post-lingual deafness (adults); implantation <2 yr age (children)
• Intensive auditory therapy; family commitment; multidisciplinary team

Implantable Bone-Conduction Devices

• Titanium fixture + abutment/magnet in mastoid; external processor snaps on
• Direct bone pathway eliminates feedback & occlusion; routes to both cochleae

Middle-Ear Implants

• External audio processor → transcutaneous link → ossicular-mounted transducer
• Avoids ear-canal occlusion; but surgical risk, cost, minimal cosmetic gain vs micro-RIC

Auditory Brain-Stem Implants (ABI)

• Electrode paddle on cochlear nucleus; for bilateral acoustic neuromas, aplasia of nerve
• Candidates as young as 18 mo; external processor similar to CI

Hearing-Assistance Technologies (HATs / ALDs)

• Target users: non-HA wearers, dexterity issues, supplemental for HA/CI
• Devices: infrared TV systems, amplified/captioned phones, doorbell/alerting (flash, vibrate), personal pocket amplifiers
• Bluetooth accessories extend modern HA connectivity (streamers, neck loops)

Remote Sound-Transmission Systems

• FM/DM/Roger – \mathbf{1:1} or group; mic near speaker → digital radio → listener’s HA/CI boots
• Infrared – line-of-sight light waves; theaters, living rooms
• Induction Loop / Electromagnetic Field – wire loop radiates magnetic field; telecoil pickup → virtually no background noise

Classroom HAT

• Personal FM/DM – tailored to student; portable receiver or HA boot
• Classroom Audio Distribution (CAD) – teacher mic → ceiling/wall speakers; benefits all learners

Ethical, Philosophical & Practical Implications

• Early pediatric fitting influences language neuroplasticity; delays risk lifetime deficits
• OTC legislation balances autonomy vs professional guidance; underscores need for public education on realistic expectations and safe use
• Implant candidacy highlights resource equity (surgery cost, therapy access)
• Continuous tech evolution requires audiologists to engage in lifelong learning & evidence-based updates