KAI｜Hearing Aids

INTRODUCTION

  • Hearing aids help overcome hearing loss, particularly sensorineural hearing loss.
  • Issues faced include:
    • Some sounds may be inaudible.
    • Some sounds may be misidentified due to missing spectrums, typically high-frequency sounds.
    • The range of levels between the weakest and most intense audible sounds is less in sensorineural hearing loss.
  • Hearing aids amplify weak sounds more than intense sounds.
  • Sensorineural loss affects:
    • Frequency, temporal, and spatial resolution of sounds making noise mask speech more.
  • Physiological origins of sensorineural loss include:
    • Loss of inner and outer hair cell function.
    • Reduced cochlear electrical potential.
    • Changes to cochlear mechanical properties.
  • A hearing-impaired person may need a greater signal-to-noise ratio (SNR) for effective communication than a normal hearing person.

HEARING AID COMPONENTS

  • Hearing aids are functional systems composed of:
    • Microphone: Converts acoustic signals to electrical signals. Can be directional or omnidirectional.
    • Amplifier: Boosts the electrical signals. Types: linear (consistently amplifies signals) and non-linear (varies amplification based on input signal level).
    • Receiver: Converts the amplified electrical signals back into sound.
    • Earmould/Earshell: Holds the aid in place and transmits sound to the ear.
  • Hearing aids are classified by size and type of fit:
    • Body, spectacle, behind-the-ear (BTE), in-the-ear (ITE), in-the-canal (ITC), and completely-in-canal (CIC).
  • The six eras of hearing aid development:
    1. Acoustic
    2. Carbon
    3. Vacuum
    4. Transistor
    5. Digital
    6. Wireless

HEARING AID SYSTEMS

  • Hearing aids can be customized or modular, with features like wireless communication between devices for coordinated amplification.
  • Amplification characteristics can be programmed via wired or wireless methods to suit individuals.
  • Methods to enhance speech intelligibility include:
    • Magnetic induction
    • Radio transmission
    • Infrared transmission
    • Acoustic transmission

ELECTROACOUSTIC PERFORMANCE AND MEASUREMENT

  • Performance is typically measured using a coupler, but real-ear measurements give a better indication of performance in practice.
  • Real-ear measurements include:
    • Real-ear aided response (REAR)
    • Real-ear aided gain (REAG)
    • Real-ear insertion gain (REIG)
  • Problematic factors in measurement: probe placement, background noise, ear wax blockage, etc.

HEARING AID EARMOLDS, EARSHELLS AND COUPLING SYSTEMS

  • Earmoulds serve to fit hearing aids securely and ensure adequate sound transmission.
  • Venting is critical:
    • Reduces occlusion effect.
    • Balances sound rebalance with low-frequency gain.
  • Common venting issues include maximizing sound path while minimizing feedback.

COMPRESSION SYSTEMS IN HEARING AIDS

  • Compression limits sound levels for comfort and intelligibility:
    • Varies amplification based on the input level of sounds.
  • Types of compression:
    • Input-controlled: Before volume control.
    • Output-controlled: After volume control.

DIRECTIONAL MICROPHONES AND ARRAYS

  • Directional microphones improve SNR by focusing on sounds from a specific direction.
  • Methods:
    • Split-band directivity (high and low frequencies).
    • Adaptive arrays that alter sensitivity based on background noise.

ADVANCED SIGNAL PROCESSING SCHEMES

  • Techniques for improving sound quality and intelligibility, like adaptive filtering and frequency compression.
  • Active occlusion reduction and intelligent switching are new advances.

ASSESSING CANDIDACY FOR HEARING AIDS

  • Clinician recommendations depend on:
    • Patient motivation, perceived disability, and adjustment capacity.
    • Accommodating central auditory processing disorders or cognitive abilities.

PRESCRIBING HEARING AID AMPLIFICATION

  • Various prescription formulas available: POGO, NAL, DSL.
  • Different rules apply for linear vs. non-linear aids, mu- lti-memory aids available for varied environments.

SELECTING, ADJUSTING AND VERIFYING HEARING AIDS

  • Selection based on individual needs, including fit, comfort, and required features.
  • Adjusting gain and performance through software, with a focus on real-ear measurements.

PROBLEM SOLVING AND FINE-TUNING

  • Common adjustments made after initial fitting weeks.
  • Solutions to feedback issues, tonal quality optimization, and adjustments based on user preferences.

PATIENT EDUCATION AND COUNSELLING FOR HEARING AID WEARERS

  • Education enhances the likelihood of proper aid usage and management.
  • Communication tactics and auditory strategies key to rehabilitation and performance.

ASSESSING THE OUTCOMES OF HEARING REHABILITATION

  • The effectiveness of outcomes assessed through self-report measures and speech tests for patient satisfaction and functional improvement.

BINAURAL AND BILATERAL CONSIDERATIONS IN HEARING AID FITTING

  • Bilateral fittings improved SNR and localization abilities.
  • Both advantages and disadvantages to consider based on individual needs.

SPECIAL HEARING AID ISSUES FOR CHILDREN

  • Timely provision of aids to infants critical for language development.
  • Unique fitting challenges due to anatomical differences in children, requiring specialized programs.