Hearing aid anatomy

CDS 631 Lecture
Hearing aid components

1. Types of Hearing Aids

2. Terminology

3. Hearing aid styles http://www.jefferson.edu/jmc/departments/otolaryngology/centers/balance_hearing/patient_services/hearing_loss.html

4. Hearing Aid Effect Johnson et al 2005

5. Hearing Aid Components

6. Batteries

7. Batteries Types Silver Oxide Mercury Zinc-Air Long shelf-life Disposable Sizes Smallest to largest 10 yellow 312 brown 13 orange 675 blue

8. Zinc-Air batteries Require air to work Battery compartment of hearing aid must be permeable to air Air activation pore may clog up Affected by humidity Low humidity dries out the electrolyte in the cell High humidity can flood the cell Teflon membrane in battery helps moderate effect of humidity separator zinc powder anode and electrolyte anode can insulator gasket cathode can air hole cathode catalyst/current collector air distribution layer semipermeable membrane

9. Batteries Flat discharge rate Capacity rating Capacity is in Amperes/hour In general, larger size batteries are designed for greater load. Size Capacity Discharge C/D 10 245 3000 Ω .08 312 215 1500 Ω .14 13 360 1500 Ω .24 675 320 620 Ω .52

10. Batteries 2320 cases of battery ingestion 1983 – 1990 952 were hearing aid batteries (45%) Of those cases, 312 (33%) were batteries removed from the hearing aid by the child Litovitz & Schmitz, 1992

11. Batteries Zinc-Air batteries are relatively benign Of 418 cases of zinc-air ingestion, only 21 (5%) had negative outcomes Minor: nausea, vomiting, fever Moderate: high fever, bloody stools, dehydration Litovitz & Schmitz, 1992

12. Batteries If anyone ingests a battery, this is what you should do: Immediately call the 24-hour National Battery Ingestion Hotline at 202-625-3333 (call collect if necessary), or call your poison center at 1-800-222-1222. If readily available, provide the battery identification number, found on the package or from a matching battery. In most cases, an x-ray must be obtained right away to be sure that the battery has gone through the esophagus into the stomach. (If the battery remains in the esophagus, it must be removed immediately. Most batteries move on to the stomach and can be allowed to pass by themselves.)  Based on the age of the patient and size of the battery, the National Battery Ingestion Hotline specialists can help you determine if an immediate x-ray is required. National Capital Poison Center

13. Batteries Don't induce vomiting.  Don't eat or drink until the x-ray shows the battery is beyond the esophagus. Watch for fever, abdominal pain, vomiting, or blood in the stools. Report these symptoms immediately. Check the stools until the battery has passed. Your physician or the emergency room may call the National Button Battery Ingestion Hotline/National Capital Poison Center collect at 202-625-3333 for consultation about button batteries. Expert advice is available 24 hours a day, 7 days a week. National Capital Poison Center

14. Microphones

15. Microphones Converts acoustic energy to an electrical signal Sound pressure waves enter front volume of microphone Diaphragm oscillates Oscillating voltage between diaphragm and backplate Voltage amplified by field effect transistor (FET) Diaphragm Charged Electret Backplate FET Barometric relief hole Damping screen

16. Types of Microphones Omnidirectional mic Directional mic Thompson, 2003

17. Directionality with a single directional microphone Thompson, 2003

18. Directionality with a two omnidirectional microphone Thompson, 2003

19. Directional sensitivity Low frequencies lose sensitivity Hearing aid must add gain to low frequency inputs to counteract reduced sensitivity May make internal noise more audible

20. Broken microphone? Listening check No feedback, no sound? Check for debris in port, and clean out. If applicable, turn hearing aid to t-coil and hold up to fluorescent light/CRT. Buzzing? Probably microphone. No buzzing? Probably receiver.

21. Receivers

22. Receivers

23. Signal Processors

24. Signal processor

25. Digital signal processor What is the stuff? Input assigned to channels (frequency ranges) Analyzed for speech characteristics Sound classification schemes Appropriate gain applied independently to each channel per programming Scheme Channels (Hz) <500 500-1500 1500-3000 >3000 Speech 10 20 30 25 Music 15 20 20 20 Comfort 15 15 5

26. 1.5 1.5 2 .5 1

27. Other aids

28. Bone conduction hearing aids Intact cochlea Air conduction hearing aids contraindicated Chronic drainage Microtia

29. Frequency (Hz) 250 500 1000 2000 4000 8000 -10                           Intensity (dB HL)                                                                               10                                                     20                                                     30                                                     40                                                     50                                                     60                                                     70                                                     80                                                     90                                                     100                                                     110                                                     120                                                     O O O O O O O O [ [ [ [ [ ] ] ] ] ] X X X X X X X X

30. CROS Contralateral Routing of Signal Microphone on one side Receiver on the other No amplification Used for unilateral loss One dead ear One normal ear

31. Frequency (Hz) 250 500 1000 2000 4000 8000 -10                           Intensity (dB HL)                                                                               10                                                     20                                                     30                                                     40                                                     50                                                     60                                                     70                                                     80                                                     90                                                     100                                                     110                                                     120                                                             O O O O O O O O < < < < <  ] ] ] ] ]