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Hearing aid anatomy

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Derek Stiles
Derek Stiles

This document discusses different types of hearing aids and their components. It describes traditional air conduction hearing aids such as behind-the-ear (BTE), in-the-ear (ITE), and in-the-canal (ITC) styles. It also discusses bone conduction hearing aids. The components of hearing aids discussed include the microphone, signal processor, receiver, and batteries. Zinc-air batteries commonly used in hearing aids and safety issues related to battery ingestion are covered. Microphone types including omnidirectional and directional microphones are described.

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TYPES OF HEARING AIDS
Terminology Traditional Hearing Aids Air Conduction Standard Behind-the-ear (BTE) Receiver-in-the-ear Receiver-in-the-canal (RITE/RIC) Custom In-the-ear (ITE) In-the-canal (ITC) Completely-in-the- canal (CIC) Bone Conduction Headband Eyeglass
Hearing aid styles http://www.jefferson.edu/jmc/departments/otolaryngology/centers/balance_hearing/patient_services/hearing_loss.html
Hearing Aid Effect Johnson et al 2005
HEARING AID COMPONENTS
Microphone Digital Sound Processor Power Source Receiver
Batteries
Batteries  Types  Silver Oxide  Mercury  Zinc-Air  Long shelf-life  Disposable  Sizes  Smallest to largest  10 yellow  312 brown  13 orange  675 blue
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 A. separator B. zinc powder anode and electrolyte C. anode can D. insulator gasket E. cathode can F. air hole G. cathode catalyst/current collector H. air distribution layer I. semipermeable membrane
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
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
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
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
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
Microphones
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
Types of Microphones Omnidirectional mic Directional mic Thompson, 2003
Directionality with a single directional microphone Thompson, 2003
Directionality with a two omnidirectional microphone Thompson, 2003
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
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.
Receivers
Receivers
Signal Processors
Signal processor Digital representation of microphone output enters digital processor Stuff Digital representation of modified signal sent to receiver
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 0 15 15 5
Hearing aid anatomy
-1 0 1 -1 0 1 250 500 1000 2000 4000
Hearing aid anatomy
OTHER AIDS
Bone conduction hearing aids  Intact cochlea  Air conduction hearing aids contraindicated  Chronic drainage  Microtia
Frequency (Hz) 250 500 1000 2000 4000 8000 -10 Intensity(dBHL) 0 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
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
Frequency (Hz) 250 500 1000 2000 4000 8000 -10 Intensity(dBHL) 0 10 20 30 40 50 60 70 80 90 100 110 120         O O O O O O O O < < < < <  ] ] ] ] ]

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Hearing aid anatomy

  • 8. Batteries  Types  Silver Oxide  Mercury  Zinc-Air  Long shelf-life  Disposable  Sizes  Smallest to largest  10 yellow  312 brown  13 orange  675 blue
  • 9. 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 A. separator B. zinc powder anode and electrolyte C. anode can D. insulator gasket E. cathode can F. air hole G. cathode catalyst/current collector H. air distribution layer I. semipermeable membrane
  • 10. 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
  • 11. 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
  • 12. 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
  • 13. 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
  • 14. 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
  • 16. 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
  • 17. Types of Microphones Omnidirectional mic Directional mic Thompson, 2003
  • 18. Directionality with a single directional microphone Thompson, 2003
  • 19. Directionality with a two omnidirectional microphone Thompson, 2003
  • 20. 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
  • 21. 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.
  • 25. Signal processor Digital representation of microphone output enters digital processor Stuff Digital representation of modified signal sent to receiver
  • 26. 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 0 15 15 5
  • 31. Bone conduction hearing aids  Intact cochlea  Air conduction hearing aids contraindicated  Chronic drainage  Microtia
  • 32. Frequency (Hz) 250 500 1000 2000 4000 8000 -10 Intensity(dBHL) 0 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
  • 33. 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
  • 34. Frequency (Hz) 250 500 1000 2000 4000 8000 -10 Intensity(dBHL) 0 10 20 30 40 50 60 70 80 90 100 110 120         O O O O O O O O < < < < <  ] ] ] ] ]