TYPES OF HEARING AIDS

Terminology

Traditional Hearing

Aids

Air Conductio

n

StandardBehind-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

Implantable hearing aids

Middle Ear

Implants

Bone-Anchor

ed Implant

s

Cochlear

Implants

Auditory

Brainstem

Implants

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. separatorB. zinc powder anode and

electrolyteC. anode canD. insulator gasketE. cathode canF. air holeG. cathode catalyst/current

collectorH. air distribution layerI. 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 .08312 215 1500 Ω .1413 360 1500 Ω .24675 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, dehydrationLitovitz & 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 processorDigital

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 programmingSche

meChannels (Hz)

<500 500-1500

1500-3000

>3000

Speech

10 20 30 25

Music 15 20 20 20Comfort

0 15 15 5

-1

0

1

250 500 1000 2000 4000 8000

-1

0

1

1.5 1.5 2.5 1

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

Intensit

y (dB HL)

0

10

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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

Intensit

y (dB HL)

0

10

20

30

40

50

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