TYPES OF HEARING AIDS

Terminology

Traditional Hearing

Aids

Air Conductio

n

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

Implantable hearing aids

Middle

Ear

Implant

s

Bone-

Anchore

d

Implant

s

Cochlear

Implant

s

Auditory

Brainste

m

Implant

s

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 .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, 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 programmingScheme

Channels (Hz)

<500 500-1500

1500-3000

>3000

Speech

10 20 30 25

Music 15 20 20 20

Comfort

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

20

30

40

50

60

70

80

90

100

110

120

O O OO O O O

O

[[ [

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XX

X X XX

XX

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

60

70

80

90

100

110

120

O O OO O O O

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