Hearing aid anatomy

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

Microphone
Digital
Sound
Processor
Power
Source
Receiver

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
 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.

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

-1
0
1
-1
0
1
250 500
1000
2000
4000

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
< < < < <

]
]
] ] ]

Hearing aid anatomy

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