CSD 3000 DEAFNESS IN SOCIETY

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24 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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CSD 5400

REHABILITATION
PROCEDURES FOR THE
HARD OF HEARING

Amplification

Hearing Aids

Preliminary Remarks

One of the most critical elements of any
rehabilitation plan (child or adult) is
the fitting of appropriate amplification

This is the only recognizable way, at this
time, to maximize residual hearing

The primary objective of modern amplification
theory is to make speech audible without
introducing distortion and to restore a more
“normal” range of loudness experience


Preliminary Remarks


Fitting of appropriate amplification should
only be done by a trained professional

Audiologist

Licensed hearing aid dispenser

Hearing aids are the most common form
of amplification, but not the only one

Selecting the

Hearing Aid Candidate

What factors do we
consider, after a
hearing assessment,
in making decisions
about
recommending
amplification?


1. Degree of Hearing Loss

Right or wrong, this continues to be a major
parameter in deciding whether or not to
recommend amplification

Many studies demonstrate poor correlation
between hearing sensitivity and self
-
perceived
handicap (activity limitation)

In general, as the degree of HL increases, the
need for amplification increases, but
overall HA benefit decreases

“Problem Fittings”

Unilateral hearing loss

Lots of residual hearing

Mild hearing loss

Normal hearing throughout a broad area
of the audiometric range

Profound degrees of HL/very little
residual hearing

Unusual/atypical configurations

2. Motivation

Very common for people to
believe they have HL, but
only a small percentage
will ever utilize HAs or
even try them

Why?

Poor advice from MDs

Lack of understanding of HL
and HAs

Financial concerns

Here are some common
factors that cause people
to try amplification

3.
Acceptance of

Hearing Loss

How realistic is the client regarding the
amount or type of communication
problems the hearing loss is causing?

Is the client still angry about the
hearing loss?

4. Cosmetic Concerns

“Hearing aid effect”

It is real

Stigma

How concerned is the client over
hearing aid size and style?

Hearing Aids

First hearing aids were manufactured
by Siemens Electronics in 1912

The two major trends that drive current
hearing aid design include

1.
Miniaturization


Overall size, components, batteries

2.
Enhanced signal processing

Hearing Aid Development

http://dept.kent.edu/hearingaidmuseu
m/development.html

Recent Advances


in Signal Processing

Multiple memories

Provides access to different amplification characteristics

Increases flexibility

Noise reduction circuits

Sophisticated ways to improve the signal
-
to
-
noise ratio

Major complaint of most listeners

Feedback control

Feedback is the audible signal related to an interaction
between certain components of a hearing aid

As hearing aids became smaller, feedback was a major
limitation and complaint

Recent Advances

in Signal Processing

Programmability

Allows the dispenser to set and control a number of different
properties of the HA

Increases “precision” of the fitting

Increases hearing aid flexibility

Digital processing

Major tech advance of the 1990’s

Talk more about this later

Tremendous improvement in HA fidelity and flexibility

Multiple channels

Allows separate frequency bands to be established and
processed (manipulated) separately and independently

Basic Components

of a Hearing Aid

Microphone

Converts sound waves to an electrical signal

Amplifier

Increases the amplitude of the electrical signal from the microphone

Receiver

Converts the amplified electrical signal back to sound

Battery

Provides power to drive the amplifier

Basic Hearing Aid
Components

Putting everything together

Let’s look at each of these components in a
little more detail…

Batteries

Drive the amplifier

Come in about 6 sizes
denoted by a number

675, 312, 13, 230, 10A, 5A

Sizes are not
interchangeable

Come in different
materials

Mercury (banned), silver
oxide, zinc
-
air

Materials can be used
interchangeable, but
battery life might vary

Microphones

Two types are used

Directional

Designed to respond
primarily to sounds
originating from in
front of the wearer,
not behind

Helps to improve S/N

Non
-
directional or
omnidirectional

Equally sensitive in all
directions


Amplifiers

The amplifier determines the “gain” of the
hearing aid

Hearing aid gain is the difference, in dB, between
the input level of an acoustic signal and the
output level

Generally matched to the degree of hearing loss
and the dynamic range of the listener

Also determines the method of output limitation
and how the hearing aid increases intensity
throughout it’s functional range

If the HA has a volume control, it is tied to the
amplifier

Receivers

Most modern hearing
aids use an internal air
conduction receiver

“G”

You can’t see them

Other receiver options
exist

Bone conduction HAs

Traditional

Implantable (later
)

Vibrotactile HAs

(later)

Other Common

Hearing Aid Features

On
-
off switch

Allows the hearing aid
to be shut off
completely

Battery saving device

Levers, switches,
feature of the VCW

Other Common

Hearing Aid Features

Telecoil

Special circuit that enhances
telephone communication
or allows compatability
with loop systems (later)

Circuit emits an
electromagnetic signal
which is picked up by the
t
-
coil

The t
-
coil signal is amplified
and sent to the receiver

“D”


Other Common

Hearing Aid Features

Telecoil

Typically the user manually
activates the circuit


T switch”

M
-
T
-
O

MT

Advantages to the telecoil

Reduces the occurance of
feedback when using the
telephone

Improves the signal
-
to
-
noise
ratio


Other Common

Hearing Aid Features

Volume Control

Directly tied to the
amplifier

Some HA styles don’t offer
a VCW

CIC

Some DSP circuits don’t
offer VC

These hearing aids
continuously adjust the
gain

Other Common

Hearing Aid Features

Earmolds

Some hearing aid styles require
a separate earmold

Body HA, BTE, eyeglass HA

Serve three purposes

Deliver the amplified sound
from the HA to the ear

Couples the HA to the wearer

Modifies the acoustics of the
amplified sound delivered to
the ear

Here’s a diagram illustrating
some common earmold
styles

CD
-
ROM Ch07.06 and Ch07.07

Hearing Aid Fitting
Considerations

There are three major considerations
that that need to be addressed when
making a recommendation of hearing
aids:

What hearing aid style will work best?

What level of technology will work best?

Monaural or binaural?

Hearing Aid Style

Currently, six styles are available from
most manufactureres

Body, eyeglass, behind
-
the
-
ear, in
-
the
-
ear,
in
-
the
-
canal, completely
-
in
-
the
-
canal

Nearly every level of technology is
available in any style

Every degree of hearing loss can be fit
appropriately with every style

Body Hearing Aid

Advantages

High gain with no
feedback

Large controls

Disadvantages

Stigma

Monaural fitting only

Y
-
cords

Microphone placement

Body baffle, clothing
noise

Newer DSP circuits not
available in this style

Eyeglass Hearing Aid

Very popular in the 1960’s
-
1980’s

Needs an earmold

Advantages

Used to be more cosmetically
appealing, especially for
people with unilateral HL

Disadvantages

Fitting issues

Breakdown and repair

True binaural fitting is not
possible

DSP circuits not available in
this style

Behind
-
the
-
Ear (BTE)

Needs an earmold

CD
-
ROM CH07.04

BTE

Advantages

Flexibility

Good choice for kids

Compatibility with direct audio input

Few problems with feedback

Microphone placement

True binaural fitting possible

Very durable/good repair record

BTE

Disadvantages

Stigma compared to custom styles

Microphone placement compared to custom
styles

Fit with glasses, etc..

Harder to manipulate, compared to custom
styles


Custom Hearing Aid Styles

ITE, ITC, CIC

All components of the
HA fit completely in
the pinna and/or
EAM

Completely changed
the approach
audiologists take
regarding the HA
fitting

CD
-
ROM CH07.05

ITE and ITC

Advantages

Cosmetic appeal

Ease of insertion/secure fit

Microphone placement is very realistic

Reduction in wind noise (especially
ITC)

Disadvantages

Feedback can be a problem

Battery size

Expense (especially as size decreases)

Some desired options might not be
available in ITC


Telecoil, directional microphones,
VCW

Completely
-
in
-
the
-
Canal
(CIC)

These hearing aids sit
very close to the TM so
the acoustic
advantages are
different than the
other styles

The major drawback is
that the listener needs
a large enough
earcanal to
accommodate the
minimum size the HA
has to be

CIC

Advantages

Easy to use and insert

Reduction/elimination of occlusion effect

Less HA gain/reduction of feedback/phone

Improved localization ability

Elimination of wind noise

Elimination of cerumen problem

Invisible

Greater high frequency emphasis is possible

Disadvantages

Expense, size, gain/output problems, custom
features not available

Level of Technology

There are currently three major levels
of hearing aid technology people are
able to access:

Analog hearing aids

Digitally controlled analog hearing aids

Sometimes referred to as
“programmable”

Digital hearing aids

Analog Hearing Aid

Oldest signal processing strategy

Signal is processed in a manner that’s continuously
varying over time

Here’s a schematic of an analog circuit

Hearing aid controls are analog, too

Digitally Controlled

Analog Hearing Aids

Uses analog signal processing with digital control
over the electroacoustic parameters

Gain, compression, frequency response, etc

Here’s a schematic of such a circuit

Major advantages include flexibility,
programmability, greater range of outputs

Digital Hearing Aids

Analog signal is converted to a digital representation

Once digitized, the signals are manipulated by
processing algorithms (programs) and converted
back to analog form

Biggest advantage is complete flexibility and
programmability

Here’s a schematic of a digital HA

Fitting One Ear or Two??

Benefits of binaural amplification

1.
Localization

2.
Binaural summation

3.
Elimination of head shadow

4.
Improved S/N through binaural
squelch

5.
Auditory deprivation

A Word About

Hearing Aid Maintenance..

This is mentioned specifically in the ASHA
Scope of Practice for SLPs

Any client of yours wearing hearing aids
needs to have their HAs checked
daily

Battery

Invest in a battery tester

Earmold and receiver checked for wax

Recommended daily HA check procedures

HA troubleshooting guide