Download Hearing Aids

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
•
2.
Overall size, components, batteries
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’s1980’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