Hearing
Aid Selection
Selection of hearing aid candidates
Pre-selection of hearing aids
Evaluation of performance with hearing
aids
Validation of selection
Selection
of Hearing Aid Candidates
 |
To assess the ______ amplification |
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To determine if the individual may
experience benefit from amplification |
Pre-Selection
of Hearing Aids
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To
determine hearing aids that have appropriate characteristics for the individual based on
previous data with _____________ |
Evaluation
of Performance
with Hearing Aids
|
|
To determine which hearing aid allows for
the greatest _______ and limits the _____ so that it is not uncomfortable |
Validation
of Selection
|
|
To confirm that the aid selected is
actually of _______ to the patient as anticipated once the patient has been properly
instructed on hearing aid use and had a ________ |
Selection
of Hearing Aid Candidates
Generalizations
Factors that
influence candidacy
Tests to
conduct
Consider
Differences Relative To...
Microphone
Possible
gain
_____ of output
Availability of options
Ease of _____
Flexibility
Arrangements
Pre-selection
of Hearing Aids
Type
of aid
Optional features
Output limiting
Gain
Methods to Determine
Maximum Output
Main goal is to avoid discomfort
Guiding Principles
Set OSPL90 equal to or just below the UCL
Set OSPL90 as high as possible without causing discomfort to avoid
excessive saturation
The minimum acceptable OSPL90 is one that would not limit slightly loud
speech (ie 75 dB SPL)
Prescriptive Approaches to Maximum Output
Cox
Set OSPL90 to 100 dB SPL plus a quarter of the hearing loss
NAL
Set OSPL90 midway between OSPL90 needed to avoid discomfort and the
OSPL90 needed to avoided excessive saturation
DSL
Set OSPL90 so that the difference between the projected level of
amplified speech is 10 dB for profound losses up to 40 dB for moderate
losses
(linear function)
Prescriptive Approaches to Maximum Output
Conductive Hearing Loss
Set OSPL90 based on Sensorineural part of loss and increase by 90% of the
Air Bone Gap
Small Ears
Measure RECD to convert 2cc values to real ear values (SPL is greater in
smaller volumes!)
When UCL cannot be measured
Use Threshold predictions, not perfect, but a starting point (ex. Skinner
tables)
Ways
to Specify Gain
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2cc
gain |
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Zwislocki
gain |
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____
gain |
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|
_____gain |
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Functional
gain |
Two
Methods to Determine Gain
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Threshold
based |
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Loudness perception based |
According to Skinner, NAL, POGO,
Berger et al. Give to much gain at 2k re: .5 KHz Cox procedure causes speech spectrum to
fall below MCL. Pascoe & Skinner best to use with any etiology
Threshold
Based
|
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Byrne & Tonnisson (1976)-NAL |
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Byrne & Dillon (1986)-NAL-R |
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Berger, Hagberg and Rane (1977) |
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McCandless & Lyregard (1983) POGO |
Loudness
Perception Based
|
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Pascoe
(1975) |
|
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Skinner
(1982) |
|
|
Cox
(1986) |
Evaluation
of Performance
With Hearing Aids
Validation
of Selection
|
|
Frequency
of use |
|
|
Frequency
of changes in fitting |
|
|
Specific
benefit provided |
|
|
Patient
acceptability ratings |
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|
Observations |
REAL EAR MEASUREMENTS
Rationale
History
Advantages
Rationale
Difficult to assess coupling modifications, such as vents, dampers
Evaluation procedures (i.e. speech recognition) are not very sensitive
All this is complicated by real ear-to-2cc coupler differences
History
1980’s: Probe mic measures at Univ. of Minnesota -- very small microphone
put into ear canal
1981: Probe tube mic--tube was inserted into the ear canal, not
whole mic
1984: Commercial probe-mic systems with HA analyzer. Allowed for ANSI
measures as well.
Advantages
Eliminate subject threshold response variability
Information across the entire frequency range, rather than just
octave freqs.
Able to avoid "contamination" of aided thresholds by internal noise of HA
or by room noise
Time savings
Advantages, continued
Measurements can be reliably done in a reverberant room
Ear-specific information
"High Tech" effect, impresses patients
REAL EAR MEASUREMENT: outline of STEPS
1) Calibrate or Level
2) Measure Ear Canal Resonance
3) Place Hearing Aid on Patient, measure Aided Response
4) Examine Insertion Gain
To see how close to the desired gain you are
5) Measure Maximum Output
1) Calibrate or Level
Present "sweep" of frequencies or broad band signal
Adjust the distance of the loud speaker to the patient’s ear/HA to get
desired output
Note: Some systems have reference mics located at the position of the
hearing aid mic to monitor SPL & adjust for reverberation, head movement
effects
2)
Measure Ear Canal Resonance
Place probe mic tube into ear canal, and insert as far as possible
Present sweep frequencies or broad-band signal at 70 dB SPL
3) With aid on patient, Measure Aided Response (
In
Situ response)
Adjust the HA to preferred settings
(i.e. run the HA on test box with 50/60 dB SPL input)
Place HA on patient with probe mic in canal as close to the same position
used in Measuring Ear Canal Resonance in previous step…use reference "sleeve"
on probe tube
Present sweep frequencies or broad-band signal at 70 dB SPL
4)
Examine Insertion Gain
To see how close to desired or target gain you are
Insertion gain = In Situ gain - Field to Ear Transfer
Function
Insertion Gain = Aided response - Canal Resonance
look at Insertion Gain and compare with gain as per Cox, NAL-R, POGO,
etc. procedures
5)
Measure Maximum Output
Present sweep frequencies 90 dB SPL
Examine levels in the real ear re: patient’s UCL
WAYS TO SPECIFY GAIN
2 cc Gain
Zwislocki Gain
In Situ Gain
Insertion Gain
Functional Gain
2cc Gain
Output in 2cc coupler minus the input
remember, 2cc measures overestimate high-freq gain re: to Real Ear
(functional gain) with a BTE aid and standard closed earmold with no vent
Zwislocki Gain
Output measured in a Zwislocki coupler minus the input
Zwislocki coupler also overestimates the Real Ear insertion gain in the
higher frequencies.
In Situ
Gain
Output in Zwislocki coupler in KEMAR
(Knowles Electronic Manikin for Acoustic Research)
Minus the input.
or, Output at the TM with hearing aid on MINUS the Input at the mic of the
hearing aid
Insertion Gain
In Situ gain minus Field-to-ear drum transfer function.
Output at TM with HA minus output at TM without the HA.
Functional Gain
Unaided threshold minus aided threshold
ACRONYMS
REUR-Real ear Unaided Response
REAR-Real ear Aided Response
REIR-Real ear Insertion Response
RESR-Real ear Saturation Response
REOR-Real ear Occluded Response
REVIEW of TERMS
ISSUES IN PROBE MIC MEASURES
1) Is the input signal next to the ear constant?
Consider the input signals in a sound booth vs. reverberant room
…The Following Figures Brought to You By…
Hawkins & Mueller (1986). Hearing Instruments, 37, 12-12.
Sound Booth vs. Reverberant Room
Issues in probe mic measures, continued
2) Do the output levels measured agree with those measured by a Zwislocki
coupler in KEMAR?
Generally, less than a 4 dB difference between Zwislocki coupler + KEMAR
(i.e. true SPL at TM) and what was measured with probe mic
KEMAR vs. Zwislocki coupler
Issues in probe mic measures, continued
3) What is the effect of probe tube insertion depth?
The closer to the TM, the more accurate the output measurement re: that
obtained with Zwislocki coupler
Accuracy of output at 3 different positions in canal measured in sound booth
Issues in probe mic measures, continued
4) Does the insertion depth affect Insertion Gain measurement?
Distance is not that critical for insertion gain measures, as long as it
is constant for UNaided vs. aided.
Probe tube Depth & Insertion Gain
Using Probe mic measurements to confirm Hearing Aid circuitry
Example 1- Input AGC
Example 2 – Low SSPL90
Custom Hearing Aids -- An Overview
Types and Styles
Candidates for Custom molded HAs
Advantages
Disadvantages
Fitting Procedures
Functional gain vs 2cc gain
Custom Hearing Aids
Types or Styles
In The Ear (ITE) 1970’s
In The Canal (ITC) 1980’s
In The Helix (ITH) 1985
Peritympanic 1990’s
Completely In the Canal (CIC) 1990’s
These styles came and went fairly quickly
In The Ear (ITE) 1970’s
For mild up to severe hearing losses
Most common are custom molded shells; some available in "modular" design
In The Canal (ITC) 1980’s
Really "kicked – off" by then-president Ronald Reagan…sales literally
tripled overnight
Manufactures claim ITCs can fit losses up to 65 dB HL; less gain required
due to longer canal portion, closer to the TM than full shell ITE styles
In The Helix (ITH) 1985
For normal hearing up to 1.5 KHz, leaves the ear canal open for natural ear
canal resonance
Also called an "open fit"
Peritympanic Hearing Aids 1990s
EXTREMELY close to the tympanic membrane
Special ear impression material and techniques needed; e.g. Siemens would
not allow you to dispense their peritympanic hearing aid without first being
trained at their special fitting seminars
Problems with patient comfort: impression taking and fit led
to demise
Completely In Canal (CIC) 1990’s -- present
Idea germinated from peritympanic HA
Not quite as deep an insertion as a peri-TM HA, but significantly deeper
than a standard ITC
Development helped with concurrent advances in component miniaturization of
mics, chips, receivers, etc.
CICs, continued
Dr. Moushegian fit with CICs
"These sound so much more natural than my other bigger (ITE
style) hearing aids…"
Candidacy for Custom Molded Hearing Aids
Age
Manual Dexterity
Degree of Hearing Loss
Size of Ear
Need for Options
Age
Typically, custom molded HAs fit on adults
Children can be fit, most manufactures will recase for a modest fee
($60-$90)
Manual Dexterity
Smaller ITEs, ITCs, and certainly CICs require greater dexterity
Battery insertion and removal an issue as well
Degree of Hearing Loss
Custom molded products typically not successful with
severe-profound losses
Manufacture "Fitting Range" guides often overestimate "fittable" loss area
ITE may tout "60 dB of Gain!", however, usable gain
will never be that great due to feedback
Size (and texture) of Ear
Small ear canal size or very sharp bends in ear canal may prevent adequate
seal/comfort with ITCs and CICs
Stiff, "cardboard" pinna
Very soft, flaccid texture
Different pinna texture affects comfort + feedback issues
Need for Options
If need for ALD great, consider BTE
Several BTEs with FM options becoming available
Telephone/T-coil use
Telecoils available and work great in ˝ shell ITE hearing
aids (#312 batt)!
Direct Audio Input (DAI) VERY important for some persons with
severe-profound losses
Custom Hearing Aids
Custom Hearing Aids-Advantages
Response peak moves up to 2000 Hz
Boost of 3-6 dB between 3 and 5 kHz
Increased localization ability
Directional mic arrangements now available from several ITE companies
Less visible (?) probably for ITE ˝ shell, ITC, CIC
full-shell ITE >visible than BTE
Custom Hearing Aids-Disadvantages
Size limits Options
Loaners difficult to fit
Less flexible if hearing changes
Cannot "pre-select"
‘Tho programmable instruments may solve some of these problems
Difficult to do Comparative evaluations
Custom Hearing Aids-
Fitting Procedures
Hearing Aid Evaluation (HAE) with BTE
Order ITE based on Audiogram
Stock Hearing Aids
Programmable Hearing Aids
Custom HA s – Functional Gain vs 2cc Gain
For ITEs, functional gain > in higher frequencies
On average, functional gain 3 dB higher than 2cc coupler gain
Concha resonance
Higher resonance peak
No horn effect with HA-1
ITE gain
vs
BTE gain
CORFIG
from
Cox &
Risberg
(1986)
STOCK HEARING AIDS
Styles
Candidates
Advantages/Disadvantages
Available Models/Cost
Stock Aids
Stock Aids
Stock Aids
Stock Aids
"Stock" Hearing Aids
Songbird Medical’s "Songbird"
9 different circuit matrixes
Disposable hearing aid with a usable life of about 30-40 days
Reported to be available Aug. 2000
"Stock" Hearing Aids
Resound’s "Avance"
BTE type with very small-diameter, flexible tube; open-fit; for
mild-to-mod HF losses
Very small size for a BTE style
Has volume control trimmer
Relatively low price (~$500)
"Patients love ‘em!"
Lee W. and Paul D. in Callier Clinic
Resound’s "Avance" Hearing Aid