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Electroacoustic Limitations in Fitting
Children with Severe to Profound
Hearing Loss
Shane Moodie, M.Cl.Sc.
Clinical Supervisor in Audiology
School of Communication Sciences and
Disorders, University of Western Ontario
[email protected]
Children with severe to profound hearing loss
often present as our most difficult population
in which to provide appropriate amplification
Limiting Factors would include:
a)
Overall degree and configuration (slope) of the
hearing loss
b)
Changes in the ear acoustics related to
growth/aging
c)
Electroacoustic performance (limitations) of hearing
instruments
We are fairly knowledgeable about the affects
of degree of loss and ear acoustics on
amplification requirements. (DSL)
However, we are not always certain of the
abilities of hearing instruments to provide
audibility of amplified long-term average
speech spectrum (LTASS) for a given hearing
loss.
We should know that a definition of hearing in sound
pressure level (SPL) is dependent on both:
a) The minimal response levels provided by
the child in HL
b) The unique acoustic transform provided by the
child’s ear (and Ear Mould)
Frequency (Hz)
250
500
1000
2000
4000 6000
8000
-10
0
10
Hearing Threshold Level (dB)
20
30
40
50
60
70
80
90
100
110
120
If at 8 months
of age the
measured
thresholds in HL
are these.
And at 8 months
of age the ear
and ear mould
acoustics are
represented by
this RECD
If you know
this
And you
have
measured
this
You can
predict this
Dial Reading HL + RETSPL+ Acoustic Transform = Threshold in SPL
They will define
these thresholds
and auditory area
in SPL
Frequency (Hz)
250
500
1000
2000
4000 6000
8000
-10
0
10
Hearing Threshold Level (dB)
20
30
You can
predict this
for average
And you
assume this
is average
If you
know this
40
50
Dial Reading HL + RETSPL+ Acoustic Transform = Threshold in SPL
60
70
80
90
100
110
120
They will also
provide this
predicted HL
audiogram
And generate
these targets
for coupler gain
for speech
Target coupler response =REAGT – mic - RECD
Ear acoustics are an important developmental factor
They will change as the child grows and can have
important implications on fitting.
And independent of a change in hearing levels, will
require that the hearing instrument gain and output be
modified.
That is, as the ear grows we have to increase the
output into the ear!
Frequency (Hz)
250
500
1000
2000
4000 6000
8000
-10
0
10
Hearing Threshold Level (dB)
20
30
40
50
60
70
80
90
100
110
120
The predicted
HL audiogram
An average
adult RECD ?
Adult
20 dB
8 months
2 cc coupler
targets for
amplified LTASS
Consequently, with changes in ear acoustics
The questions is not … can we meet the hearing needs
of the 6 month old ?
But rather …
Can we meet the hearing needs of the child as they
grow?
In today’s world, many children with Permanent
Childhood Hearing Impairment (PCHI) will be fitted by 6
months of age.
They will wear their “first” hearing instruments for up to
three years.
Therefore, we need to account for potential growth in
the ear canal and ear mould in the hearing instrument
fit.
With this in mind, if we want to determine the fitting range of a
hearing instrument…
the question is not …
If I have a three year old with given ear acoustics ….
what Hearing Instrument works for this hearing loss?
BUT rather …. If I have a three year old with given ear acoustics…
What loss works for this hearing instrument?
Case Example outline:
1) Assume an average 3 year old RECD
2) Set hearing instrument to maximum at all frequencies
3) Set hearing instrument to maximum in mid to high
frequency area and reduce lows as much as possible
(Slope)
4) Determine what loss can be fitted (audibility of LTASS
at all relevant audiometric frequencies)
4 Hearing Instruments:
1) Digital Power – WDRC
2) Digital Power – Output compression
3) Analogue Programmable Power – Output
Compression
4) Mini Digital Power - WDRC
3 year old
average RECD
Bagatto et al. 2002
4 HI responses
Frequency (Hz)
250
500
1000
2000
4000 6000
8000
-10
0
10
Hearing Threshold Level (dB)
20
HL Corrected
audiogram (flat) for
optimal match to
targets
30
40
50
60
70
80
90
100
110
120
Limits for amplification
Receiver
Resonance
Frequency (Hz)
250
500
1000
2000
4000 6000
8000
-10
0
10
Hearing Threshold Level (dB)
20
30
40
50
60
70
80
90
100
110
120
HL Corrected
audiogram
(sloping) for
optimal match to
targets
Frequency (Hz)
250
500
1000
2000
4000 6000
8000
-10
0
10
Hearing Threshold Level (dB)
20
30
40
50
60
70
80
90
100
110
120
HL fitting
range
RECD = 0
Speech
Input
140
130
120
110
Ear Canal SPL (dB)
100
90
80
70
60
50
40
30
20
10
0
100
1000
Frequency (Hz)
10000
Adult
8 months
What the HI
can provide for
speech like
input versus
what we want
Best Fit?
HI gain
for 50
dB SPL
swept
pure
tone
53
50
53
59
57 47 Max Gain for Speech
Gain for Speech is not the same as gain for Pure tone!
Whether gain or output is measured, if we are
going to predict audibility for speech, it has to be
measured with speech!
In the case of severe to profound hearing loss…
Unless we can maintain the acoustical advantage of an
infant’s ear….
It does not appear that we can optimize speech
perception in the growing child beyond 2000 Hz.
15
Undamped
Response,
36 mm
tubing
10
5
0
-5
-10
-15
100
1000 1900 2800 3700 4600 5500 6400 7300
CFA#2
CFA#4
REVERSE HORN
Ear mould modifications will only provide minimal benefit.
15
Damped
Response,
36 mm
tubing
10
5
0
`
-5
-10
-15
100 1000 1900 2800 3700 4600 5500 6400 7300
CFA#2 (damped)
CFA#4
REVERSE HORN
Summary
1) We are limited in our ability to provide high
frequency amplification for children with severe to
profound hearing loss, primarily due to the receiver
resonance in the hearing instrument.
Summary
2) High power for hearing instruments appears to mean
higher output for low frequency sounds.
Summary
3) Our ability to provide audibility of amplified high
frequency speech sounds will decrease with changes
in the ear and ear mould acoustics.
Summary
4) Given compression, if we are to predict audibility for
speech, we need to use speech input.
Summary
5) We can not expect to enhance the high frequency
response of a hearing instrument significantly
through the use of an ear mould.
Summary
6)
This has important implications for counseling and
planning treatment?
- predicting the HL audiogram
- understanding the limitations of hearing instruments
- what you can provide to the infant, is not what you can
provide to the 3 year old.
- with severe to profound hearing loss, responsiveness will
change with time.