1. LINEAR HEARING INSTRUMENT
FITTING METHODS
• Hearing instrument fitting methods
have evolved over the past
seventy years as a result of a
mixture of hearing instrument
technology available at any
particular time; and the
experimental trials and errors of
scientific inquiry at that time
period.

2. LINEAR HEARING INSTRUMENT
FITTING METHODS
• History
Initial attempts to create fitting
formulae began in 1935. The first
premise was to “mirror” the
audiogram. In other words restore
one decibel of gain for every
decibel of measured hearing loss
by frequency.

3. LINEAR HEARING INSTRUMENT
FITTING METHODS
• Mirroring the audiogram seems
intuitively correct because we could
make all thresholds 0dbHL for the
patient/client’s hearing loss.
• If their tolerance for loudness grew
with the hearing loss, we could fit a
sixty decibel hearing loss with a sixty
decibel gain hearing instrument.

4. LINEAR HEARING INSTRUMENT
FITTING METHODS
Most of our patient/clients will not
have a conductive hearing loss;
they will have a sensorineural loss—
a damaged cochlea where
sensitivity to soft sounds is reduced
while the tolerance for loud sounds
remains unchanged or even
reduced, as well (recruitment).

5. LINEAR HEARING INSTRUMENT
FITTING METHODS
• In 1946 Ray Carhart developed a
comparative clinical procedure
by trying several different hearing
aids upon patients to see which
hearing instrument they liked the
best.
• His approach involved various
speech test measures in aided
and unaided conditions.

6. LINEAR HEARING INSTRUMENT
FITTING METHODS
• It soon became clear that this
approach was difficult to teach
and replicate from one clinic to
another.
• Clinicians began to feel the need
for a systematic prescription
approach to fitting hearing
instruments.

7. LINEAR HEARING INSTRUMENT
FITTING METHODS
• History
The next thought was to determine
the patient/client’s most comfortable
listening level (MCL) and create
enough amplified gain to make
sound audible and comfortable.
For some reason, the dynamic range
variations of speech energy were not
considered.

8. LINEAR HEARING INSTRUMENT
FITTING METHODS
• History
This investigation of MCL created
the observation that most satisfied
hearing aid users were desiring the
gain of their instruments to be
established at approximately half
of the revealed threshold of
hearing loss.

9. LINEAR HEARING INSTRUMENT
FITTING METHODS
• In 1963, Sam Lybarger proposed a
half-gain rule for linear hearing
instrument gain.

10. LINEAR HEARING INSTRUMENT
FITTING METHODS
• During the 1960’s, 1970’s, and
1980’s, subsequent linear fitting
formulae were developed. All
with a similar premise of the half-
gain rule. All concluded that
mirroring the audiogram was not
an appropriate hearing instrument
gain formulae for cochlear
pathologies.

11. LINEAR HEARING INSTRUMENT
FITTING METHODS
• Linear based fitting formulae
offered an alternative to the
comparative approach to hearing
instrument fitting.
• These fitting formulae created
“prescriptive” gain targets rather
than speech testing comparisons.

12. LINEAR HEARING INSTRUMENT
FITTING METHODS
Let’s review some of the more
popular fitting formulae as
described in Venema, chapter
three, pages #57--#59.

13. LINEAR HEARING INSTRUMENT
FITTING METHODS
It is interesting that most fitting
formulae are based upon the
scientific information gained from
research upon non-pathologic
hearing ability.
However, the majority of hearing
instruments are fit to pathologic
conditions—mostly cochlear
pathologies.

14. LINEAR HEARING INSTRUMENT
FITTING METHODS
The importance of measuring
threshold by frequency and
determining supra-threshold
loudness perception has existed for
over fifty years.

15. LINEAR HEARING INSTRUMENT
FITTING METHODS
Most research into appropriate
fitting formulae was based upon
linear hearing instruments where
the increase in HI gain directly
increased the HI output.

16. LINEAR HEARING INSTRUMENT
FITTING METHODS
The target for any particular
hearing loss and for any particular
threshold based fitting method
arises out of a compromise
between the reduced dynamic
range encountered with the
sensorineural hearing loss and
linear hearing instrument
technology.

17. LINEAR HEARING INSTRUMENT
FITTING METHODS
Finding a simple relationship between hearing
loss and target gain has not been easy for
four primary reasons. They are:
1. The optimum target gain is dependent upon
the input signal.
2. The optimum gain by frequency may depend
upon supra-threshold loudness by frequency.
3. The optimum gain by frequency may depend
on the perception formed by many years of
hearing loss.
4. The patient/client’s individual communication
goals i.e. comfort in noise, speech intelligibility
in noise, etc.

18. LINEAR HEARING INSTRUMENT
FITTING METHODS
Harvey Dillon has stated that
hearing instrument candidacy
should not be solely based upon a
pure tone threshold audiogram.
As Hearing Instrument Specialists,
we will learn to not only measure
hearing loss but, how to define
residual hearing ability.

19. LINEAR HEARING INSTRUMENT
FITTING METHODS
Hearing Instrument Specialists will
also find that the pathologic nature
of one single frequency of your
patient/client’s hearing loss can
determine the overall perceptual
loudness of their hearing
instrument.

20. LINEAR HEARING INSTRUMENT
FITTING METHODS
• Fitting Formulae
Most fitting formulae have been
using targets based upon the
thresholds of hearing loss and using
linear amplification.
Certainly some digital hearing
instruments may be programmed
based upon these same targets and
result in digitized analog/linear
outputs.

21. LINEAR HEARING INSTRUMENT
FITTING METHODS
As dispensing professionals, we now
have the ability to address loudness
by frequency discreetly thus,
customizing the electroacoustic
information to our patient/client’s
pathologic ear.