Linear hearing instrument fitting methods

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.


• 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.


•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.


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).


• 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.


• 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.


• 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.


• 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.


• In 1963, Sam Lybarger proposed a half-gain rule for linear hearing instrument gain.


•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.


•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.


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


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.


The importance of measuring threshold by frequency and determining supra-threshold loudness perception has existed for over fifty years.


Most research into appropriate fitting formulae was based upon linear hearing instruments where the increase in HI gain directly increased the HI output.


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.


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.


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.


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.


• 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.


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.

Linear hearing instrument fitting methods

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