Hearing Instrument Fitting Formulae History and Overview

HI Fitting Formulae—History & OverviewHI Fitting Formulae—History & Overview

HistoryHistory

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


HistoryHistory

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


HistoryHistory

This realization of MCL created the This realization of MCL created the observation that most satisfied hearing observation that most satisfied hearing aid users were desiring the gain of the aid users were desiring the gain of the instruments to be established at instruments to be established at approximately half of the revealed approximately half of the revealed threshold of hearing loss.threshold of hearing loss.


It is interesting that most fitting formulae It is interesting that most fitting formulae are based upon the scientific information are based upon the scientific information gained from research upon non-gained from research upon non-pathologic hearing ability. pathologic hearing ability.

However, the majority of hearing However, the majority of hearing instruments are fit to pathologic instruments are fit to pathologic conditions—mostly cochlear pathologies.conditions—mostly cochlear pathologies.


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


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


Most of the HI fitting formulae which Most of the HI fitting formulae which include supra-threshold information in include supra-threshold information in their calculations utilize loudness their calculations utilize loudness perception information “normative” perception information “normative” averages—based upon the study of averages—based upon the study of loudness perception of non-pathologic loudness perception of non-pathologic ears!ears!


Finding a simple relationship between hearing loss Finding a simple relationship between hearing loss and gain has not been easy for four primary reasons. and gain has not been easy for four primary reasons. They are: They are:

1.1. The optimum gain is dependant upon the input The optimum gain is dependant upon the input signal.signal.

2.2. The optimum gain by frequency may depend upon The optimum gain by frequency may depend upon supra-threshold loudness by frequency.supra-threshold loudness by frequency.

3.3. The optimum gain by frequency may depend on the The optimum gain by frequency may depend on the perception formed by many years of hearing loss.perception formed by many years of hearing loss.

4.4. The patient/client’s individual communication goals The patient/client’s individual communication goals i.e. comfort in noise, speech intelligibility in noise, etc.i.e. comfort in noise, speech intelligibility in noise, etc.


As Dillon has stated hearing instrument As Dillon has stated hearing instrument candidacy should not be solely based candidacy should not be solely based upon an audiogram. upon an audiogram.

With electroacoustic selections--With electroacoustic selections--

Why should the HI fitting formulae?Why should the HI fitting formulae?


As Hearing Instrument Specialists, you As Hearing Instrument Specialists, you will find that the pathologic nature of will find that the pathologic nature of one one single frequencysingle frequency will determine the will determine the perceptual loudness to your perceptual loudness to your patient/client.patient/client.


The new paradigm in hearing instrument The new paradigm in hearing instrument fitting is not that digital amplification has fitting is not that digital amplification has created increased adjustability, or greater created increased adjustability, or greater clarity of the speech signal; it has been clarity of the speech signal; it has been the ability to separate the amplified gain the ability to separate the amplified gain from the amplified output.from the amplified output.


As dispensing professionals, we now As dispensing professionals, we now have the ability to address loudness by have the ability to address loudness by frequency frequency discreetlydiscreetly thus, customizing thus, customizing the electroacoustic information to a the electroacoustic information to a pathologic ear.pathologic ear.


NAL-NL1NAL-NL1

This is a popular fitting formulae This is a popular fitting formulae developed by Harvey Dillon et. al. developed by Harvey Dillon et. al.

It uses the “little dots” of the articulation It uses the “little dots” of the articulation index to predict speech intelligibility. index to predict speech intelligibility.

Its recommended frequency pattern/slope Its recommended frequency pattern/slope is based upon—you guessed it—non is based upon—you guessed it—non pathologic hearing ability.pathologic hearing ability.


Other fitting formulaeOther fitting formulae

Most other fitting formulae have been Most other fitting formulae have been targeting linear amplification—not digital targeting linear amplification—not digital compression.compression.

Certainly some digital hearing Certainly some digital hearing instruments may be programmed linearly instruments may be programmed linearly resulting is digitized analog/linear resulting is digitized analog/linear outputs.outputs.


HI fitting formulae & cochlear pathologyHI fitting formulae & cochlear pathology

The impaired cochlea may be compared The impaired cochlea may be compared to a “bottleneck”. There may be an to a “bottleneck”. There may be an abundance of information in the upper abundance of information in the upper pathways of auditory processing; pathways of auditory processing; however, the impaired cochlea can pass however, the impaired cochlea can pass only a restricted amount of information.only a restricted amount of information.


HI fitting formulae & other physiologic conditions.HI fitting formulae & other physiologic conditions.

We have examined the individual resonances We have examined the individual resonances of our ear canals and found significant of our ear canals and found significant resonant differences among us. resonant differences among us.

It has been found that when considering the It has been found that when considering the frequency slope of the frequency slope of the individual ear canal individual ear canal resonancesresonances we will have created a greater we will have created a greater acceptance to amplification by the HI acceptance to amplification by the HI patient/client.patient/client.

(ref. Dillon pg 248)(ref. Dillon pg 248)


Fitting formula for the Digital ageFitting formula for the Digital age

With the electroacoustic programming With the electroacoustic programming flexibility, perhaps “non-traditional” test flexibility, perhaps “non-traditional” test protocols by the “treating” professional protocols by the “treating” professional may be in order?may be in order?


Fitting formula for the digital ageFitting formula for the digital age

Perhaps making programming Perhaps making programming adjustments to digital hearing adjustments to digital hearing instruments based upon test information, instruments based upon test information, will create a better understanding of how will create a better understanding of how to prescribe HIs to RESIDUAL to prescribe HIs to RESIDUAL PATHOLOGICPATHOLOGIC HEARING ABILITY. HEARING ABILITY.


Please review the fitting formulae in Please review the fitting formulae in Dillon pgs 236-259—both linear and non-Dillon pgs 236-259—both linear and non-linear. linear.

You will notice in his discussion of non-You will notice in his discussion of non-linear fitting formulae, that more testing linear fitting formulae, that more testing information is required to meet those information is required to meet those formulae requirements.formulae requirements.


A more detailed discussion of fitting A more detailed discussion of fitting formulae and compression will be in formulae and compression will be in the fall semester coursework.the fall semester coursework.

Education

Hearing Instrument Fitting Formulae History and Overview