HIS 140 - HI Fitting Formula History and Overview

HI Fitting Formulae—History & Overview

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.


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 realization of MCL created the

observation that most satisfied hearing

aid users were desiring the gain of the

instruments to be established at

approximately half of the revealed

threshold of hearing loss.


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.


Most of the HI fitting formulae which

include supra-threshold information in

their calculations utilize loudness

perception information “normative”

averages—based upon the study of

loudness perception of non-pathologic

ears!


Finding a simple relationship between hearing loss

and gain has not been easy for four primary reasons.

They are:

1. The optimum gain is dependant 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.


As Dillon has stated hearing instrument

candidacy should not be solely based

upon an audiogram.

With electroacoustic selections--

Why should the HI fitting formulae?


As Hearing Instrument Specialists, you

will find that the pathologic nature of one

single frequency will determine the

perceptual loudness to your

patient/client.


The new paradigm in hearing instrument

fitting is not that digital amplification has

created increased adjustability, or greater

clarity of the speech signal; it has been

the ability to separate the amplified gain

from the amplified output.


As dispensing professionals, we now

have the ability to address loudness by

frequency discreetly thus, customizing

the electroacoustic information to a

pathologic ear.


NAL-NL1

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

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

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


Other fitting formulae

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

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


HI fitting formulae & cochlear pathology

The impaired cochlea may be compared

to a “bottleneck”. There may be an

abundance of information in the upper

pathways of auditory processing;

however, the impaired cochlea can pass

only a restricted amount of information.


HI fitting formulae & other physiologic conditions.

We have examined the individual resonances

of our ear canals and found significant

resonant differences among us.

It has been found that when considering the

frequency slope of the individual ear canal

resonances we will have created a greater

acceptance to amplification by the HI

patient/client.

(ref. Dillon pg 248)


Fitting formula for the Digital age

With the electroacoustic programming

flexibility, perhaps “non-traditional” test

protocols by the “treating” professional

may be in order?


Fitting formula for the digital age

Perhaps making programming

adjustments to digital hearing

instruments based upon test information,

will create a better understanding of how

to prescribe HIs to RESIDUAL

PATHOLOGIC HEARING ABILITY.


Please review the fitting formulae in

Dillon pgs 236-259—both linear and non-

linear.

You will notice in his discussion of non-

linear fitting formulae, that more testing

information is required to meet those

formulae requirements.


A more detailed discussion of fitting

formulae and compression will be in

the fall semester coursework.

HIS 140 - HI Fitting Formula History and Overview

Health & Medicine

Transcript of HIS 140 - HI Fitting Formula History and Overview