1. 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.

2. HI Fitting Formulae—History &
Overview
 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.

3. HI Fitting Formulae—History &
Overview
 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.

4. HI Fitting Formulae—History &
Overview
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.

5. HI Fitting Formulae—History &
Overview
The importance of measuring threshold
by frequency and determining supra-
threshold loudness perception has
existed for over fifty years.

6. HI Fitting Formulae—History &
Overview
Most research into appropriate fitting
formulae was based upon linear hearing
instruments where the increase in HI
gain directly increased the HI output.

7. HI Fitting Formulae—History &
Overview
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!

8. HI Fitting Formulae—History &
Overview
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.

9. HI Fitting Formulae—History &
Overview
As Dillon has stated hearing instrument
candidacy should not be solely based
upon an audiogram.
With electroacoustic selections--
Why should the HI fitting formulae?

10. HI Fitting Formulae—History &
Overview
As Hearing Instrument Specialists, you
will find that the pathologic nature of one
single frequency will determine the
perceptual loudness to your
patient/client.

11. HI Fitting Formulae—History &
Overview
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.

12. HI Fitting Formulae—History &
Overview
As dispensing professionals, we now
have the ability to address loudness by
frequency discreetly thus, customizing
the electroacoustic information to a
pathologic ear.

13. HI Fitting Formulae—History &
Overview
 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.

14. HI Fitting Formulae—History &
Overview
 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.

15. HI Fitting Formulae—History &
Overview
 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.

16. HI Fitting Formulae—History &
Overview
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)

17. HI Fitting Formulae—History &
Overview
 Fitting formula for the Digital age
With the electroacoustic programming
flexibility, perhaps “non-traditional” test
protocols by the “treating” professional
may be in order?

18. HI Fitting Formulae—History &
Overview
 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.

19. HI Fitting Formulae—History &
Overview
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.

20. HI Fitting Formulae—History &
Overview
A more detailed discussion of fitting
formulae and compression will be in
the fall semester coursework.