1. NAL & COMPRESSION
The evolution of NAL fitting formulae
have always been based upon the
theory of creating enough gain for
audibility.
It is a formula which evolved from it’s
early days as a linear fitting formula with
one gain target to it’s current
compression formula with three gain
targets.
2. NAL & COMPRESSION
The research used to evolve the linear
formula into a compression formula
revealed that, for gently sloping hearing
loss audiograms ranging from mild to
severe in degree, similar gain targets
were required for sixty-five decibel
inputs.
3. NAL & COMPRESSION
Formost types of hearing loss, the mid
frequencies of speech are aided so that
they will sound similar in loudness to the
lower and higher adjacent speech
frequencies bands. This suggests that
maximal speech intelligibility should be
received.
4. NAL & COMPRESSION
Although normal hearing people hear
vowels louder than high frequency
consonants, NAL proposes that speech
intelligibility is maximized if all
frequencies of speech are equalized
rather than normalized.
5. NAL & COMPRESSION
In other words, NAL proposes to
preserve the loudness relationships
(soft, average, and loud inputs) of the
speech frequency spectrum; compared
to DSL which proposes to preserve
unaided loudness relationships of all
soft, average, and loud outputs.
In effect “equalize” instead of
normalize.
6. NAL & COMPRESSION
With this “equalization” approach to
speech audibility, it is theorized that
maximum speech intelligibility should
occur. Simply preserving loudness
relationships of all unaided speech
frequency outputs has not been shown
to improve speech intelligibility.
7. NAL & COMPRESSION
Infact, non-speech frequency
information is not even used to calculate
an NAL formula. This is why you will
not find any NAL frequency gain targets
above 4K.
8. NAL & COMPRESSION
Speech intelligibility for NAL is based
upon the importance of phonemic
information as calculated from the
Articulation Index. However, formula
exceptions are created based upon
other factors such as:
1. Severe to profound hearing loss
2. Ear canal volumes of children vs.
adults
9. NAL & COMPRESSION
There are two primary features which
characterize the NAL compression fitting
formula. They are:
1. Equalizing rather than normalizing the
loudness of adjacent speech frequencies.
2. Providing less gain for frequencies where
the hearing loss is worst and more gain
where hearing is best.
10. NAL & COMPRESSION
Audibility vs. Effective Audibility
If actual hearing thresholds are known,
audibility can be measured in terms of
sensation level.
Effective audibility refers to how much
speech information can be extracted from
speech sounds, once they become audible.
11. NAL & COMPRESSION
Audibility vs. Effective audibility
It is theorized, that as hearing loss thresholds
increase, the hearing impaired tend to have
more effective audibility with less audibility.
For example: for those with a severe or greater
hearing loss, a small sensation level for
audibility might give speech understanding;
while a greater sensation level will not provide
much more effective audibility.
12. NAL & COMPRESSION
Audibility vs. Effective Audibility
When attempting to calculate frequency
specific NAL gain targets for speech, a
severe hearing loss will not have a gain
target available—the target gain
calculation just disappears!
13. NAL & COMPRESSION
Gain + Input = Output
Let’s compare the two compression fitting
formula (NAL and DSL).
The gain for audibility calculations will be
established between these two formulae
using a sixty decibel flat configuration
hearing loss.
14. NAL & COMPRESSION
Gain + Input = Output
We will convert the gain for audibility to
output for comfort curves to better
compare/contrast NAL and DSL fitting
formulae.
Let’s review Venema, pages #84 thru #86
and compare/contrast these two formulae
using four different audiogram
configurations.
15. NAL & COMPRESSION
NAL vs. DSL
You will notice that the greatest difference
between these two compression based
fitting methods concerns the low
frequency gain/output.
16. NAL & COMPRESSION
From a physiological perspective, it is
not presently possible to model what is
really going on in the aided cochlea with
any one particular fitting method.
17. NAL & COMPRESSION
WDRC hearing instruments are meant
to imitate the role of the outer hair cells
after being driven through the middle
ear system.
18. NAL & COMPRESSION
The application of electroacoustic
information to various hearing
pathologies will always remain a
challenge to standardized fitting
formulae.
19. NAL & COMPRESSION
Thefitting of hearing instruments
continues to be both an art and science.
