1. DSL & COMPRESSION
 Audibility vs. Loudness
The patient/client hearing thresholds up to
the loudest sounds the patient/client can
tolerate is defined as the patient/client’s
dynamic range of hearing ability—for each
ear.

2. DSL & COMPRESSION
 Audibility vs. Loudness
The typical dynamic range for the mid-
frequencies is about one hundred decibels.
Soft—barely apparent sounds to loud but
tolerated sound falls within this typical
dynamic range area.

3. DSL & COMPRESSION
 Audibility vs. Loudness
With hearing loss, the soft sounds become
more difficult to hear; while loud sounds
continue to be tolerated at the same decibel
level.
Thus, hearing loss results in a reduced
dynamic range for the hearing impaired
patient/client.

4. DSL & COMPRESSION
 Audibility vs. Loudness
The challenge, as a hearing instrument
specialist, is to place the perception of soft,
average, and loud sounds into this residual
dynamic range area.

5. DSL & COMPRESSION
 Audibility vs. Loudness
Some types of hearing loss even reduce the
tolerance for loud sounds which even further
reduces the residual dynamic range area.
When this abnormal growth of loudness
(recruitment) is encountered, this creates a
greater and more critical necessity for
compression.

6. DSL & COMPRESSION
 Audibility & HI Gain
As previously discussed, various fitting
formulae described a single gain target for
linear hearing instrument fittings.
With compression hearing instruments, three
gain targets are normally described/defined.
They are: Soft, Average, and Loud.

7. DSL & COMPRESSION
 Audibility & HI Gain
A linear fitting formula would provide enough
gain for soft speech however, when a loud
sound was received it was too much gain.
Remember: Gain + Input = Output (this
formula is true for both compression and linear HIs)
Output is what the patient/client perceives as
comfortable sound.

8. DSL & COMPRESSION
 Audibility & HI Gain
Compression based fitting formulae provide
for different amounts of gain based upon the
instrument’s received input intensity levels.
These gain targets are based upon Soft,
Average, and Loud inputs.
Remember: Gain + Input = Output

9. DSL & COMPRESSION
 Compression & Loudness Growth
HI’s can not replace normal cochlear function.
However, the goal for fitting most HI’s is to
restore normal loudness growth for mild to
moderate hearing loss.
As we learned, the cochlea is a WDRC
amplifier. Thus, WDRC HI’s are appropriate
for mild to moderate hearing loss encountered
by the hearing instrument specialist.

10. DSL & COMPRESSION
 Compression & Loudness Growth
Let’s review Venema, figure 4-1, on page #67
and figure 4-2, on page #68.
Notice the impaired loudness growth curve
for the fifty-five decibel hearing loss and the
perceived loudness is the same as the normal
loudness growth curve.

11. DSL & COMPRESSION
 Compression & Loudness Growth
From figure 4-2 in Venema, please notice how
the loudness growth frequency contours for
normal hearing begin to flatten with increased
intensity levels.
At one hundred decibels the frequency
contour is almost flat.

12. DSL & COMPRESSION
 Compression & Loudness Growth
These psychoacoustic loudness growth curves
reveal that the cochlea is a non-linear organ.
They also reveal that the physical acoustic
properties of the outer and middle ear
perceptually influence the minimal audibility
frequency curves in loudness growth
measurement.

13. DSL & COMPRESSION
 Compression & Loudness Growth
Most digital compression instruments have
algorithms designed to increase gain for soft
level inputs and reduce and/or produce little
to no gain for high level inputs (similar to the
amplification function of outer hair cells in a
cochlea).

14. DSL & COMPRESSION
 Compression & Loudness Growth
Digital hearing instruments will often employ
two types of compression within their
algorithms. They are:
1. Compression output limiting
2. Wide Dynamic Range Compression

15. DSL & COMPRESSION
 DSL Fitting Method
The Desired Sensation Level fitting formula
was based upon creating audibility for speech
information in order to enhance language
development of hearing impaired children.

16. DSL & COMPRESSION
 DSL Fitting Method
The goal was to place as much of the
conversational speech input signal into the
residual dynamic range of the children’s
hearing ability using compression hearing
instruments.

17. DSL & COMPRESSION
 DSL Fitting Method
They chose to measure long term average
speech (LTASS) as received at the ear
(hearing) rather than had been previously
measured at the mouth (spoken word).
This resulted in more low and high frequency
outputs required for this fitting formula as
contrasted by others.

18. DSL & COMPRESSION
 DSL Fitting Method
This long term average speech spectrum
(LTASS) information is, of course, to be
presented to the auditory system without
distortion and comfortably—not too loud!

19. DSL & COMPRESSION
 DSL Fitting Method
What sets this fitting method apart from
others, is that its final interest is not
necessarily with the gain component of the
formula but, with the hearing instrument’s
output. This is why you will often see its
targets measured in dBSPL not dBgain.

20. DSL & COMPRESSION
 DSL Fitting Method
In other words, this formula looks at the
output delivered to the eardrum (both
acoustically and electroacoustically)--not just
the gain for audibility, or the in-situ gain.

21. DSL & COMPRESSION
 DSL Fitting Method
When attempting to transform dBSPL output
accurately into a fitting formula, there are
three transforms to consider. They are:
1. Real ear to dial difference (REDD)—audiometer
headset
2. Real ear to coupler difference (RECD)--ANSI 2cc
coupler
3. Microphone location effect (MLE)--Style of HI
microphone location/placement

22. DSL & COMPRESSION
 DSL Fitting Method
Let’ review Venema, pages #76 thru #79.
This will provide information regarding the
output transforms as well as the measurement
decibel levels for soft, average, and loud
speech signals

23. DSL & COMPRESSION
 DSL Fitting Method
To measure outputs of wide dynamic range
hearing instruments, Canada and the United
States have chosen three common levels of
standardized measurement input. They are:
1. Soft inputs presented at fifty decibels
2. Moderate inputs presented at seventy decibels
3. Loud inputs presented at eighty-five decibels

24. DSL & COMPRESSION
 DSL Fitting Method
The three input levels used to measure HI
output are measured by frequency using the
long term average speech spectrum (LTASS).

25. DSL & COMPRESSION
 DSL Fitting Method
These decibel levels by frequency result in the
DSL fitting formula. Output is recorded
from consistent input levels (50dB, 70dB,
85dB) using the LTASS frequencies.
GAIN + INPUT = OUTPUT