COMPRESSION:
Historical Development
& Use Today
Take the Continuing Education
quiz on page 59.
By Ted Venema, PhD
“Many professionals struggle with
or are intimidated by the concept of
compression. This excellent article
provides simple but very useful
explanations. It is also an excellent
resource for anyone who is training
new hearing aid specialists.”
— Sandy Hubbard, BS, ACA, BC-HIS
IHS Editorial Advisory Committee Member
T
he word “sensori-neural” has two parts. “Sensory” refers
to mild-moderate sensori-neural hearing loss (SNHL)
caused by mostly outer hair cell (OHC) damage. “Neural”
refers more to SNHL caused by inner hair cell (IHC) damage.
In most cases, OHC damage usually occurs before IHC
damage. This is why “neural” SNHL is also a more severe
degree of SNHL. Here, we specifically discuss the technical
aspects of compression used in hearing aids, in order to
accommodate both types of SNHL.
Compression per se is a gain issue. Inasmuch as input +
gain = output, compression also then affects the output.
Always remember that providing gain and output, however,
is only half the challenge of hearing aid fittings. The other
half is a matter of increasing the signal-to-noise ratio for the
client, making speech (the signal) louder and hence more
distinct from the background noise. Today’s approach to
this endeavor involves the usage of digital noise reduction
and directional microphones, but these topics will not be
examined in this article.
Here, we will examine compression types for mild-tomoderate “sensory” hearing loss versus compression
for severe “neural” hearing loss. We will also discuss the
“dynamic” aspects of compression, which consist of
attack and release times. Readers should not think that
compression began with today’s digital hearing aids.
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Compression began—and flourished—way back in the
analog era of hearing aids several decades ago. The
decade of the 1990s at the tail end of the reign of analog
hearing aids, was actually the “golden age” of compression.
Hearing aids then used one specific type of compression or
another. Changing to another type of compression meant
choosing an entirely different hearing aid! Wide dynamic
range compression (WDRC) was beginning to emerge,
and seasoned clinicians had to learn it as a new type of
compression. Most types of compression developed in the
analog era have simply been adapted and implemented in
the software of today’s digital hearing aids.
Compression Amplification
Input/Output Functions Input/output (I/O) functions are
the “language” of compression, and so it may be a good
idea to make them “your friends.” On an I/O function, the
horizontal or X axis show input sound pressure level (SPLs)
and the vertical or Y axis show output SPLs. The diagonal
lines show the difference between corresponding input
and output levels. In other words, they show the gain of a
hearing aid that takes place for different input SPLs.
Before compression emerged on to the scene, hearing aids
only provided what was called “linear gain.” This kind of
gain is represented by a 45° diagonal line (Figure 1). The
point at which that line suddenly takes a bend is called the
Linear Amplification
Figure 1. In this example, the gain is linear (shown by the 45° line); eg; for 20-dB
inputs, the output is 80 dB SPL; for 60-dB SPL inputs, the output is 120 dB SPL. For
100-dB SPL inputs, the output would theoretically be 160 dB SPL. Due to limiting
by means of peak clipping, however, the output is maintained at a maximum here
of 120 dB SPL.
Figure 2. The output shown here is linear up until the knee-point. Beyond (to the
right of) the knee-point, the output still increases with input increases, but this
increase is no longer at a corresponding 1:1 rate. For example, a 20-dB input
increase from 60 dB SPL to 80 dB SPL results in only about a 5-dB output increase,
making the compression ratio 20:5 or 4:1. Here, the MPO is limited by compression.
“knee-point,” and it shows the input where “compression”
begins. The gain shown in Figure 1 is linear to the left of (or
below) the knee-point because for any increase of input
SPL, there is a correspondingly equal increase of output
SPL. This is a 1:1 input/output ratio. In other words, for
every one dB input increase there is a corresponding 1 dB
output increase. For example, if the hearing aid has a gain
of 60 dB, then a 10-dB SPL input will result in a 70-dB SPL
output, a 20-dB SPL input results in an 80-dB SPL output,
and so on, up until an input level of 60 dB SPL. Past this,
something called “peak clipping” was utilized to ensure
that the maximum power output (MPO) never exceeds
a certain amount; for example, 120 dB SPL. The MPO
could be raised or lowered to accommodate the client’s
loudness discomfort levels.
The main problem with limiting the MPO with peak
clipping was that when the output sound exceeded the set
MPO, the hearing aid became “saturated,” thus distorting
the sound. With peak clipping, the receiver diaphragm (like
the cone of a speaker) is literally restricted in its back-andforth movements by the walls of the receiver. When this
happens, sine waves of sounds are literally “clipped” or
turned into square waves, which themselves are complex
sounds. In short, simple sinusoids containing single
Continued on page 50
49
frequencies are converted into complex sounds containing
more than one frequency. This is how harmonic distortion
(as measured by ANSI procedures) is produced. Linear
hearing aids were the “state of the art” in hearing aid
technology well into the 1980s and they still lingered
about in the early 1990s.
Now compare and contrast compression shown in Figure 2
to the linear gain shown in Figure 1. For the sake of clarity
in explanation, both are shown to have a knee-point at
an input level of 60 dB SPL, and both provide 60 dB of
linear gain up until this input level. The differences appear
past or to the right of the knee-point. In Figure 2, once
the input level exceeds 60 dB SPL, compression serves to
limit the MPO. Note how the MPO does not take a straight
horizontal direction to the right; instead, it rises but with a
shallower slope. The MPO is limited, but in a slight “giving”
kind of way. Like peak clipping did for linear hearing aids,
compression also determines the MPO of the hearing aid.
As in Figure 1, the MPO is shown by the general “height”
of any line that is to the right of the knee-point.
Compression “ratios” are the amount of compression
provided by the hearing aid once compression begins at
the knee-point. It can be visualized on an I/O function
by the slant of the line after (or to the right of) the kneepoint. A 10:1 compression ratio means that for every 10-dB
increase of input SPL, there is only a 1-dB corresponding
increase to the output SPL. A 2:1 compression ratio
means that for every 10-dB increase of input SPL, there
is a corresponding 5-dB increase to the output SPL of
the hearing aid. Higher compression ratios indicate more
compression; lower compression ratios indicate less
compression. In general, one can think of the knee-point
as the “when” of compression and the ratio as the “how
much” of compression.
Always remember too that I/O functions display only
inputs and outputs. To find the gain for some specific
input, one must always look at the corresponding output
and then subtract the input from that output. Readers
are also advised that the length of any gain function
(showing either linear gain or compression) has precious
little (read “nothing”) to do with the amount of gain. It
is only the position of the gain functions themselves—
right or left—along the horizontal axis that shows an
increase or a decrease in gain.
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Figure 2 shows that a right-ward shift in gain functions
actually shows a decrease in gain, while a left-ward shift
would show an increase in gain. See the vertical line going
down from the knee-point to the input of 60 dB SPL. Note
also the horizontal line going left from the knee-point to
the output axis. This shows that an input of 60 gives an
output of 120. Now look at the dotted parallel 450 line
from the input of 20 until it meets with the darker line
showing compression. On this line, it becomes clear that
an input of 60 gives an output of only 100. The gain has
now decreased.
Output Limiting Compression We now come to a
major fork in the road concerning compression—namely,
output limiting compression (OLC) and WDRC. These are
essentially two different compression schemes, and they
refer to separate ranges of compression threshold kneepoints and compression ratios. Since OLC emerged first,
it will be described first, followed by WDRC.
Output Limiting Compression
and
MPO Adjustment
Figure 3. An I/O function (left) and MPO adjustment (right) are shown for OLC. Note
the relatively high knee-point and high compression ratio. Maximum (linear) gain
is provided for soft and average input levels. Past the knee-point, however, a high
compression ratio dramatically limits the MPO. Note that lowering the knee-point
also lowers the MPO.
The salient features of OLC are shown in the I/O function
in Figure 3 (left). OLC has relatively high compression
knee-points (e.g., 60 dB SPL or more) and high
compression ratios (greater than 3 or 4:1). A high
knee-point means that the hearing aid begins to use
compression only at high input SPLs. For soft and
moderate inputs, below or to the left of the knee-point,
the OLC hearing aid provides linear gain.
Figure 3 shows a 10:1 compression ratio, resulting in an
almost completely horizontal line to the right of the kneepoint. Compare this to the linear gain shown in Figure 1. The
two figures look very similar. OLC can be thus considered to
be a “close cousin” to linear gain. With linear gain, the MPO
is limited by means of “hard peak clipping.” With OLC, the
MPO is limited with a bit of “give,” in other words, by means
of a high compression ratio. The numerical values on the
figures here are kept the same, mainly for illustration and
comparison. Linear hearing aids actually came in all kinds
of strengths, with greater and lesser amounts of gain. The
thing they all did, however, was to limit the MPO by means
of peak clipping. OLC hearing aids simply utilized a high
degree of compression to accomplish the same thing. This
is how the first compression (OLC) emerged in the 1980s;
it was a method of limiting the MPO without the distortion
caused by peak clipping!
OLC is especially useful for clients with neural (severe)
SNHL who would benefit from high-power hearing aids.
These clients have a very narrow dynamic range. They
tend to prefer a strong, linear gain over a wide range of
input SPLs, at least until the output SPL becomes close
to their loudness tolerance or uncomfortable loudness
levels. High-power OLC hearing aids gave lots of gain for
soft sounds and the same “lots of gain” for average input
sounds, making average conversational speech quite
audible. Figure 3 (left) shows that OLC provides a strong
degree of compression over a narrow range of intense
inputs. In other words, it “waits” for a fairly high-input SPL
to go into compression, but once it goes into compression,
it really goes into compression. In this way, OLC could be
said to focus on the “ceiling” of a client’s dynamic range.
Figure 3 (right panel) shows a similar MPO adjustment
to that of linear hearing aids. Again, the purpose is to
best address the client’s loudness tolerance levels. Some
clinicians opt to set the MPO (measured in dB SPL) to be
about 15 dB higher than the client’s reported loudness
tolerance levels (measured in dB HL). The rationale here
is that the difference in dB HL versus dB SPL over various
intensity levels and across the speech frequencies is close
to an average of about 15 dB (with dB SPL showing the
greater dB values).
hearing loss. Note that unlike severe SNHL, the dynamic
range of conductive hearing loss is not normally
diminished by much (conductive hearing loss is much like
a plug in the ear). Since the thresholds as well as loudness
tolerance levels for this clinical population would both be
elevated from normal, they too would also benefit from
linear gain—along with a high MPO. WDRC per se would
not be the best fit for conductive hearing loss.
Wide Dynamic Range Compression (WDRC) WDRC
hearing aids became extremely popular during the 1990s.
As such, it was a newcomer to the world of compression.
Many clinicians who were used to OLC and its adjustments
were initially quite confused by WDRC and especially how
it was adjusted.
Wide Dynamic Range Compression
and
Gain Adjustment



Figure 4. An I/O function (left) and MPO adjustment (right) are shown for WDRC.
Note the relatively low knee-point and low compression ratio, and also, that the
linear gain here is only 40 dB. Maximum (linear) gain is thus provided only for soft
input levels. Past the knee-point, a weak (2:1) ratio of compression gradually limits
the MPO. Note that lowering the knee-point increases the gain for soft inputs.
A typical I/O function for WDRC is shown in Figure 4 (left).
In contrast to OLC, WDRC is associated with low threshold
knee-points (below 60 dB SPL) and low compression ratios
(less than 4:1). In fact, WDRC most commonly utilizes
a 2:1 compression ratio. A look at Figure 4 (left) shows
that due to its low knee-point, the WDRC hearing aid is
in compression over a relatively wide range of inputs. As
such, it is almost always in compression.
Look at the slope of WDRC as shown in Figure 4 (left panel)
It should be added here that another hearing loss that
might be mentioned as a candidate for OLC is conductive
Continued on page 52
51
There is no real rule for adjusting the TK control; the main
reason it was adjustable in the first place was because
a TK setting for maximum gain in quiet environments
could result in the client being able to hear the internal
amplifier and microphone noise of the hearing aid itself.
The audible hiss can be annoying, especially for the client
who has excellent low-frequency hearing. In today’s digital
hearing aids, “expansion” (also to be discussed later) is
commonly used along with WDRC, in order to reduce the
audibility of the “hissing” sounds in quiet.
WDRC is normally used for the client who has OHC
damage and consequently, “sensory” SNHL, which
presents with a mild-to-moderate SNHL along with fair
speech discrimination. The OHCs amplify soft sounds
(approximately less than 40–50 dB SPL) so that the IHCs
can sense them. WDRC was intellectually construed as
an attempt to electro-acoustically imitate the function
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The goal of amplification for this population is to restore
normal loudness growth. To accomplish this goal, we need
to amplify soft sounds by a lot and loud sounds by little or
nothing at all. The low knee-point and low compression
ratio serve to reduce a normally large dynamic range into
the smaller one associated with mild-to-moderate SNHL.
For example, a low compression ratio of 2:1 will compress
a dynamic range of 100 dB into one of 50 dB. This is
literally why this type of compression was called “wide
dynamic range compression.”
Output Limiting vs WDRC:
Displayed as Frequency Responses
In analog hearing aids with WDRC, a “threshold kneepoint (TK) control adjusted the amount of linear gain
given for very soft input sounds (Figure 4, right panel).
The TK adjustment shifts the 450 linear gain function to
the right or left. Note how this is so very different from the
adjustment of the MPO with OLC (Figure 3, right panel).
The left-most 450 linear gain line shows the greatest gain
for soft inputs. The right-most 450 linear gain line shows
the least amount of gain for soft inputs. In summary, as
the knee-point with the TK control is lowered, the gain
for low-intensity input sounds is increased. As the kneepoint is raised, the gain for low-intensity input sounds
is decreased. On today’s digital hearing aids, the TK
adjustment works the same way, but it may not always be
called a “TK” control. It may be seen on digital software
simply as the left or right adjustment of the left-most,
knee-point on a rather complex-looking I/O function (to be
described in the next section).
of the OHCs of the cochlea. Here is some food for clinical
thought: It is no coincidence that oto-acoustic emissions
and the knowledge of the OHCs, as well as the KAmp and
WDRC became clinically popular at around the same
time, namely, the late 1980s and early 1990s. WDRC was
commonly associated with hearing aids using the KAmp
circuit.
and compare that to the slope of OLC, as shown in the left
panel of Figure 3. It is evident that WDRC provides a weak
degree of compression over a wide range of inputs. Unlike
linear or OLC, WDRC gradually reduces the output (and
hence the gain) for a wide range of moderate to intense
input SPLs. Only for very soft inputs is linear (maximum)
gain provided; all other inputs are given compression (less
gain). In contrast to OLC, WDRC can be seen as having a
focus on the “floor” of hearing sensitivity.
Figure 5. OLC (left) provides the same linear gain for both soft (40) and average
(60) input levels (the lines are shown slightly apart only to make the results for 40and 60-dB inputs both visible). Once the input level exceeds the knee-point, there
is a dramatic reduction in gain. The knee-point for WDRC (right) is much lower,
and the compression ratio is also comparatively lower. WDRC thus provides very
different amounts of gain for the different input intensities of 40, 60, and 80 dB SPL.
Both WDRC and OLC can also be described in terms of
their effects upon the frequency response of a hearing
aid (Figure 5). The left panel of Figure 5 shows that OLC
provides its maximum gain for both soft and average
input levels and then suddenly reduces its gain once the
input level becomes more intense than its relatively high
knee-point. The right panel of Figure 5 shows that WDRC
gradually reduces its gain over a wide range of increasing
input sound levels above its low knee-point.
Summary
A Clinical “Spectrum” of Compression
A Multi-Kneepoint

 
Input/Output Function


Figure 6. For linear, OLC, and WDRC, two sets of horizontal lines and three
arrows are shown. The left horizontal lines represent inputs, the arrows represent
gain, and the right horizontal lines represent output. The dotted horizontal line
represents the loudness tolerance level for some particular client. Note how
for linear gain and OLC, the gain is the same for soft and average input levels.
Linear hearing aids used peak clipping to limit the MPO, resulting in distortion
of sound quality. OLC uses compression to limit the MPO, shown by the lines
squeezed together. Note how for WDRC, both the input and output lines are evenly
spread apart. This is because the gain is gradually reduced as the input intensity
increases. A large dynamic range is more “evenly” shrunk into a smaller one.
Figure 6 summarizes where we have come so far. In the
beginning, the signal processing in hearing aids involved
simple linear gain along with peak clipping. Next came
OLC, followed in time by WDRC. The left-most panel shows
linear gain, where equal amounts of gain are applied to
all input. Peak clipping is employed when the output
would be excessive, although this caused distortion. The
middle panel shows OLC, a very similar type of signal
processing to linear gain except that compression instead
of using peak clipping is used to limit the MPO. The right
panel shows WDRC, where progressively less and less
gain is applied to increasingly more intense inputs. In this
manner, a wide dynamic range is neatly shrunk into a
smaller one.
Compression in Digital Hearing Aids Digital hearing aids
simply combine all sorts of compression types that were
found separately on yesterday’s analog hearing aids. In
any one of its channels, the fitting software for a digital
hearing aid may very well show I/O functions that have
two or more knee-points (Figure 7). As with all previous I/O
functions, the greatest amount of gain is seen below—or
to the left of—the left-most knee-point. Here, the gain is
linear. In any channel, the knee-points can typically be
Figure 7. The software for fitting many digital hearing aids often shows I/O
functions that have more than two knee-points; each can be adjusted. Below the
left-most knee-point, either linear gain or expansion can be selected. WDRC is
found to the right of the left-most knee-point. Note how linear gain reappears to
the right of the middle knee-point.
moved either horizontally or vertically, thus completely
affecting the compression characteristics.
In all truth however, most clinicians do not adjust
compression in this manner. Instead, the frequency
response (not I/O functions) is usually the main focus.
This is largely because frequency response is the most
readily understood display by most clinicians. In most
fitting software, adjustments in hearing aid gain/output
across the channels actually changes the underlying
compression; that is, the I/O function for each channel.
Back to Figure 7; linear gain and “expansion” (to be
described in the next section) appear below the first or
left-most knee-point, which is shown at soft input. WDRC
appears next, between the first and second knee-points
(for moderate inputs). From 65 to about 80 dB SPL,
however, the gain becomes once again linear! Past 80-dB
SPL inputs, the compression ratio is then dramatically
increased, in order to limit the MPO.
Let’s look more closely at that “second” use of linear
gain here. This has been utilized by various hearing aid
manufacturers over the past decade. It is a means whereby
to provide “extra” gain for average to slightly greater
Continued on page 54
53
than average inputs, such as speech in a somewhat
noisy environment. The reasoning here is at these levels,
speech and noise are commonly mixed together. Most
people generally prefer increased gain for these levels, so
as to hear speech better in these more difficult listening
situations. It is one solution to address the complaint
given by many clients who wear WDRC hearing aids, such
as, “I can hear people at other tables better than the
person sitting right across from me!”
Expansion Expansion is the opposite of compression. On
the basis of everything discussed so far—especially when
considering compression, and the reduced dynamic range
that results from SNHL—one might wonder when this
would ever be of use. Basically, expansion is a technique
whereby to reduce internal microphone and amplifier
noise that sometimes becomes audible to the listener
in quiet. This is especially noticeable by those who have
good low-frequency hearing.
Expansion
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Figure 8. Expansion provides greater than linear gain for very soft inputs below the
left-most knee-point of compression. The vertical output axis is extended below
the horizontal input axis to show how, with expansion, the gain increases as the
inputs increase from 0 dB SPL up to the compression knee-point. For increasing
inputs beyond the knee-point, the gain once again decreases, because of the use
of WDRC.
Here’s how and why it works: Figure 8 shows expansion
superimposed on an I/O function showing typical WDRC,
as offered by some fictitious hearing aid. In this example,
straight WDRC without the use of expansion would provide
40 dB of linear gain for all inputs below the knee-point.
Now look at the function for expansion. The vertical
54
output axis is extended downward on this figure to show
where the function of expansion would terminate. At this
point, the output for a 0-dB SPL input is 0 dB SPL, and so
the gain is 0 dB! The gain dramatically increases, however,
as the inputs increase, up until the knee-point shown. To
accomplish this, expansion provides greater than 1:1 linear
gain. In the case here it has a 1:2 input/output ratio; that
is, for each added decibel of input, there are two decibels
of added output! When used along with WDRC, expansion
thus provides maximum gain at (and only at) the kneepoint. In an I/O function with multiple knee-points, this
would be the left-most knee-point. The idea is to have
this left-most knee-point set at an input level typical to
very soft conversational speech, because then this soft
speech is provided with the greatest amount of gain for
the listener.
The reasoning behind expansion is quite simple. Without
it, WDRC supplies maximum gain for all soft inputs, and
any and all internal microphone and amplifier noise is
also given maximum amplification. This results in an
unwanted audibility of internal hearing aid noise. By
providing less gain below the knee-point, expansion thus
acts like an internal “noise squelch” feature.
Dynamic Aspects of Compression Until now,
compression has been discussed in terms of threshold
knee-points and compression ratios. These are sometimes
known as the “static” aspects of compression. Sound
in the environment, however, is constantly changing in
intensity over time, and compression has to respond
to these changes in intensity over time. The “dynamic”
aspects of compression concerning reaction times of
compression are known as the “attack” and the “release”
times (Figure 9). When the input SPL exceeds the kneepoint of compression, the hearing aid “attacks” the
sound by going into compression and reducing the
gain. Once the input sound falls below the knee-point of
compression, the hearing aid “releases” from compression
and restores the linear gain.
Hearing aids are not the only electrical devices that use
compression, nor are they the first to have attack/release
times. Audiovisual equipment has used OLC and WDRC,
along with various schemes of attack/release times for
many years. We have all heard the effects too. Recall,
for example, television broadcasts where the sports
Dynamic
Compression Characteristics
Figure 9. The top shows input sound changing in intensity (vertical dimension)
over time (horizontal dimension). The bottom shows the response of a
compression hearing aid to the changes in sound input intensity over time (top).
Compression circuitry takes some amount of time to respond to these changes.
announcer is talking; when a score is made and the
audience suddenly cheers, listeners may notice a slight lag
in time for the audiovisual equipment to reduce its gain
for the noise. Similarly, with sudden drops in intensity, it
may again take some time for the system to release from
compression.
Most attack and release times have been set to achieve
a best compromise between two undesirable extremes.
Times that are too fast will cause the gain to fluctuate
rapidly, and this may cause a jarring acoustical perception
by the listener. Times that are too slow may make the
compression act too slowly and cause a real lagging
perception on the part of the listener. Poor management
of attack/release times can cause a “fluttering” perception
on the part of the listener.
Most analog compression hearing aids initially used
a technique called “peak detection” to track the peak
intensity of input sound and thereby, signal the hearing
aid to go into or out of compression. With peak detection,
the attack and release times were constant and fixed
for any incoming sound intensity patterns. Most peak
detection systems in hearing aids were adjusted to
provide quick (50 ms) attack times and longer, slower (150
ms ) release times. This was seen as the best compromise
between objective effectiveness of compression and
subjective listening comfort.
In comparison to peak detection, automatic volume
control (AVC) has a relatively long attack and long release
times of several seconds. It thus does not respond to rapid
fluctuations of sound input. The long attack/release times
were actually intended to imitate the length of time it
takes for a listener to react to sudden noise increases by
physically raising a hand to manually adjusting the VC on
a hearing aid; hence, its name! Widex promoted the use of
AVC on its first digital hearing aid (the Senso); the reason
was because field trial subjects who first tried the Senso
liked it best.
“Syllabic compression” refers to the exact opposite of
AVC—namely, relatively short attack and release times,
shorter than the duration of the typical speech syllable.
This enables the hearing aid to compress or reduce the
gain for the peaks of more intense speech (usually the
vowel sounds), thus providing more uniformity in the
intensity of ongoing speech syllables. The main premise
of syllabic compression is to allow a hearing aid to
make the softer sounds of speech more audible without
simultaneously making the normally louder parts of
speech from becoming too loud.
“Adaptive compression” had fixed, quick attack times,
and release times that varied with the length of time it
took for a loud sound to become quiet again. For short
(transient) intense sound inputs like a door slam, the
attack and release times were short. For sound inputs
that took longer to become quiet again, the attack time
remained quick but the release times were longer. The
desired result was a reduction of compression “pumping”
heard by the listener. Adaptive compression was originally
patented by a now long-gone company called Telex; this
company was also associated with FM systems of the day.
Later on, adaptive compression became most commonly
associated with the KAmp circuit, which was utilized by
many of the hearing aid manufacturers.
“Average detection” was a further development beyond
adaptive compression. Unlike the peak detection method
that tracked the peak amplitude of incoming sound
waves, the average detection method employed both
long average amplitudes and short average amplitudes of
sound inputs over a given length of time. When the slow
Continued on page 57
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average SPL exceeded the knee-point of compression,
then the gain was reduced. When the fast average
exceeded the slow average by several dB, it also signaled
the circuit to go into compression. The main advantage
here is that both the attack and release times varied with
the length of the incoming intense sounds.
Today’s digital hearing aids mostly use syllabic compression
and average detection. Most often, syllabic compression
is the default for the low-frequency channels, and average
detection is the default for the higher-frequency channels.
It should also be noted that an additional focus over the
past 10 or so years has become the instant compression of
sudden, very loud transient sounds.
Regarding dynamic compression characteristics, the
“jury is out”; that is, there does not seem to be a round
consensus as to the “best” set of attack/release times.
The complexity of the matter is complicated further when
considering what set of attack/release times might be
best for different types and degrees of hearing loss. Most
clinicians do not tend to adjust or make changes to the
default dynamic compression characteristics. Clinicians
are often strongly ill-advised by the manufacturer
to tamper with the default dynamic compression
characteristics provided. n
Ted Venema, earned a BA in Philosophy at Calvin
College (1977), an MA in Audiology at Western
Washington University (1988), and a PhD in Audiology
at the University of Oklahoma
(1993). He has worked as a
clinical audiologist, and also in
the hearing aid manufacturing
sector (Unitron). He has also
taught audiology at Auburn
University in Alabama (1993-95)
and also at Western University
in Ontario Canada (2001-06). In
2006 he initiated, developed and implemented the HIS
program at Conestoga College in Kitchener Ontario. As
of September 2015, Ted is associated with the online
HIS program at Ozarks Technical Community College
in Springfield Missouri. Ted is the author of a textbook,
Compression for Clinicians, which is now being
rewritten as a 3rd edition.
Remember to take the IHS Continuing Education
test on page 59.
57
IHS Continuing Education Test
Compression: Historical Development & Use Today—article on page 48
(Sketching I/O functions for Questions 4 to 10 may be very helpful.)
1. On an I/O function, longer 45° lines
represent more linear gain. a. true
b. false
2. On an I/O function, moving a 45° line to
the right represents more linear gain.
a. true
b. false
3. With WDRC, lowering the knee-point
increases linear gain for soft inputs.
a. true
b. false
4. A compression hearing aid provides
90 dB SPL output with 40 dB SPL input;
the gain here is _____ dB.
a. 25 dB
b. 45 dB
c. 50 dB
d. 52 dB
5. Same hearing aid: knee-point at
50 dB SPL, compression ratio of 2:1;
the output for a 60-dB SPL input is:
a. 55 dB SPL.
b. 90 dB SPL.
c. 105 dB SPL.
d. 120 dB SPL.
6. Same hearing aid: knee-point at 50 dB
SPL, compression ratio of 2:1; the gain
for a 60-dB SPL input is:
a. 45 dB.
b. 50 dB.
c. 52 dB.
d. 70 dB.
9. Same hearing aid: knee-point at
70 dB SPL, compression ratio of 10:1;
the output for a 90-dB SPL input is:
a. 45 dB SPL.
b. 78 dB SPL.
c. 141 dB SPL.
d. 142 dB SPL.
7. A compression hearing aid provides
120 dB SPL output with 50 dB SPL input;
the gain here is _____ dB.
a. 45 dB.
b. 50 dB.
c. 52 dB.
d. 70 dB.
10. Same hearing aid: knee-point at
70 dB SPL, compression ratio of 10:1;
the gain for a 90-dB SPL input is:
a. 25 dB.
b. 35 dB.
c. 42 dB.
d. 52 dB.
8. Same hearing aid: knee-point at
70 dB SPL, compression ratio of 10:1;
the output for an 80-dB SPL input is:
a. 45 dB SPL.
b. 78 dB SPL.
c. 141 dB SPL.
d. 142 dB SPL.
For continuing education credit, complete this test and send the answer section to:
International Hearing Society • 16880 Middlebelt Rd., Ste. 4 • Livonia, MI 48154
• After your test has been graded, you will receive a certificate of completion.
• All questions regarding the examination must be in writing and directed to IHS.
• Credit: IHS designates this professional development activity for one (1) continuing education credit.
• Fees: $29.00 IHS member, $59.00 non-member. (Payment in U.S. funds only.)
COMPRESSION: HISTORICAL DEVELOPMENT & USE TODAY
Name _____________________________________________________________________________
Address ___________________________________________________________________________
City ___________________________________State/Province _____ Zip/Postal Code __________
Email _____________________________________________________________________________
Office Telephone ____________________________________________________________________
Last Four Digits of SS/SI # _____________________________________________________________
Professional and /or Academic Credentials _______________________________________________
Please check one: o $29.00 (IHS member) o $59.00 (non-member)
Payment:
o Check Enclosed (payable to IHS)
(PHOTOCOPY THIS
FORM AS NEEDED.)
Charge to: o American Express o Visa o MasterCard o Discover
Card Holder Name ___________________________________________________________________
Card Number _______________________________________________ Exp Date _______________
Signature __________________________________________________________________________
Answer Section
(Circle the correct response from the test questions above.)
1. a
b
2. a
b
6.a b c d
7. a b c d
3.a b
8.a b c d
4.a b c d
9.a b c d
5. a
10. a
b
c d
b
c d
59