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Ampclusion Management 101:
Understanding Variables
A tutorial on occlusion, its causes, and how to reduce/eliminate its effects
The term “ampclusion”
denotes the occlusion effect
experienced during hearing
aid use and audiometric
testing, when the physical
occlusion of the ear canal and
the hearing aid amplifier gain
are included in the wearers’
perception. Complaints about
ampclusion may originate
from shell blockage of the ear
canal, to sub-optimal
amplification settings for
amplifying the wearers’ voice,
to an over-expectation for
hearing aid performance. This
article provides a summary of
the factors that can contribute
to the wearers’ complaint of
the ampclusion problem, and
a follow-up article details
remedies for the various
causes of ampclusion.
By Francis Kuk, PhD, & Carl Ludvigsen, MS
uch of the literature on hearing
instrument fitting has focused on
the wearers’ perception of externally generated sounds. This is reasonable
because the primary objective of a hearing
instrument is to improve the audibility
(and hopefully intelligibility) of external
signals. On the other hand, a potential
negative side-effect of hearing instrument
use—the occlusion or “hollow voice”
effect—has been cited by as many as 30%
of hearing aid wearers as a reason for their
dissatisfaction (or discontinued use) of
their hearing instruments.1,2 It is reasonable to expect that any methods that can
effectively minimize this complaint would
significantly improve satisfaction with
hearing aids.
M
Variables Affecting the
Wearers’ Own Voice
During audiometric bone conduction
testing, the physical occlusion of the ear
canals by the headphones (or inserts)
improves the low frequency bone conduction thresholds of the test subjects.
Tonndorf3 termed this improvement in low
frequency bone conduction thresholds the
occlusion effect (OE). Hearing aid wearers
experience the occlusion effect when they
speak while wearing their hearing aids
because the insertion of an earmold or of a
hearing aid lowers the bone conducted
speech reception threshold (SRT) of the
wearer.4 A difference between the occlusion effect experienced during hearing aid
Francis Kuk, PhD, left, is
director of audiological services at the Widex Office of
Research in Clinical Amplification, Lisle, Ill, and Carl
Ludvigsen, MS, is director of
audiology at Widex APS,
Vaerloese, Denmark.
use and that during audiometric testing is
that the effect of the hearing aid amplifier
is also included in the wearers’ perception.
To distinguish between these two
origins of occlusion effect, Painton 5
used the term ampclusion to describe
the dual origins of the complaint reported by hearing aid wearers. The term
ampclusion will be used in this article to
include the contribution of shell occlusion and hearing aid amplifier gain to
the perception of unnaturalness in the
wearers’ own voice. Other related
observations like hearing oneself chew
crunchy, crispy food, or the thumping
of one’s footstep have similar origins.
Hearing aid wearers express the ampclusion effect by using various descriptors,
like “hollowness,” “talking through the
nose,” “plugged,” etc. Some of these
descriptors may reveal the underlying origin of the wearers“ ampclusion complaint.
These variables include shell and amplifier
origin and user experience/expectations.
Shell Origin
Occlusion of the ear canal. During
vocalization, the vibration of the vocal folds
sets the skull into motion along with the air
within the ear canal.3 Much of this sound
pressure in the ear canal is eventually transmitted to the cochlea via the tympanic
membrane and ossicular chain. In the open
ear condition, this transmission is less effective in the low frequencies than in the high
frequencies since the open ear canal constitutes a more effective “short circuit” at low
frequencies than at high frequencies (ie,
much of the low frequencies dissipate out
of the ear canal). When the ear is occluded
(as in the use of a hearing aid/earmold),
the low frequency sound pressure level
(SPL) in the ear canal is “trapped” by the
earmold/hearing aid, resulting in increases
in low-frequency SPL.
Other changes also occur. The resonance characteristics of the occluded ear
canal change from the quarter-wavelength
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THE HEARING REVIEW
Kuk
&
Ludvigsen:
Ampclusion
Management
actions with the
phase response of
the direct signal and
that of the hearing
aid). Below the
Helmholtz resonance frequency
(which is determined by the complex
relations
between the length
Figure 2. Effect of vent diameter on the measured occlusion effect.
and diameter of the
vent and the residual ear canal volume) minimize shell occlusion. This may have
sounds entering through the vent may the same mechanism as venting. Mueller16
cancel sounds leaking through the vent. reported that tapering a CIC hearing aid
On the other hand, above the Helmholtz lowered the real-ear SPL relative to a
resonance frequency, sounds leaking bone-conducted stimulus from 125 Hz to
through the vent may add to sounds enter- 300 Hz but increased its output at 400 Hz.
ing the vent, increasing (rather than No change in the response to external
decreasing) the measured SPL in the ear sounds was noted. However, Staab 17
canal. This may partly explain the ‘incon- reported the opposite finding.
sistent’ effects of venting on the occlusion
The seal of the hearing aid at the
effect and the difficulty in controlling the bony section of the ear canal is another
amount of low frequency gain in hearing variable that may affect the perception of
aids through venting.10 This prompted the ampclusion effect. Bryant et al. 18
Schweitzer & Smith11 to propose an “elec- compared ITEs built in the traditional
tronic vent” where the dispensers can manner with ITEs that are built with
electronically adjust the phase response of minimal-contact-technology (MCT) but
the circuit to cancel out frequencies with a deep canal length and total
around 500 Hz where the wearer’s voice acoustic seal in the bony portion. Their
results showed a reduction of the occludominates.
Despite the occasional inconsisten- sion effect by 10 dB at 200 Hz. While
cy, the general effect of venting has this technique may be effective,
been positive in alleviating the occlu- Pirzanski 19 estimated that 80% of CIC
sion effect and improving sound wearers may not accept such a solution
quality.12 However, it should be recog- because of potential physical discomfort.
Insufficient canal length beyond secnized that venting could compromise
the advantages offered at low frequen- ond bend. Tonndorf,3 in explaining the
cies by directional microphones13 and occlusion effect (OE), noted that the measured OE decreases as the depth of inseralter the real-ear compression ratios.14
When working with nonlinear direc- tion of the occluding device (eg, insert
tional hearing aids, one may consider earphone) increases. He attributed this to
starting with the smallest vent size that an increase in impedance at the tympanic
is just sufficient to minimize the occlu- membrane as well as to the modified resosion effect so significant compromise on nance frequency of the occluded ear
the directivity index and compression canal. This explanation has also been
ratio will not occur. The use of a Select- used by Berger20 who showed that the
A-Vent (SAV) system may relative amount of occlusion effect varied
be desirable to achieve as a function of the depth of insertion of
this purpose. It is also of the hearing protectors (Figure 3).
Killion et al 21 extended Zwislocki’s
interest to note that some
preliminary work sug- observation of the occlusion effect into
gests a possible relation- hearing aid fittings. These authors
ship between the middle showed a significant reduction in the meaear compliance of the sured real-ear OE when the canal portion
wearers and the minimum of the custom hearing aid extended far
vent size that is required beyond the second bend of the ear canal.
A hearing aid that extends beyond the
for minimal occlusion.15
Similarly, leakage of second bend with a tight seal in the bony
sound from a loosely fitted portion of the ear canal was recommendFigure 1. Real ear response of /i/ measured in an open-ear earmold/hearing aid shell ed to overcome the occlusion effect. This
and an occluded ear condition.
has also been reported to observation was supported by subsequent
resonator to a half-wavelength resonator.
The impedance at the tympanic membrane is modified along with the damping
characteristics of the ear canal wall. 3
These changes lead to a greater low-frequency output during vocalization in the
occluded condition than in the unoccluded
condition.6 Figure 1 shows the real-ear output for an open ear and an occluded ear
during sustained vocalization of the sound
/i/. The additional low frequency output
may have led hearing aid wearers to use
adjectives like “hollow,” “echoing,” “talking in a barrel/closed room,” or “boomy”
to describe their perception.
Insufficient venting/leakage. When
the origin of the ampclusion complaint
is primarily shell occlusion, minimizing
the physical occlusion or increasing the
leakage by using larger vents could alleviate the complaint.
Westermann7 indicated that vents act
like acoustic short-circuits in a hearing
aid shell (ie, allowing “trapped” low frequency sound to escape). He estimated
that a vent size of 2 mm is sufficient to
remove the effect of shell occlusion.
Revit, 8 however, showed that a 2 mm
vent reduced the occlusion effect by 8.5
dB at 200 Hz but had no effect at 500 Hz.
In addition, as the vent size decreases,
more occlusion effect is noted. Kampe &
Wynne9 also showed that, as the vent size
increased, the low frequency SPL measured in the subjects’ ear canals decreased.
However, no correlation was seen between
the measured low frequency and the subjective impression of occlusion. Figure 2
shows OE as a function of vent diameter.
A particularly interesting observation
in Kampe & Wynne’s study9 was that, in
some subjects, the introduction of a vent
increased (instead of decreased) the measured occlusion effect. A possible explanation for these somewhat inconsistent
effects of vents may have to do with the
phase response of the vent (and its inter-
THE HEARING REVIEW
AUGUST 2002
Figure 3. Berger’s20 chart (reprinted with permission) showing the effect of
different hearing protectors (and the insertion depths) on the occlusion effect.
investigators.8,16 Figure 4 shows the realear measurement of the open ear and a
deep canal seal shell during vocalization.
As indicated earlier, a deep canal seal
hearing aid may be physically uncomfortable to some wearers. Unless the ampclusion effect can only be resolved with a
long ear canal, many dispensers would opt
for a more comfortable solution.
Bongiovanni et al.22 examined the difference in patients’ subjective preference for
two versions of a CIC that were matched
in target gain (and venting) but differed in
canal lengths. In one version, the canal tip
terminated 2 mm before the second bend
of the ear canal. In the other version, the
canal tip terminated 2 mm beyond the second bend. As predicted, the shorter version was more comfortable to the wearers.
However, no group difference in the quality of the wearer’s own voice was evident.
This suggests that, while canal lengths
may affect the amount of shell occlusion
within the ear canal, it may not be the sole
factor governing the perception of ampclusion for all wearers.
Manufacturing variation. Pirzanski19
speculated that if the effect of shell diameter is significant on the observed occlusion, then one may expect variation in the
observed occlusion effect when using different impression materials, different
impression-taking techniques, and different manufacturing processes for the
same ear impression/client. Consequently, Pirzanski took ear impressions
of one individual in a chewing position, a
closed-jaw position, and an open-jaw position. Two types of inserts were made: one
with a hard shell and the other with a silicon impression material with a low Shore
value. The lengths of the canal inserts
ranged from 0-21 mm from the canal
opening. Additionally, some of the inserts
were vented. Real-ear measures of the
occlusion effect were made while the subject produced a sustained /i/ sound. The
results showed significant variability in
Figure 4. Real ear responses of /i/ measured in an open ear
and a deep canal sealed ear.
the measured occlusion effect as a function of the canal length, venting condition, how the impression was taken, as
well as the manufacturing process (number of coating).
On the other hand, some of the variations observed in Pirzanski’s study19 were
not easily explained on theoretical
grounds. More data would be needed to
define how each variation could affect the
measured occlusion effect. Nevertheless,
the study points to the complexity of
managing the occlusion effect.
Amplifier Origin
Excessive low frequency output in
the ear canal. While physical occlusion
increases the low frequency SPL in the
ear canal during vocalization, such is
not the only source for the increase in
low frequency SPL. The input level of
one’s own voice at the position of the
hearing aid microphone, assuming a
normal vocal effort, is higher than that
measured at a conversational distance
of 1 meter. This is due to the shorter
distance between the speaker’s mouth
and his/her own ears compared to the
typical conversational distance.
In addition, the radiation characteristics of the mouth also affect the SPL
measured at the ear level.23 Indeed, if
one measures the SPL at the ear level, it
is frequently 15-20 dB higher than that
measured at a distance of 1 meter.
Figure 5 shows the difference in the
long-term speech spectrum measured at
a distance of 1 meter and at the ear
level. An increase in the low frequency
and a slight decrease in the high frequency are seen. This knowledge is
important when it comes to fine-tuning
the acoustic parameters of a hearing aid
to minimize the ampclusion effect.
A second factor that increases the
low frequency output in the ear canal is
the low frequency gain provided by the
hearing aid. These two factors (ie,
increase in low frequency input and
available low frequency gain) further
increase the low frequency energy in the
wearers’ ear canals during vocalization.
In order to determine if the amount of
low frequency output plays a role in the
wearers’ perception of their own voice,
Kuk24 instructed hearing aid wearers to
select their preferred frequency-gain
responses on a programmable hearing aid
while they listened to their own vocalization and to externally presented speech.
The results showed that the average preferred frequency response for the listening task had significantly more low frequency gain than the preferred frequency
response selected for the vocalization task.
In a follow-up study, Kuk et al. 25
demonstrated that a hearing aid that
adaptively changed its low frequency
gain according to input levels resulted
in a reduced perception of hollowness.
Excessive sound pressure level in
the ear canal. On the other hand, some
individuals with a high frequency hearing
loss who were wearing what is essentially
an open mold with no low frequency gain
on the hearing aid still reported the ampclusion effect.25 This suggests that low
frequencies per se may not be the only
contributor to the ampclusion effect.
Rather, it may be the overall loudness
that contributes to such a perception.
The low frequency effect is more
noticeable because low frequency sounds
contribute more to loudness than mid and
high frequency sounds. In the absence of
low frequency, the amplified mid and high
frequency sounds may contribute to the
overall loudness and thus to the ampclusion effect. Wimmer26 reported that the
ampclusion effect was diminished by a
reduction of overall gain. In Kuk’s study,24
the average subject also preferred a lower
overall gain for the speaking condition
than the listening condition.
Insufficient output SPL. It is not infrequent to find hearing aid wearers whose
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THE HEARING REVIEW
Kuk
&
Ludvigsen:
Ampclusion
Figure 5. Long-term speech spectra measured
at the distance of 1 meter and at ear level.
ampclusion complaint is due to insufficient
output. This may occur in an under-fit situation where the insertion loss created by
the physical occlusion of the hearing aid is
not overcome (or is only partially overcome) by the gain of the hearing aid, or
the hearing loss in the low frequency is
not adequately compensated.
An example of circumstances where
this may occur is the use of a minimally
vented completely-in-the-canal (CIC) hearing aid for a mild-to-moderate hearing loss.
Sweetow & Valla27 explored this possibility
in 10 CIC wearers (15 ears) and found that
almost half of their subjects required a gain
increase to minimize the ampclusion effect
whereas half required a gain decrease. Frequently, wearers whose ampclusion complaint originates from this cause may
describe it as “talking through the nose,”
“closed,” “plugged,” “can’t hear own
voice,” “unnatural,” “own voice too soft,”
“muffled,” or “distorted.” Ampclusion complaint that originates from insufficient output requires more low frequency gain than
the settings on the hearing aids provide.
Imbalance of sound pressure output at different frequencies. One possibility for the unnatural voice during
vocalization may be a tonal imbalance
between high and low frequency output
during vocalization where such balance
is achieved for listening to external
sounds. Killion28 improved a hearing aid
fitting from the occlusion effect by
increasing the mid and high frequency
gain on the hearing aid.
Wearers may comment that their
vocalizations sound “unnatural,” “muffled,” “too loud,” “too much bass,”
“boomy,” or “too tinny.” Although the
same comments may be made for external sounds, it is important to recognize
that these comments are made relative
to a high input level. Under this hypothesis, remediation may be difficult because
tonal balance for vocalization may disrupt the tonal balance for listening to
external sounds. Wearer counseling and
THE HEARING REVIEW
AUGUST 2002
Management
re-training may be
necessary.
Distortion of
input/output.
Although not truly
a possibility for the
ampclusion effect,
some wearers may
report distortion
from their hearing
aids during vocalization as “unnatural.” The high input
level at the microphone opening during
vocalization suggests the possibility that
the wearers’ voice may saturate the hearing aid at the input stage.
Saturation may be especially likely for
individuals who speak at a higher than
normal volume. Another possibility is saturation distortion at the output stage
when the output limit of the hearing aid
does not increase with gain settings. This
would be especially true for linear hearing aids that use peak clipping as the output limiting method. Subjective comments like ‘distorted,” “unnatural,” “static” are frequently used to describe voice
problems with such origin. Gain reduction at high input levels and the use of a
compression limiting circuit at the output
stage may solve such complaints.
In contrast, some wearers react negatively to gain reduction at high input or
to a compression limiting circuit. They
may say that their own voice sounds
“muffled,” “unnatural,” or “not loud
enough.” This is especially a possibility
for those who are accustomed to linear
hearing aids with peak clipping. The
dispensing professional may need to
increase gain at high inputs (ie,
increase MPO setting or decrease compression ratio) to satisfy these wearers.
Multichannel specificity. A concern
in minimizing the ampclusion effect is
that any gain reduction should not affect
intelligibility. This should not be a major
concern because the ampclusion effect is
primarily below 500 Hz where minimal
intelligibility information is available.
In practice, it is often impossible to
achieve complete ampclusion reduction
without accompanying intelligibility
reduction. This is because conventional
single-channel aids lack channel specificity (or channel independence), so
that low frequency gain reduction also
leads to gain reduction in the nearby
frequencies. To preserve intelligibility,
one may have to compromise the effectiveness of ampclusion reduction by limiting the amount of gain reduction.
Historically, this fact has fueled the popularity and use of some multichannel hearing instruments. Due to the independent
compressors, a multichannel hearing aid
has a higher potential for allowing specificity in low frequency adjustment without
affecting intelligibility. On the other hand,
a multichannel aid with highly overlapping
filters (shallow filter slopes) may not
improve the situation because gain adjustment to one frequency region may
“spread” to nearby frequency regions.
Consequently, there is no guarantee that a
multichannel device will provide better
management of the ampclusion effect.
Rather, one may hypothesize that the multichannel devices with narrower bandwidths (especially in the low frequencies)
and steeper filter slopes may offer more
specificity (and thus more effective occlusion management) than ones with a broader bandwidth and shallower filter slopes.
Artifacts of some DSP hearing
aids. In contrast to analog and programmable hearing aids, digital signal processing results in a delay of the
processed signal (relative to the
unprocessed signal).
The exact amount of delay (or group
delay) varies from one DSP design to
another. (For an example of how to test
for this phenomena, see Frye.29) While
short delays are not noticeable to the
wearers, longer delays are noticeable
and result in an echoic sensation when
the wearers listen to external speech
and to their own voice, depending on
the earmold/shell condition. This
echoic sensation occurs when there is a
mixture of processed sounds and direct
sounds in the wearers’ ear canals.
In a completely occluding earmold,
group delay may only produce an echoic
sensation of one’s own voice because
there is no vent to provide a direct
sound path for external sound. The
wearer’s own voice that is trapped within
the ear canal via bone conduction interacts with the hearing-aid-processed
sounds in the ear canal to produce the
echoic sensation. By contrast, in a vented earmold/hearing aid shell, the direct
acoustic signal enters the ear through
the vents/leakage in the hearing aid
shell/earmold. When this signal is
mixed with the processed sound from
the hearing aid, the partial overlap could
result in an “echoic” or “hollow” sound.
Stone & Moore 30 and Agnew &
Thornton31 showed that a group delay as
little as 5-10 ms could result in perceptible artifacts. This perception is less like-
ly in the severe losses because these
wearers are less likely to use large
vents in the earmolds. Obviously, to
minimize such perception, the choice of
DSP hearing aids with minimum group
delay is important.
Inexperience and
Expectations of the Wearers
It is a common observation that new
hearing aid wearers complain of the
ampclusion effect more frequently than
experienced hearing aid wearers. One
reason for such an observation may be
that new wearers are not familiar with
their hearing aids, whereas experienced
wearers have resided to the experience
that ampclusion is a necessary side effect
of hearing aid wear.
Compared to experienced wearers,
new wearers are exposed to a new set of
challenges when they wear their hearing
aids. They may not expect the occlusion
of the ear canal to bring such changes in
the quality of their own voice. This raises
the importance of pre-fitting counseling
for hearing aid wearers so that proper and
realistic expectations can be set. Pogash
& Williams32 have also indicated such possibility.
Clinical Implications
Because of its myriad nature, resolution
of the ampclusion complaint would necessarily need a precise identification of its origin so that proper and efficient solutions
can be applied. The next article will consider a systematic approach to managing ampclusion effects in hearing aid wearers. ◗
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Reprinted with permission. “Ampclusion Management 101: Understanding Variables”,
Hearing Review, August 2002; Volume 9, Number 8: Pages 22, 24, 26, 30, 31, & 32.
AUGUST 2002
THE HEARING REVIEW