1. Traditional definitions of the
occlusion effect have focused on the
increase in bone conducted sound in
the low frequency range when the
cartilaginous meatus is occluded.

2.  When placing a block (occluding) the
ear canal, the former acoustic resonant
pattern of the ear is lost.
 With the high frequency emphasis
taken away, the low frequencies, which
carry the greatest potential sound
power, will be heard internally more
easily.

3. Increased sensitivity to bone conducted
stimuli varies considerably from
individual to individual, it occurs
predominantly in the low frequencies,
and has been measured as a BC threshold
improvement of up to 30 dB.
Note: A Weber effect may also occur
when one ear is occluded.

4. Increased sensitivity to bone
conduction (BC) under occlusion is of
particular concern when considering
the voice of a hearing instrument
wearer.

5.  The human voice results from
vibrations caused by movement of the
vocal folds as air rushes past them.
 These vibrations are modified by the
resonant cavities of the skull, which are
surrounded predominantly by bone.
 Sound can then be transmitted through
the bone to the cochlea.

6. Sound that is transmitted through bone can
stimulate cochlear response in one of two
ways:
1. compressional bone conduction,
where sound passes from the temporal
bone through the outer shell of the
cochlea.
2. inertial bone conduction, caused
when the bony portion of the external
auditory meatus transmits sound to the
tympanic membrane through the
annulus or the air in the canal.

7.  The loudness of inertial bone
conducted sound is increased
through occlusion.
 The human voice is capable of
producing sound pressures
(measured in the throat) of 140 dB
SPL (Killion et al, 1988).

8. Plugging the outer ear canal causes
the delivery of high frequencies to be
reduced.

9.  The low frequency (long wavelength
sounds), will travel through the bone
structures with the least amount of loss;
will be delivered to the bony portion of
the meatus with the greatest facility.
 This occurs at sound pressure levels that
have been measured, under
occlusion, at near 100 dB SPL (Killion et
al, 1988).

11.  FIGURE 4 (the previous slide)
illustrates some in-situ measurements,
and the increase in low frequencies is
well defined using the vocalized OO &
EE .
 Killion and his associates called the
results of this process "self-masking."

12.  “Self-masking” refers to the fact that the
low frequency sounds emphasized by
occlusion could cause other signals to
be reduced to in-audibility.
 This is the result of low frequency
sounds having more acoustic
power, and thus masking the higher
frequencies.

13. Examples include not only the user’s
own voice, but sounds made while
eating, shaving, etc.
The introduction of any low
frequency sound input can result in
the 'upward spread of masking'.

14. A sound of 500Hz frequency has a
wavelength of about two and two-tenths
feet.
One cycle of such a sound takes up over
two feet of space in the atmosphere as it
travels forth from its source.
Note: The cochlea is about 31mm in total
length from base to apex (Zemlin, 1988).

15.  A low frequency sound wave has
wavelengths which are so great that
they will excite not just a single area of
low frequency responsive hair cells, but
also high frequency responsive hair
cells.
 Due to the overall areas of pressure, and
the introduction of harmonic, or
multiple frequency components
masking of those high frequencies
easily occur.

16. This spread of masking into the
higher frequencies, called the
'upward spread of masking', is
probably further exacerbated by the
fact that the basilar membrane is
narrower at the base of the
cochlea, and grows wider at the apex
(Zemlin, 1988). Reference next slide.

18. After review of the previous slide, it
is indeed a valid assumption that a
narrower membrane will be more
easily set into motion than a wider
one, causing the high frequency
sensory cells at the base of the basilar
membrane to also be set into motion.

19. Not only is the upward spread of
masking considered to be one of the
reasons why exposure to noise, which has
been classed as a predominantly low
frequency event, results in so many high
frequency hearing losses. It is also one
reason why low frequency sounds can
cover over high frequency sounds, the
concept previously referred to by Killion
as 'self-masking.'

20.  A common complaint of hearing aid
users is that their own voices sound too
loud (Dempsey, 1990).
 This is often assumed to be the result of
a sensorineural loss in which the person
literally did not hear themselves for a
long period of time, and find this re-
acquaintance with their own voice to be
something of a 'rude awakening'.

21. However, since occlusion of the ear canal
causes such dramatic increases in the
sound pressure of the patient/client's own
voice (as we have just described), this
loudness growth may have less to do with
a loss of reception of their own
vocalizations over a long time period, than
it does with the change in delivery of their
currently modified vocalization reception.

22.  Recruitment occurs not only in the high
frequencies, but in the low frequencies
as well.
 The response of the human auditory
system to low frequencies requires
more sound pressure be present before
actual audition occurs.
 Once a sound is heard, recruitment can
occur at a faster rate than in any other
frequency range (Humes, 1985).

23. Let’s closely view this next slide.
There is a lot of information on it.
For now let’s learn the sound
pressures required for each frequency
to be audible to the human auditory
system.

25. The magnification of a
patient/client's own voice through
occluded bone conduction
(BC), results in a strong potential for
low frequency recruitment.

26.  The patient/client’s voice is received at
the ear through both AC and BC.
 In air conduction (AC), vocalizations
must travel further distance at much
slower speeds, resulting in airborne
speech sounds arriving milliseconds
later than those traveling to the ear via
bone conduction (BC).

27. Estimates of the difference in time of
transmission, based on average distance
traveled and velocity of sound in a given
medium (Zemlin, 1988; Speaks, 1992), are
that AC speech sounds arrive at the ear
approximately 6 msec later than BC
speech sounds.

28.  Minimal time interval resolution
(MTIR), has been shown to occur at
intervals as brief as 3-4 msecs (Muchnik
et al, 1985).
 MTIR generally slows with increasing
age (Muchnik et al, 1985).
 Fastest resolutions occur at higher
intensity levels (Muchnik et al, 1985);
and in high frequency ranges (Irwin et
al, 1981).

29. Sensorineural hearing loss (SNHL)
has been shown to decrease
resolution times (Irwin et al, 1981).

30. However, the increased intensities
involved in BC vocalizations while the ear
is occluded, combined with the difference
in transmission time between air and
bone conducted sound could cause some
of the complaints of "echo effects"
reported by hearing instrument users.

31.  Research on the effects of air pressure on
the ears has indicated that increased air
pressure causes reduction of acuity for low
frequency AC sounds, (Weaver &
Lawrence, 1954).
 Reduction in otoacoustic emission
amplitudes, (Naeve et al, 1992).
 Continued subjective reports of individual
ability to discriminate fine differences in
barometric pressure based on feelings of
pressure changes in the ear (Vernon, 1992).

32. This research suggests that there is a
group of individuals who will have
difficulty in dealing with air pressure
changes brought on by occlusion.

33. Occlusion of the ear canal can also cause
an increase in the loudness of
tinnitus, resulting from either of two
situations:
1. Reduction of the masking effects of
external sound input can cause the
loudness of tinnitus to become
exaggerated.
2. The increase in the loudness of tinnitus
may be due to occluding the ear canal
itself (Vernon, 1992).

34.  The cases where the introduction of
occlusion causes a marked increase in
the loudness of tinnitus are relatively
rare.
 However, awareness of the role of
occlusion in tinnitus management is
essential to the potential alleviation of
tinnitus through hearing instruments.

35. Traditional definitions of occlusion have
sometimes suggested that occlusion is a
single effect, most often associated with
changes in BC thresholds (Silman &
Silverman, 1991).
While bone conduction is clearly an
important part of physical occlusion, this
discussion has attempted to clarify the fact
that occlusion involves multiple effects to
include neural occlusion (we will discuss
more on neural occlusion next week).