1. For the purposes of this discussion,
"occlusion" is operationally defined as
the complete or partial blockage of the
auricle and/or the external auditory
meatus.
Implicit in this definition are the
psychoacoustic and/or physical
perceptions resulting from such
conditions.
2. Occluding the outer ear canal causes high
frequency hearing loss for air conducted
sounds.
The audiograms in the next slide show air
conduction thresholds obtained with
varying degrees of physical occlusion of
the external ear canal.
3.
4. This effect was controlled through varying
ear insert vent size to extend from 80% to
100% occlusion. Note: Ranges of
occlusion at or above 80% are not
uncommon for many hearing instrument
fittings.
The subject involved in the Chandler
study had "normal" hearing (all measured
thresholds were at zero when un-
occluded).
5. High frequencies are affected first, and,
regardless of the degree of occlusion, to
the greatest extent.
Note: At or above 95% occlusion the low
frequency thresholds (below 1K Hz)
shift as well.
6. This same effect can also occur with
any configuration of hearing.
Audiogram A in the next slide
shows a moderate to severe
sensorineural hearing loss.
7.
8. Audiogram B (in the previous slide)
shows the same subject while
completely occluded.
The result is a mixed loss which
significantly reduces the level at
which the subject can perceive sound.
9. This is of considerable consequence in
hearing aid fitting because any amount of
hearing capacity reduced by the body of
the acoustic coupling, (traditionally called
"insertion loss"), must be replaced
artificially by the amplifier of the hearing
aid.
10. The artificial replacement of lost
sounds (HI amplification) occurs in
ear canal spaces which are made
significantly smaller by the presence
of the acoustic coupler (earmold or
HI).
11. This limitation of space can easily
result in higher delivered sound
intensities through decreased
distance of sound source to the TM
(remember Boyle’s law of physics).
12. The result of combining high frequency
hearing loss (made worse by
occlusion), with high frequency
emphasis hearing instruments delivered
at close proximity to the TM; creates a
good potential for high frequency
recruitment (frequency specific abnormal growth in
the perception of loudness).
13. Recruitment, in sensorineural hearing
loss, begins at threshold.
Threshold represents a point of very low
perceived loudness. Yet, a shift in
magnitude from a sound presented at a
threshold level of 70 dB HL to a
presentation level of 80 dB HL results in a
sound pressure increase of nearly 200%.
14. This ten decibel (70-80dB HL)
difference, requiring almost a 200%
increase in sound pressure
loudness, is typical of high
frequency thresholds in many SN
losses.
15. When there is an attempt to provide HI
stimulation, there is so much pressure
involved with the application of
amplification, that the cochlea is dealing
with tremendous amounts of energy.
This will (in turn) often yield sudden
increases in the patient/client’s received
perception of loudness.
16. Amplification in the high frequency range
(located where that group of sounds is
routinely described by "normally hearing"
people as “piercing” or “tinny”) makes
recruitment management in SN hearing
loss much more challenging when the
ear is occluded!
17. Along with insertion loss and the
potential for "overdriving" those
recruiting ears, excess high frequency
input (through amplification) can
produce cochlear distortions.
21. Summation Tones
With the introduction of multiple
frequency sounds, the primary
frequencies can add to each other
producing a summation tone.
22. Difference Tones
With the introduction of multiple
frequency sounds, the primary
frequencies can subtract from each
other producing a difference tone.
23. The introduction of additional input
sounds can cause the inner ear to
"naturally produce" additional sounds.
Cochlear distortion products, including
harmonics, summation and difference
tones, can occur even at low input levels.
Although cochlear distortions vary
depending on input, high intensity inputs
produce higher intensity distortion
products sub-harmonics are known to
occur at input levels of 90 dB HL at
frequencies of 2 KHz or more.
24. The perceived pitch changes created
by increased intensity are detailed on
the next slide.
25.
26. The occurrence of cochlear distortions is
probably worsened by the cyto-
architectural changes involved with the
additional presence of SNHL.
It is also likely contributing to the
reported background noise and distortion
complaints by users of hearing
instruments.
28. Occlusion causes loss of external
ear acoustic resonance.
This removal of the “familiar”
resonance pattern represents a
fundamental change regarding the
emphasis with which acoustic
information was previously
presented to the auditory system.
