Ear Canal Occlusion—the Physical Challenges I

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.

Ear Canal Occlusion—the Physical Challenges I

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.

Ear Canal Occlusion—the Physical Challenges I

Ear Canal Occlusion—the Physical Challenges I

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).

Ear Canal Occlusion—the Physical Challenges I

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.

Ear Canal Occlusion—the Physical Challenges I

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.

Ear Canal Occlusion—the Physical Challenges I

Ear Canal Occlusion—the Physical Challenges I

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.

Ear Canal Occlusion—the Physical Challenges I

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.

Ear Canal Occlusion—the Physical Challenges I

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).

Ear Canal Occlusion—the Physical Challenges I

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).

Ear Canal Occlusion—the Physical Challenges I

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).

Ear Canal Occlusion—the Physical Challenges I

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%.

Ear Canal Occlusion—the Physical Challenges I

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.

Ear Canal Occlusion—the Physical Challenges I

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.

Ear Canal Occlusion—the Physical Challenges I

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!

Ear Canal Occlusion—the Physical Challenges I

Along with insertion loss and the potential for "overdriving" those recruiting ears, excess high frequency input (through amplification) can produce cochlear distortions.

Ear Canal Occlusion—the Physical Challenges I

Commonly created cochlear distortions include: Harmonics Sub-harmonics Summation tones Difference tones

Ear Canal Occlusion—the Physical Challenges I

Cochlear Harmonics

A harmonic occurs when a given wavelength divides into shorter wavelengths or higher frequencies.

Ear Canal Occlusion—the Physical Challenges I

Cochlear Sub-Harmonics

A sub-harmonic occurs when a wavelength multiplies into longer wavelengths, or lower frequency sounds.

Ear Canal Occlusion—the Physical Challenges I

Summation Tones

With the introduction of multiple frequency sounds, the primary frequencies can add to each other producing a summation tone.

Ear Canal Occlusion—the Physical Challenges I

Difference Tones

With the introduction of multiple frequency sounds, the primary frequencies can subtract from each other producing a difference tone.

Ear Canal Occlusion—the Physical Challenges I

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.

Ear Canal Occlusion—the Physical Challenges I

The perceived pitch changes created by increased intensity are detailed on the next slide.

Ear Canal Occlusion—the Physical Challenges I

Ear Canal Occlusion—the Physical Challenges I

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.

Ear Canal Occlusion—the Physical Challenges I

A typical external ear acoustic resonance pattern is shown below:

Ear Canal Occlusion—the Physical Challenges I

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.