Ear canal occlusion -physical challenges ii

Ear Canal Occlusion—the Physical Challenges II

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.


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.


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.


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


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.


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.


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).


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


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).



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."


“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.


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'.


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).


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.


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.



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.


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.'


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'.


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.


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).


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.



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


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).


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.


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).


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


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.


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).


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


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).


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.


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).

Education

Ear canal occlusion -physical challenges ii