by Mark Ross, Ph.D.
This article first appeared in Hearing Loss (Jan/Feb 2004)
The occlusion effect is not a new phenomenon and has been described here before. Still, it is a serious and continuing problem for some hearing aid users and thus is the kind of topic that merits some repetition now and then. It relates to the complaint that some hearing aid users make, newer ones in particular, of their own voice sounding a bit “funny” when they talk. Sometimes people feel like they’re talking in a barrel, that what they’re hearing is echoes of their own voice, or that their voice sounds “hollow” or “booming”. Whatever adjectives people use to describe their perceptions of their own voice while talking, it is almost invariably negative. They just don’t like the way their own voice sounds.
And it is not just their voice that can be disturbing. People who have a large occlusion effect may also feel a sense of pressure or blockage in the ear when an earmold is inserted. Chewing one’s food may sound noisy or unpleasant. It is disturbing enough so that if the occlusion effect is not solved or made acceptable in a reasonable time frame, some people will reject hearing aids. For them, the benefits they receive with hearing aid amplification are overshadowed by the problems the occlusion effect causes. If they’re a reluctant hearing aid candidate (dragged into the clinic by spouse or adult child) then the sensations caused by an occlusion effect may give the person a convenient excuse to reject hearing aids.
An occlusion effect occurs when some object (like an unvented earmold) completely fills the outer portion of the ear canal. What this does is trap the bone-conducted sound vibrations of a person’s own voice in the space between the tip of the earmold and the eardrum. Ordinarily, when people talk (or chew) these vibrations escape through an open ear canal and the person is unaware of their existence. But when the ear canal is blocked by an earmold, the vibrations are reflected back toward the eardrum and increases the loudness perception of their own voice. Compared to a completely open ear canal, the occlusion effect may boost the low frequency (usually below 500 Hz) sound pressure in the ear canal by 20 dB or more.
This is a real and measurable increase in sound. One way that it can be determined is with a probe-tube microphone (a device that should be used, in my judgment, in just about every hearing aid fitting). A probe-tube microphone consists of a very fine, flexible tube that is connected to various types of sound measurement equipment. The tube is inserted in the ear canal and the audiologist measures the sound levels in the ear canal while the hearing aid user utters some standard vowel (like “ee”). Then, with the tube still in place, an earmold (or hearing aid shell) is inserted in the ear canal, making sure that the tube extends a few millimeters past the earmold tip. Then, with the hearing aid turned off, the audiologist again measures the sound levels when the hearing aid user says the same vowel (at the same loudness level). The difference between the sound levels occurring when the ear is open and when it is closed with a hearing aid is a measure of the amount of the occlusion effect. As I noted above, differences of 20 dB or more are common.
There are basically only two ways to reduce or remove the occlusion effect. The most effective way is to not completely block the ear canal with an earmold. This permits the bone-conduction sound generated in the ear canal to escape the ear the way it is supposed to. When someone is wearing hearing aids, the only way to do this is to create a vent hole in the earmold. For those who are unfamiliar with an earmold vent, it is a hole drilled completely through the earmold from the outer surface to the inner surface. The amount of sound that escapes, and thus the magnitude of the occlusion effect, depends upon the size of the vent. The larger the vent, the more the occlusion effect can be reduced.
The smallest vent is termed a “pressure” vent. By allowing air to enter the ear canal, a “pressure” vent will alleviate the feeling of fullness in the ear that can occur when an ear is completely blocked by an earmold. It does this by equalizing the atmospheric air pressure to that occurring in the ear canal. While a pressure vent will not reduce the occlusion effect or influence the pattern of amplification, it will help a hearing aid user be more comfortable when wearing the aid. Pressure vents are very common in hearing aids, except for smallest completely-in-the canal (CIC) hearing aids. These aids are so chock full of electronics that they have no spare space left to accommodate a vent.
Slightly larger vents will not only begin to reduce the occlusion effect, which is one of our primary goals, but it will also shunt some of the amplified low frequency sounds out of the ear. That is, some of the low frequency sounds amplified by the hearing aid will not be transmitted through the middle ear into the inner ear. Instead, these amplified low frequency sounds in the ear canal will find that the acoustical path of least resistance is out through the vent hole and into the atmosphere, rather than through the middle ear. Thus a vented earmold effectively reduces the degree of low frequency amplification delivered to a hearing aid user. These sounds are simply shunted out of the ear canal. The acoustical consequence of this action may be positive or negative depending upon the audiologist’s fitting goals. It will be positive for those people with little or no hearing loss in the low frequencies, for whom little or no amplification of the low frequencies is required, but not so good for people with more severe hearing losses across the audiogram. The larger the vent, the greater the reduction in the occlusion effect and the more amplified low frequency sounds that are shunted out of the ear canal.
Even though vents will reduce the occlusion effect and can make wearing a hearing aid a bit more comfortable, there are several potential downsides. Hearing aid users with moderate-to-severe hearing loss or greater frequently experience acoustical squeal when their earmolds are vented. The sounds that escape from the vent are picked up again by the hearing aid microphone and re-amplified, thus beginning the acoustical oscillations that produce the feedback squeal. On the positive side (if you can call it that) people with more severe hearing losses generally do not require vents, except for, perhaps, a pressure vent and thus vent produced feedback is not a problem. (Which is not to say that feedback from other causes is not an issue, of course it is.) Insofar as the occlusion effect is concerned, however, they are less likely to be bothered by it than people with lesser degrees of hearing loss. The people who seem most susceptible to the occlusion effect are those with relatively good hearing at the lower frequencies.
Recently, new and improved feedback management circuits have been developed. By reducing or eliminating acoustical feedback electronically, hearing aids with this feature permit the use of larger vents in earmolds. While there are several types feedback management systems available, the one I think most effective is the type that effectively cancels the squeal without changing the amplification pattern of the hearing aid. When feedback is eliminated electronically, people can enjoy the comfort of a larger vent without being bothered by acoustical feedback. But since a large vent reduces low frequency amplification, it may be necessary in some instances for the hearing aid to provide additional amplification at these frequencies to compensate for the acoustical impact of a vent. This type of action is ordinarily part of the hearing aid fitting process.
Another way of reducing the occlusion effect is to use a very long and tight earmold or to locate a completely-in-the canal (CIC) hearing aid far down in the ear canal, very close to the eardrum. What this does, essentially, is to prevent the sound vibrations produced by one’s own voice from being developed in the ear canal. This is because the earmold tip or CIC aid is already filling the ear canal space in which the vibrations are generated. Several years ago, I discussed the elimination of the occlusion effect as one major advantage of CIC hearing aids. Since that time, however, some authorities have pointed out that this presumed advantage may be a bit overstated. Evidently, fewer CIC hearing aids are now being made with long enough canal portions necessary to eliminate the occlusion effect. Sometimes, too, a really tight and long earmold situated deep into ear canal can be uncomfortable and possibly irritate the skin in the canal.
There is a phenomenon known as “ampclusion” that affects people in a similar way as occlusion and sometimes gets confused with it. Ampclusion, however, is produced by hearing aid amplification and not a closed earmold. It occurs when a hearing aid provides a great deal, possibly too much, low frequency amplification, particularly if a person has a relatively mild hearing loss in the low frequencies. This would cause hearing aid users to hear the low frequency components of their voice much louder than they would hear someone talking three or four feet away from them (since their own mouths are much closer to their own ears). In this instance, they would indeed complain that their voices sound booming” or “echoing.” One way to make the distinction is to consider whether it is only the person’s own voice that is disturbing and not other people’s voices as well. If only their own voice, then the problem is due to the occlusion effect. If they are disturbed by both their conversational partner’s voice and their own, then the problem is likely to be ampclusion. Of course, both ampclusion and the occlusion effect may occur in an individual case. Then the challenge of the hearing aid dispenser is to attempt to reduce both, or at least determine which is the worst offender and reduce that one. In practice, a fair amount of trial and error can be involved.
One often hears comments from hearing aid dispensers that people with complaints about how their own voice sounds must “get used” to it. Of course, a certain amount of acclimatization is necessary whenever anybody is suddenly exposed to a different pattern of amplified sound. For a new hearing aid user, there are likely to be changes in how one’s voice sounds. This is simply a by-product of people hearing themselves differently when they talk. But before a dispenser puts the complete burden of adjusting to something like an occlusion effect on the shoulders of the hearing aid user, he/she must first go through the process of trying to find an audiological solution. It is easy enough to reduce the low frequency amplification gain of a hearing aid and to determine what difference this makes in a person’s perception of his/her own voice. It is a little more difficult to systematically modify an earmold vent, and to do this without producing acoustic feedback, but this too can be tried. It is only when these steps are taken and complaints persist, that a hearing aid user should embark on a program of “getting used to an aid.”