Feedback Cancellation Systems and Open-Ear Hearing Aid Fitting - Part 1

By Mark Ross, Ph.D.

The presence, or threat, of acoustic feedback has long been one of the major problems in the fitting and wearing of hearing aids. Acoustic feedback occurs when some of the amplified sound leaks from the ear canal and is picked up by the hearing aid microphone and then re-amplified. This starts the cycle of leakage and re-amplification (the "feedback loop") that results in the squeal we know as "acoustic feedback."

The traditional solution for reducing acoustic feedback has been to increase the acoustic seal in the ear canal, usually by fabricating tighter, longer, but often more uncomfortable earmolds. For some hearing-impaired people, particularly those with moderate or moderate-to-severe hearing losses, this may take care of the problem. However, there is a limit to the amount of sound isolation that any earmold can provide; even with the tightest mold; given enough amplification, sound is going to leak from the ear canal and will start the feedback cycle.

This would be particularly true for those people with the most severe hearing losses. They are often unable to achieve the desired amplification targets because of the occurrence of acoustic feedback, no matter how well fit the earmold. To minimize feedback, they will often reduce the gain level of their hearing aids, yet squealing may still occur when they chew, talk, put on a hat, or even comb their hair. These people require an effective solution to their feedback problem that entails more than simply fabricating tighter earmolds.

The Occlusion Effect

People whose hearing losses are less than about 40 dB in the lower frequencies have their own set of issues relating to snug fitting earmolds. One of their perennial complaints when they first start wearing hearing aids is that their own voice sounds "hollow" or "booming," as if they're talking in a barrel. This is due to the acoustic phenomenon known as the "occlusion effect." It occurs when an earmold completely fills the outer portion of the ear canal. What this does is trap the amplified, bone-conducted sound vibrations of a person's own voice in the space between the tip of the earmold and the eardrum. Instead of exiting through the ear canal into the environment as would normally occur, the sound is instead reflected back toward the eardrum, thus increasing the loudness perception of a talker's own voice.

The resulting sound experience can be unpleasant. Compared to a completely open canal, the occlusion effect may boost the low frequency energy by 20 dB or even more. Additionally, these people may feel a sense of pressure or blockage when an earmold is inserted. These auditory experiences can be sufficiently disturbing to cause some people to reject hearing aids, and others to obtain much less benefit than they otherwise could have achieved. While the occlusion effect can also be reduced when an earmold (or canal hearing aid) is inserted deep into the ear canal, right next to the eardrum, this often brings its own set of comfort and wearing problems.

The typical solution for the occlusion effect is to vent an earmold, thus permitting the amplified sound to escape into the environment rather than being directed back into the ear canal. (Note: A vent is a channel drilled through the earmold, extending from its external surface to the tip.) Venting, however, presents its own set of quandaries. A vent is designed to permit sound leakage, but this is precisely what we are trying to avoid when confronting the threat of acoustic feedback. The larger the vent, the more the occlusion effect can be reduced, and this is positive. But the larger the vent, the greater the susceptibility to acoustic feedback, and this is bad.

Often, because the occlusion effect can be so intolerable, people will use vented earmolds and then be forced to reduce the gain of their hearing aids in order to eliminate acoustic feedback. Gain reduction is not an appropriate way to eliminate acoustic feedback. While acoustic feedback can be controlled in this way, it is being achieved by compromising desired amplification goals, particularly in the higher frequencies. Clinicians (and their clients) often find themselves trying to achieve a workable balance -between a tolerable occlusion effect and a minimally acceptable pattern of amplification. This is not something that we should have to compromise on. And, as will be discussed below, we may no longer have to.

In addition to reducing or eliminating the occlusion effect, there are several other relevant acoustic and audiological implications of venting. As already noted, a vented earmold permits the amplified low frequencies to escape from the ear canal. It does this by opening up a less resistant acoustic path for these low frequencies to exit the ear canal rather than continuing forward to the eardrum. In reality, therefore, it is mainly the amplified higher frequencies that are actually transmitted through the middle ear. The larger the vent, the more the low frequencies are shunted out of the ear canal and the more the amplification focus becomes the higher frequencies. These acoustic effects of vented earmolds have long been applied in hearing aid fitting practices. However, vented earmolds not only emphasize the amplified higher frequencies; they also permit the natural reception of the low frequencies directly through the vent to the eardrum.

Here, too, as with the occlusion effect, the reality situation has often necessitated a compromise between the size of the vent for the necessary low frequency reduction and the amplification goal for the higher frequencies. Hearing aid users with relatively good low frequency hearing require a larger vent for maximum low frequency reduction. But since this larger vent increases sound leakage, and thus feedback, it becomes difficult to meet amplification targets for the more impaired higher frequencies. But this is exactly where most of the sound amplification is required for people with this type of hearing loss. The resulting compromise may necessitate more low and less high frequency amplification than is desirable. So it can be a fitting dilemma.

Vents also have a number of advantages in addition to those already reviewed, such as permitting the natural aeration of the ear canal so that it is not perennially moist. To be precise, however, it is not the vent itself that produces these acoustic and non-acoustic effects, but the fact that we are reducing the impact of inserting a foreign body (the earmold) into the ear canal. For people with lesser degrees of hearing loss, a vent is just one step toward obtaining all the acoustic and comfort benefits of a completely "open" ear canal. People with the most severe hearing losses, for whom earmolds will be required for the foreseeable future, need to receive feedback-free amplification at the target output levels. For both these groups, an electronic solution for feedback is necessary in order to realize these goals. Earmold modification by itself will not do it.