Audiologists and hearing aid specialists often classify a hearing aid fitting as an “open fit” or a “closed fit” device. An open fit simply means that the earbud/dome or earmold allows sound to pass through it and into the ear canal mostly unhindered, so you receive both the natural sound in addition to the sound processed/amplified by the hearing aid. The two key advantages to this are more natural sound, especially with regard to you own voice (when you plug up your ear canals with your finger or an earplug, your voice can sound like you’re talking inside a barrel due to bone conduction inside your skull—a phenomenon known as occlusion).
A potential disadvantage to an open fit can be feedback or a squealing sound. This happens when the microphone picks up sound from the receiver and reamplifies it, creating a “feedback loop”—the same thing that happens when a presenter or singer gets too close to a microphone. Another potential disadvantage is that the processed sound comes out of the receiver milliseconds after the natural sound has already passed by the receiver in the ear canal, creating a timing delay between the processed sound versus the natural sound. When the delay is large enough, this can result in lower sound fidelity. When the delay is very long, you get an annoying echo and people’s lip movements might not match up with the words they’re speaking, making it feel like you’re watching TV with bad reception.
A closed fit means your ear canal, which is a conduit for moving sound to the eardrum, is completely or mostly closed off from outside sound. In this case, what you primarily hear is processed sound. A closed fitting can be particularly effective for people with more severe hearing losses who need more volume or a “power aid,” as well as people who need help with hearing lower frequencies. (If you need a lot of volume, as in the case for a more severe hearing loss, that increases the chance of creating a feedback loop.) In many or most cases, the fidelity of a closed fitting can be every bit as good or better than an open fit (provided that the fit of the earmold and hearing aid processing is good). However, as mentioned above, because the ear canal is mostly sealed off, occlusion can be a bigger challenge with closed fittings. Again, that might result in your own voice sounding echoic or barrel-like, and people with occlusion sometimes complain about abnormally loud chewing.
The good news is that today’s advanced hearing aids have excellent signal processing programs that, in most cases, eliminate feedback and occlusion problems almost completely. If you’re having problems with feedback or occlusion, tell your hearing aid provider about it. For occlusion, they might end up adjusting the lower frequencies or perhaps changing the hearing aid earmold/dome to a different type. In some cases, you may have to “get used to” the new sound.
Regarding feedback (squealing), you should never have to deal with feedback except for very short bursts (eg, from a scarf or hat getting too close to the microphone, or when you reach up to touch your ear or hair, all of which can create a “feedback loop”).
If someone is walking around with a continually squealing hearing aid, there is definitely something not right. Trouble-shooting steps can include:
- Check to see if the hearing aid is being worn properly. If the earmold or dome is out of the ear canal and/or the receiver has something in front of it, feedback can result.
- Check the earmold/dome for wax or anything that might be causing blockage of the receiver. If wax is present, use the cleaning tools provided or change the dome/wax guard.
- If the volume is very high and there is leakage between the microphone and receiver, feedback could be experienced. If you’re constantly turning up the volume to these levels, it’s probably time to see your hearing care provider for refitting.
- A worn out earmold or crimped tube can cause feedback. Again, your hearing care provider should be able to fix these problems.
If these don’t work, insist that your hearing care professional fixes it!
While it’s true that the most-common fitting today is an “open fit,” there are several reasons (not discussed above) why a hearing care professional might instead recommend a “closed fit” hearing aid. Additionally, some types of hearing aids—like most custom-fit in-the-ear or ITE-type devices—offer only closed fittings.