The Occlusion Effect – that muffled sound in your head

Occlusion Effect

The Occlusion Effect

The Occlusion Effect — what you can do

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.”

An occlusion effect occurs when some object (like an unvented ear mold) 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 ear mold 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 ear mold, 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.

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